2409 lines
95 KiB
Groff
2409 lines
95 KiB
Groff
.TH LIBPNG 3 "March 9, 1998"
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.SH NAME
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libpng \- Portable Network Graphics (PNG) Reference Library
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.SH SYNOPSIS
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#include <png.h>
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int png_check_sig (png_bytep sig, int num);
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void png_chunk_error (png_structp png_ptr, png_const_charp
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error);
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void png_chunk_warning (png_structp png_ptr, png_const_charp
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message);
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void png_convert_from_struct_tm (png_timep ptime, struct tm FAR
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* ttime);
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void png_convert_from_time_t (png_timep ptime, time_t ttime);
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png_charp png_convert_to_rfc1123 (png_structp png_ptr,
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png_timep ptime);
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png_infop png_create_info_struct (png_structp png_ptr);
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png_structp png_create_read_struct (png_const_charp
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user_png_ver, voidp error_ptr, png_error_ptr error_fn,
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png_error_ptr warn_fn);
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png_structp png_create_write_struct (png_const_charp
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user_png_ver, voidp error_ptr, png_error_ptr error_fn,
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png_error_ptr warn_fn);
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void png_debug_free (png_structp png_ptr, png_voidp ptr);
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png_voidp png_debug_malloc (png_structp png_ptr, png_uint_32
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size);
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void png_destroy_info_struct (png_structp png_ptr, png_infopp
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info_ptr_ptr);
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void png_destroy_read_struct (png_structpp png_ptr_ptr,
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png_infopp info_ptr_ptr, png_infopp end_info_ptr_ptr);
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void png_destroy_write_struct (png_structpp png_ptr_ptr,
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png_infopp info_ptr_ptr);
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void png_error (png_structp png_ptr, png_const_charp error);
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void png_free (png_structp png_ptr, png_voidp ptr);
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png_byte png_get_bit_depth (png_structp png_ptr, png_infop
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info_ptr);
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png_uint_32 png_get_bKGD (png_structp png_ptr, png_infop
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info_ptr, png_color_16p *background);
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png_byte png_get_channels (png_structp png_ptr, png_infop
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info_ptr);
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png_uint_32 png_get_cHRM (png_structp png_ptr, png_infop
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info_ptr, double *white_x, double *white_y, double *red_x,
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double *red_y, double *green_x, double *green_y, double
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*blue_x, double *blue_y);
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png_byte png_get_color_type (png_structp png_ptr, png_infop
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info_ptr);
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png_byte png_get_compression_type (png_structp png_ptr,
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png_infop info_ptr);
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png_voidp png_get_error_ptr (png_structp png_ptr);
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png_byte png_get_filter_type (png_structp png_ptr, png_infop
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info_ptr);
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png_uint_32 png_get_gAMA (png_structp png_ptr, png_infop
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info_ptr, double *file_gamma);
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png_uint_32 png_get_hIST (png_structp png_ptr, png_infop
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info_ptr, png_uint_16p *hist);
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png_uint_32 png_get_image_height (png_structp png_ptr,
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png_infop info_ptr);
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png_uint_32 png_get_image_width (png_structp png_ptr, png_infop
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info_ptr);
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png_byte png_get_interlace_type (png_structp png_ptr, png_infop
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info_ptr);
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png_voidp png_get_io_ptr (png_structp png_ptr);
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png_uint_32 png_get_IHDR (png_structp png_ptr, png_infop
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info_ptr, png_uint_32 *width, png_uint_32 *height, int
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*bit_depth, int *color_type, int *interlace_type, int
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*compression_type, int *filter_type);
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png_uint_32 png_get_oFFs (png_structp png_ptr, png_infop
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info_ptr, png_uint_32 *offset_x, png_uint_32 *offset_y, int
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*unit_type);
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png_uint_32 png_get_pCAL (png_structp png_ptr, png_infop
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info_ptr, png_charp *purpose, png_int_32 *X0, png_int_32 *X1,
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int *type, int *nparams, png_charp *units, png_charpp *params);
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png_uint_32 png_get_pHYs (png_structp png_ptr, png_infop
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info_ptr, png_uint_32 *res_x, png_uint_32 *res_y, int
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*unit_type);
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float png_get_pixel_aspect_ratio (png_structp png_ptr,
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png_infop info_ptr);
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png_uint_32 png_get_pixels_per_meter (png_structp png_ptr,
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png_infop info_ptr);
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png_voidp png_get_progressive_ptr (png_structp png_ptr);
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png_uint_32 png_get_PLTE (png_structp png_ptr, png_infop
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info_ptr, png_colorp *palette, int *num_palette);
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png_uint_32 png_get_rowbytes (png_structp png_ptr, png_infop
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info_ptr);
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png_uint_32 png_get_sBIT (png_structp png_ptr, png_infop
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info_ptr, png_color_8p *sig_bit);
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png_bytep png_get_signature (png_structp png_ptr, png_infop
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info_ptr);
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png_uint_32 png_get_sRGB (png_structp png_ptr, png_infop
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info_ptr, int *intent);
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png_uint_32 png_get_text (png_structp png_ptr, png_infop
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info_ptr, png_textp *text_ptr, int *num_text);
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png_uint_32 png_get_tIME (png_structp png_ptr, png_infop
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info_ptr, png_timep *mod_time);
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png_uint_32 png_get_tRNS (png_structp png_ptr, png_infop
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info_ptr, png_bytep *trans, int *num_trans, png_color_16p
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*trans_values);
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png_uint_32 png_get_valid (png_structp png_ptr, png_infop
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info_ptr, png_uint_32 flag);
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png_uint_32 png_get_x_offset_microns (png_structp png_ptr,
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png_infop info_ptr);
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png_uint_32 png_get_x_offset_pixels (png_structp png_ptr,
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png_infop info_ptr);
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png_uint_32 png_get_x_pixels_per_meter (png_structp png_ptr,
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png_infop info_ptr);
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png_uint_32 png_get_y_offset_microns (png_structp png_ptr,
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png_infop info_ptr);
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png_uint_32 png_get_y_offset_pixels (png_structp png_ptr,
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png_infop info_ptr);
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png_uint_32 png_get_y_pixels_per_meter (png_structp png_ptr,
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png_infop info_ptr);
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void png_info_init (png_infop info_ptr);
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void png_init_io (png_structp png_ptr, FILE *fp);
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png_voidp png_malloc (png_structp png_ptr, png_uint_32 size);
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voidp png_memcpy (png_voidp s1, png_voidp s2, png_size_t size);
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png_voidp png_memcpy_check (png_structp png_ptr, png_voidp s1,
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png_voidp s2, png_uint_32 size);
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voidp png_memset (png_voidp s1, int value, png_size_t size);
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png_voidp png_memset_check (png_structp png_ptr, png_voidp
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s1, int value, png_uint_32 size);
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void png_process_data (png_structp png_ptr, png_infop info_ptr,
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png_bytep buffer, png_size_t buffer_size);
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void png_progressive_combine_row (png_structp png_ptr,
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png_bytep old_row, png_bytep new_row);
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void png_read_destroy (png_structp png_ptr, png_infop info_ptr,
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png_infop end_info_ptr);
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void png_read_end (png_structp png_ptr, png_infop info_ptr);
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void png_read_image (png_structp png_ptr, png_bytepp image);
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void png_read_info (png_structp png_ptr, png_infop info_ptr);
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void png_read_row (png_structp png_ptr, png_bytep row,
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png_bytep display_row);
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void png_read_rows (png_structp png_ptr, png_bytepp row,
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png_bytepp display_row, png_uint_32 num_rows);
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void png_read_update_info (png_structp png_ptr, png_infop
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info_ptr);
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void png_set_background (png_structp png_ptr, png_color_16p
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background_color, int background_gamma_code, int need_expand,
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double background_gamma);
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void png_set_bgr (png_structp png_ptr);
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void png_set_bKGD (png_structp png_ptr, png_infop info_ptr,
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png_color_16p background);
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void png_set_cHRM (png_structp png_ptr, png_infop info_ptr,
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double white_x, double white_y, double red_x, double red_y,
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double green_x, double green_y, double blue_x, double blue_y);
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void png_set_compression_level (png_structp png_ptr, int
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level);
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void png_set_compression_mem_level (png_structp png_ptr, int
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mem_level);
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void png_set_compression_method (png_structp png_ptr, int
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method);
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void png_set_compression_strategy (png_structp png_ptr, int
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strategy);
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void png_set_compression_window_bits (png_structp png_ptr, int
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window_bits);
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void png_set_crc_action (png_structp png_ptr, int crit_action,
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int ancil_action);
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void png_set_dither (png_structp png_ptr, png_colorp palette,
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int num_palette, int maximum_colors, png_uint_16p histogram,
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int full_dither);
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void png_set_error_fn (png_structp png_ptr, png_voidp
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error_ptr, png_error_ptr error_fn, png_error_ptr warning_fn);
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void png_set_expand (png_structp png_ptr);
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void png_set_filler (png_structp png_ptr, png_uint_32 filler,
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int flags);
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void png_set_filter (png_structp png_ptr, int method, int
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filters);
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void png_set_filter_heuristics (png_structp png_ptr, int
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heuristic_method, int num_weights, png_doublep filter_weights,
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png_doublep filter_costs);
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void png_set_flush (png_structp png_ptr, int nrows);
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void png_set_gamma (png_structp png_ptr, double screen_gamma,
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double default_file_gamma);
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void png_set_gAMA (png_structp png_ptr, png_infop info_ptr,
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double file_gamma);
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void png_set_gray_to_rgb (png_structp png_ptr);
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void png_set_hIST (png_structp png_ptr, png_infop info_ptr,
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png_uint_16p hist);
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int png_set_interlace_handling (png_structp png_ptr);
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void png_set_invert_alpha (png_structp png_ptr);
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void png_set_invert_mono (png_structp png_ptr);
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void png_set_IHDR (png_structp png_ptr, png_infop info_ptr,
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png_uint_32 width, png_uint_32 height, int bit_depth, int
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color_type, int interlace_type, int compression_type, int
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filter_type);
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void png_set_oFFs (png_structp png_ptr, png_infop info_ptr,
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png_uint_32 offset_x, png_uint_32 offset_y, int unit_type);
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void png_set_packing (png_structp png_ptr);
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void png_set_packswap (png_structp png_ptr);
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void png_set_pCAL (png_structp png_ptr, png_infop info_ptr,
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png_charp purpose, png_int_32 X0, png_int_32 X1, int type, int
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nparams, png_charp units, png_charpp params);
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void png_set_pHYs (png_structp png_ptr, png_infop info_ptr,
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png_uint_32 res_x, png_uint_32 res_y, int unit_type);
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void png_set_progressive_read_fn (png_structp png_ptr,
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png_voidp progressive_ptr, png_progressive_info_ptr info_fn,
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png_progressive_row_ptr row_fn, png_progressive_end_ptr
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end_fn);
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void png_set_PLTE (png_structp png_ptr, png_infop info_ptr,
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png_colorp palette, int num_palette);
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void png_set_read_fn (png_structp png_ptr, png_voidp io_ptr,
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png_rw_ptr read_data_fn);
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void png_set_read_status_fn (png_structp png_ptr, png_read_status_ptr
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read_row_fn);
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void png_set_read_user_transform_fn (png_structp png_ptr,
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png_user_transform_ptr read_user_transform_fn);
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void png_set_rgb_to_gray (png_structp png_ptr);
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void png_set_sBIT (png_structp png_ptr, png_infop info_ptr,
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png_color_8p sig_bit);
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void png_set_shift (png_structp png_ptr, png_color_8p
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true_bits);
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void png_set_sig_bytes (png_structp png_ptr, int num_bytes);
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void png_set_sRGB (png_structp png_ptr, png_infop info_ptr, int
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intent);
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void png_set_sRGB_gAMA_and_cHRM (png_structp png_ptr, png_infop
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info_ptr, int intent);
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void png_set_strip_16 (png_structp png_ptr);
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void png_set_strip_alpha (png_structp png_ptr);
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void png_set_swap (png_structp png_ptr);
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void png_set_swap_alpha (png_structp png_ptr);
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void png_set_text (png_structp png_ptr, png_infop info_ptr,
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png_textp text_ptr, int num_text);
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void png_set_tIME (png_structp png_ptr, png_infop info_ptr,
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png_timep mod_time);
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void png_set_tRNS (png_structp png_ptr, png_infop info_ptr,
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png_bytep trans, int num_trans, png_color_16p trans_values);
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void png_set_write_fn (png_structp png_ptr, png_voidp io_ptr,
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png_rw_ptr write_data_fn, png_flush_ptr output_flush_fn);
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void png_set_write_status_fn (png_structp png_ptr, png_write_status_ptr
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write_row_fn);
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void png_set_write_user_transform_fn (png_structp png_ptr,
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png_user_transform_ptr write_user_transform_fn);
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int png_sig_cmp (png_bytep sig, png_size_t start, png_size_t
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num_to_check);
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void png_start_read_image (png_structp png_ptr);
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void png_warning (png_structp png_ptr, png_const_charp
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message);
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void png_write_chunk (png_structp png_ptr, png_bytep
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chunk_name, png_bytep data, png_size_t length);
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void png_write_chunk_data (png_structp png_ptr, png_bytep data,
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png_size_t length);
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void png_write_chunk_end (png_structp png_ptr);
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void png_write_chunk_start (png_structp png_ptr, png_bytep
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chunk_name, png_uint_32 length);
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void png_write_destroy (png_structp png_ptr);
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void png_write_destroy_info (png_infop info_ptr);
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void png_write_end (png_structp png_ptr, png_infop info_ptr);
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void png_write_flush (png_structp png_ptr);
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void png_write_image (png_structp png_ptr, png_bytepp image);
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void png_write_info (png_structp png_ptr, png_infop info_ptr);
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void png_write_row (png_structp png_ptr, png_bytep row);
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void png_write_rows (png_structp png_ptr, png_bytepp row,
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png_uint_32 num_rows);
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.SH DESCRIPTION
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The
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.I libpng
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library supports encoding, decoding, and various manipulations of
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the Portable Network Graphics (PNG) format image files. It uses the
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.IR zlib(3)
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compression library.
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Following is a copy of the libpng.txt file that accompanies libpng.
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.SH LIBPNG.TXT
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libpng.txt - A description on how to use and modify libpng
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libpng version 1.0.1 March 9, 1998
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Updated and distributed by Glenn Randers-Pehrson
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<randeg@alumni.rpi.edu>
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Copyright (c) 1998, Glenn Randers-Pehrson
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For conditions of distribution and use, see copyright
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notice in png.h.
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based on:
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libpng 1.0 beta 6 version 0.96 May 28, 1997
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Updated and distributed by Andreas Dilger
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Copyright (c) 1996, 1997 Andreas Dilger
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libpng 1.0 beta 2 - version 0.88 January 26, 1996
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For conditions of distribution and use, see copyright
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notice in png.h. Copyright (c) 1995, 1996 Guy Eric
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Schalnat, Group 42, Inc.
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Updated/rewritten per request in the libpng FAQ
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Copyright (c) 1995 Frank J. T. Wojcik
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December 18, 1995 && January 20, 1996
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.SH I. Introduction
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This file describes how to use and modify the PNG reference library
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(known as libpng) for your own use. There are five sections to this
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file: introduction, structures, reading, writing, and modification and
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configuration notes for various special platforms. In addition to this
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file, example.c is a good starting point for using the library, as
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it is heavily commented and should include everything most people
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will need. We assume that libpng is already installed; see the
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INSTALL file for instructions on how to install libpng.
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Libpng was written as a companion to the PNG specification, as a way
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of reducing the amount of time and effort it takes to support the PNG
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file format in application programs. The PNG specification is available
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as RFC 2083 <ftp://ftp.uu.net/graphics/png/documents/> and as a
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W3C Recommendation <http://www.w3.org/TR/REC.png.html>. Some
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additional chunks are described in the special-purpose public chunks
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documents at <ftp://ftp.uu.net/graphics/png/documents/>. Other information
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about PNG can be found at the PNG home page, <http://www.cdrom.com/pub/png/>.
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Most users will not have to modify the library significantly; advanced
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users may want to modify it more. All attempts were made to make it as
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complete as possible, while keeping the code easy to understand.
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Currently, this library only supports C. Support for other languages
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is being considered.
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Libpng has been designed to handle multiple sessions at one time,
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to be easily modifiable, to be portable to the vast majority of
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machines (ANSI, K&R, 16-, 32-, and 64-bit) available, and to be easy
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to use. The ultimate goal of libpng is to promote the acceptance of
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|
the PNG file format in whatever way possible. While there is still
|
|
work to be done (see the TODO file), libpng should cover the
|
|
majority of the needs of its users.
|
|
|
|
Libpng uses zlib for its compression and decompression of PNG files.
|
|
The zlib compression utility is a general purpose utility that is
|
|
useful for more than PNG files, and can be used without libpng.
|
|
See the documentation delivered with zlib for more details.
|
|
You can usually find the source files for the zlib utility wherever you
|
|
find the libpng source files.
|
|
|
|
Libpng is thread safe, provided the threads are using different
|
|
instances of the structures. Each thread should have its own
|
|
png_struct and png_info instances, and thus its own image.
|
|
Libpng does not protect itself against two threads using the
|
|
same instance of a structure.
|
|
|
|
|
|
.SH II. Structures
|
|
|
|
There are two main structures that are important to libpng, png_struct
|
|
and png_info. The first, png_struct, is an internal structure that
|
|
will not, for the most part, be used by a user except as the first
|
|
variable passed to every libpng function call.
|
|
|
|
The png_info structure is designed to provide information about the
|
|
PNG file. At one time, the fields of png_info were intended to be
|
|
directly accessible to the user. However, this tended to cause problems
|
|
with applications using dynamically loaded libraries, and as a result
|
|
a set of interface functions for png_info was developed. The fields
|
|
of png_info are still available for older applications, but it is
|
|
suggested that applications use the new interfaces if at all possible.
|
|
|
|
The png.h header file is an invaluable reference for programming with libpng.
|
|
And while I'm on the topic, make sure you include the libpng header file:
|
|
|
|
#include <png.h>
|
|
|
|
.SH III. Reading
|
|
|
|
Reading PNG files:
|
|
|
|
We'll now walk you through the possible functions to call when reading
|
|
in a PNG file, briefly explaining the syntax and purpose of each one.
|
|
See example.c and png.h for more detail. While Progressive reading
|
|
is covered in the next section, you will still need some of the
|
|
functions discussed in this section to read a PNG file.
|
|
|
|
You will want to do the I/O initialization(*) before you get into libpng,
|
|
so if it doesn't work, you don't have much to undo. Of course, you
|
|
will also want to insure that you are, in fact, dealing with a PNG
|
|
file. Libpng provides a simple check to see if a file is a PNG file.
|
|
To use it, pass in the first 1 to 8 bytes of the file, and it will
|
|
return true or false (1 or 0) depending on whether the bytes could be
|
|
part of a PNG file. Of course, the more bytes you pass in, the
|
|
greater the accuracy of the prediction.
|
|
|
|
If you are intending to keep the file pointer open for use in libpng,
|
|
you must ensure you don't read more than 8 bytes from the beginning
|
|
of the file, and you also have to make a call to png_set_sig_bytes_read()
|
|
with the number of bytes you read from the beginning. Libpng will
|
|
then only check the bytes (if any) that your program didn't read.
|
|
|
|
(*): If you are not using the standard I/O functions, you will need
|
|
to replace them with custom functions. See the discussion under
|
|
Customizing libpng.
|
|
|
|
|
|
FILE *fp = fopen(file_name, "rb");
|
|
if (!fp)
|
|
{
|
|
return;
|
|
}
|
|
fread(header, 1, number, fp);
|
|
is_png = png_check_sig(header, 0, number);
|
|
if (!is_png)
|
|
{
|
|
return;
|
|
}
|
|
|
|
|
|
Next, png_struct and png_info need to be allocated and initialized. In
|
|
order to ensure that the size of these structures is correct even with a
|
|
dynamically linked libpng, there are functions to initialize and
|
|
allocate the structures. We also pass the library version, optional
|
|
pointers to error handling functions, and a pointer to a data struct for
|
|
use by the error functions, if necessary (the pointer and functions can
|
|
be NULL if the default error handlers are to be used). See the section
|
|
on Changes to Libpng below regarding the old initialization functions.
|
|
|
|
png_structp png_ptr = png_create_read_struct
|
|
(PNG_LIBPNG_VER_STRING, (void *)user_error_ptr,
|
|
user_error_fn, user_warning_fn);
|
|
if (!png_ptr)
|
|
return;
|
|
|
|
png_infop info_ptr = png_create_info_struct(png_ptr);
|
|
if (!info_ptr)
|
|
{
|
|
png_destroy_read_struct(&png_ptr,
|
|
(png_infopp)NULL, (png_infopp)NULL);
|
|
return;
|
|
}
|
|
|
|
png_infop end_info = png_create_info_struct(png_ptr);
|
|
if (!end_info)
|
|
{
|
|
png_destroy_read_struct(&png_ptr, &info_ptr,
|
|
(png_infopp)NULL);
|
|
return;
|
|
}
|
|
|
|
|
|
The error handling routines passed to png_create_read_struct() are only
|
|
necessary if you are not using the libpng supplied error handling
|
|
functions. When libpng encounters an error, it expects to longjmp back
|
|
to your routine. Therefore, you will need to call setjmp and pass the
|
|
jmpbuf field of your png_struct. If you read the file from different
|
|
routines, you will need to update the jmpbuf field every time you enter
|
|
a new routine that will call a png_ function.
|
|
|
|
See your documentation of setjmp/longjmp for your compiler for more
|
|
handling in the Customizing Libpng section below for more information on
|
|
the libpng error handling. If an error occurs, and libpng longjmp's
|
|
back to your setjmp, you will want to call png_destroy_read_struct() to
|
|
free any memory.
|
|
|
|
if (setjmp(png_ptr->jmpbuf))
|
|
{
|
|
png_destroy_read_struct(&png_ptr, &info_ptr,
|
|
&end_info);
|
|
fclose(fp);
|
|
return;
|
|
}
|
|
|
|
Now you need to set up the input code. The default for libpng is to
|
|
use the C function fread(). If you use this, you will need to pass a
|
|
valid FILE * in the function png_init_io(). Be sure that the file is
|
|
opened in binary mode. If you wish to handle reading data in another
|
|
way, you need not call the png_init_io() function, but you must then
|
|
implement the libpng I/O methods discussed in the Customizing Libpng
|
|
section below.
|
|
|
|
png_init_io(png_ptr, fp);
|
|
|
|
If you had previously opened the file and read any of the signature from
|
|
the beginning in order to see if this was a PNG file, you need to let
|
|
libpng know that there are some bytes missing from the start of the file.
|
|
|
|
png_set_sig_bytes(png_ptr, number);
|
|
|
|
At this point, you can set up a callback function that will be
|
|
called after each row has been read, which you can use to control
|
|
a progress meter or the like. It's demonstrated in pngtest.c.
|
|
You must supply a function
|
|
|
|
void read_row_callback(png_ptr, png_uint_32 row, int pass);
|
|
{
|
|
/* put your code here */
|
|
}
|
|
|
|
(You can give it another name that you like instead of "read_row_callback")
|
|
|
|
To inform libpng about your function, use
|
|
|
|
png_set_read_status_fn(png_ptr, read_row_callback);
|
|
|
|
In PNG files, the alpha channel in an image is the level of opacity.
|
|
If you need the alpha channel in an image to be the level of transparency
|
|
instead of opacity, you can invert the alpha channel (or the tRNS chunk
|
|
data) after it's read, so that 0 is fully opaque and 255 (in 8-bit or
|
|
paletted images) or 65535 (in 16-bit images) is fully transparent, with
|
|
|
|
png_set_invert_alpha(png_ptr);
|
|
|
|
This has to appear here rather than later with the other transformations
|
|
because the tRNS chunk data must be modified in the case of paletted images.
|
|
If your image is not a paletted image, the tRNS data (which in such cases
|
|
represents a single color to be rendered as transparent) won't be changed.
|
|
|
|
Finally, you can write your own transformation function if none of
|
|
the existing ones meets your needs. This is done by setting a callback
|
|
with
|
|
|
|
png_set_read_user_transform_fn(png_ptr,
|
|
read_transform_fn);
|
|
|
|
You must supply the function
|
|
|
|
void read_transform_fn(png_ptr ptr, row_info_ptr
|
|
row_info, png_bytep data)
|
|
|
|
See pngtest.c for a working example. Your function will be called
|
|
after all of the other transformations have been processed.
|
|
|
|
You are now ready to read all the file information up to the actual
|
|
image data. You do this with a call to png_read_info().
|
|
|
|
png_read_info(png_ptr, info_ptr);
|
|
|
|
Functions are used to get the information from the info_ptr:
|
|
|
|
png_get_IHDR(png_ptr, info_ptr, &width, &height,
|
|
&bit_depth, &color_type, &interlace_type,
|
|
&compression_type, &filter_type);
|
|
|
|
width - holds the width of the image
|
|
in pixels (up to 2^31).
|
|
height - holds the height of the image
|
|
in pixels (up to 2^31).
|
|
bit_depth - holds the bit depth of one of the
|
|
image channels. (valid values are
|
|
1, 2, 4, 8, 16 and depend also on
|
|
the color_type. See also
|
|
significant bits (sBIT) below).
|
|
color_type - describes which color/alpha channels
|
|
are present.
|
|
PNG_COLOR_TYPE_GRAY
|
|
(bit depths 1, 2, 4, 8, 16)
|
|
PNG_COLOR_TYPE_GRAY_ALPHA
|
|
(bit depths 8, 16)
|
|
PNG_COLOR_TYPE_PALETTE
|
|
(bit depths 1, 2, 4, 8)
|
|
PNG_COLOR_TYPE_RGB
|
|
(bit_depths 8, 16)
|
|
PNG_COLOR_TYPE_RGB_ALPHA
|
|
(bit_depths 8, 16)
|
|
|
|
PNG_COLOR_MASK_PALETTE
|
|
PNG_COLOR_MASK_COLOR
|
|
PNG_COLOR_MASK_ALPHA
|
|
|
|
filter_type - (must be PNG_FILTER_TYPE_BASE
|
|
for PNG 1.0)
|
|
compression_type - (must be PNG_COMPRESSION_TYPE_BASE
|
|
for PNG 1.0)
|
|
interlace_type - (PNG_INTERLACE_NONE or
|
|
PNG_INTERLACE_ADAM7)
|
|
Any or all of interlace_type, compression_type, of
|
|
filter_type can be
|
|
NULL if you are not interested in their values.
|
|
|
|
channels = png_get_channels(png_ptr, info_ptr);
|
|
channels - number of channels of info for the
|
|
color type (valid values are 1 (GRAY,
|
|
PALETTE), 2 (GRAY_ALPHA), 3 (RGB),
|
|
4 (RGB_ALPHA or RGB + filler byte))
|
|
rowbytes = png_get_rowbytes(png_ptr, info_ptr);
|
|
rowbytes - number of bytes needed to hold a row
|
|
|
|
signature = png_get_signature(png_ptr, info_ptr);
|
|
signature - holds the signature read from the
|
|
file (if any). The data is kept in
|
|
the same offset it would be if the
|
|
whole signature were read (i.e. if an
|
|
application had already read in 4
|
|
bytes of signature before starting
|
|
libpng, the remaining 4 bytes would
|
|
be in signature[4] through signature[7]
|
|
(see png_set_sig_bytes())).
|
|
|
|
|
|
width = png_get_image_width(png_ptr,
|
|
info_ptr);
|
|
height = png_get_image_height(png_ptr,
|
|
info_ptr);
|
|
bit_depth = png_get_bit_depth(png_ptr,
|
|
info_ptr);
|
|
color_type = png_get_color_type(png_ptr,
|
|
info_ptr);
|
|
filter_type = png_get_filter_type(png_ptr,
|
|
info_ptr);
|
|
compression_type = png_get_compression_type(png_ptr,
|
|
info_ptr);
|
|
interlace_type = png_get_interlace_type(png_ptr,
|
|
info_ptr);
|
|
|
|
|
|
These are also important, but their validity depends on whether the chunk
|
|
has been read. The png_get_valid(png_ptr, info_ptr, PNG_INFO_<chunk>) and
|
|
png_get_<chunk>(png_ptr, info_ptr, ...) functions return non-zero if the
|
|
data has been read, or zero if it is missing. The parameters to the
|
|
png_get_<chunk> are set directly if they are simple data types, or a pointer
|
|
into the info_ptr is returned for any complex types.
|
|
|
|
png_get_PLTE(png_ptr, info_ptr, &palette,
|
|
&num_palette);
|
|
palette - the palette for the file
|
|
(array of png_color)
|
|
num_palette - number of entries in the palette
|
|
|
|
png_get_gAMA(png_ptr, info_ptr, &gamma);
|
|
gamma - the gamma the file is written
|
|
at (PNG_INFO_gAMA)
|
|
|
|
png_get_sRGB(png_ptr, info_ptr, &srgb_intent);
|
|
srgb_intent - the rendering intent (PNG_INFO_sRGB)
|
|
The presence of the sRGB chunk
|
|
means that the pixel data is in the
|
|
sRGB color space. This chunk also
|
|
implies specific values of gAMA and
|
|
cHRM.
|
|
|
|
png_get_sBIT(png_ptr, info_ptr, &sig_bit);
|
|
sig_bit - the number of significant bits for
|
|
(PNG_INFO_sBIT) each of the gray,
|
|
red, green, and blue channels,
|
|
whichever are appropriate for the
|
|
given color type (png_color_16)
|
|
|
|
png_get_tRNS(png_ptr, info_ptr, &trans, &num_trans,
|
|
&trans_values);
|
|
trans - array of transparent entries for
|
|
palette (PNG_INFO_tRNS)
|
|
trans_values - transparent pixel for non-paletted
|
|
images (PNG_INFO_tRNS)
|
|
num_trans - number of transparent entries
|
|
(PNG_INFO_tRNS)
|
|
|
|
png_get_hIST(png_ptr, info_ptr, &hist);
|
|
(PNG_INFO_hIST)
|
|
hist - histogram of palette (array of
|
|
png_color_16)
|
|
|
|
png_get_tIME(png_ptr, info_ptr, &mod_time);
|
|
mod_time - time image was last modified
|
|
(PNG_VALID_tIME)
|
|
|
|
png_get_bKGD(png_ptr, info_ptr, &background);
|
|
background - background color (PNG_VALID_bKGD)
|
|
|
|
num_text = png_get_text(png_ptr, info_ptr, &text_ptr);
|
|
text_ptr - array of png_text holding image
|
|
comments
|
|
text_ptr[i]->key - keyword for comment.
|
|
text_ptr[i]->text - text comments for current
|
|
keyword.
|
|
text_ptr[i]->compression - type of compression used
|
|
on "text" PNG_TEXT_COMPRESSION_NONE
|
|
or PNG_TEXT_COMPRESSION_zTXt
|
|
num_text - number of comments
|
|
|
|
png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y,
|
|
&unit_type);
|
|
offset_x - positive offset from the left edge
|
|
of the screen
|
|
offset_y - positive offset from the top edge
|
|
of the screen
|
|
unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
|
|
|
|
png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y,
|
|
&unit_type);
|
|
res_x - pixels/unit physical resolution in
|
|
x direction
|
|
res_y - pixels/unit physical resolution in
|
|
x direction
|
|
unit_type - PNG_RESOLUTION_UNKNOWN,
|
|
PNG_RESOLUTION_METER
|
|
|
|
The data from the pHYs chunk can be retrieved in several convenient
|
|
forms:
|
|
|
|
res_x = png_get_x_pixels_per_meter(png_ptr,
|
|
info_ptr)
|
|
res_y = png_get_y_pixels_per_meter(png_ptr,
|
|
info_ptr)
|
|
res_x_and_y = png_get_pixels_per_meter(png_ptr,
|
|
info_ptr)
|
|
aspect_ratio = png_get_pixel_aspect_ratio(png_ptr,
|
|
info_ptr)
|
|
|
|
(Each of these returns 0 [signifying "unknown"] if
|
|
the data is not present or if res_x is 0;
|
|
res_x_and_y is 0 if res_x != res_y)
|
|
|
|
For more information, see the png_info definition in png.h and the
|
|
PNG specification for chunk contents. Be careful with trusting
|
|
rowbytes, as some of the transformations could increase the space
|
|
needed to hold a row (expand, filler, gray_to_rgb, etc.).
|
|
See png_read_update_info(), below.
|
|
|
|
A quick word about text_ptr and num_text. PNG stores comments in
|
|
keyword/text pairs, one pair per chunk, with no limit on the number
|
|
of text chunks, and a 2^31 byte limit on their size. While there are
|
|
suggested keywords, there is no requirement to restrict the use to these
|
|
strings. It is strongly suggested that keywords and text be sensible
|
|
to humans (that's the point), so don't use abbreviations. Non-printing
|
|
symbols are not allowed. See the PNG specification for more details.
|
|
There is also no requirement to have text after the keyword.
|
|
|
|
Keywords should be limited to 79 Latin-1 characters without leading or
|
|
trailing spaces, but non-consecutive spaces are allowed within the
|
|
keyword. It is possible to have the same keyword any number of times.
|
|
The text_ptr is an array of png_text structures, each holding pointer
|
|
to a keyword and a pointer to a text string. Only the text string may
|
|
be null. The keyword/text pairs are put into the array in the order
|
|
that they are received. However, some or all of the text chunks may be
|
|
after the image, so, to make sure you have read all the text chunks,
|
|
don't mess with these until after you read the stuff after the image.
|
|
This will be mentioned again below in the discussion that goes with
|
|
png_read_end().
|
|
|
|
After you've read the header information, you can set up the library
|
|
to handle any special transformations of the image data. The various
|
|
ways to transform the data will be described in the order that they
|
|
should occur. This is important, as some of these change the color
|
|
type and/or bit depth of the data, and some others only work on
|
|
certain color types and bit depths. Even though each transformation
|
|
checks to see if it has data that it can do something with, you should
|
|
make sure to only enable a transformation if it will be valid for the
|
|
data. For example, don't swap red and blue on grayscale data.
|
|
|
|
The colors used for the background and transparency values should be
|
|
supplied in the same format/depth as the current image data. They
|
|
are stored in the same format/depth as the image data in a bKGD or tRNS
|
|
chunk, so this is what libpng expects for this data. The colors are
|
|
transformed to keep in sync with the image data when an application
|
|
calls the png_read_update_info() routine (see below).
|
|
|
|
Data will be decoded into the supplied row buffers packed into bytes
|
|
unless the library has been told to transform it into another format.
|
|
For example, 4 bit/pixel paletted or grayscale data will be returned
|
|
2 pixels/byte with the leftmost pixel in the high-order bits of the
|
|
byte, unless png_set_packing() is called. 8-bit RGB data will be stored
|
|
in RGBRGBRGB format unless png_set_filler() is called to insert filler
|
|
bytes, either before or after each RGB triplet. 16-bit RGB data will
|
|
be returned RRGGBBRRGGBB, with the most significant byte of the color
|
|
value first, unless png_set_strip_16() is called to transform it to
|
|
regular RGBRGB triplets.
|
|
|
|
The following code transforms grayscale images of less than 8 to 8 bits,
|
|
changes paletted images to RGB, and adds a full alpha channel if there is
|
|
transparency information in a tRNS chunk. This is most useful on
|
|
grayscale images with bit depths of 2 or 4 or if there is a multiple-image
|
|
viewing application that wishes to treat all images in the same way.
|
|
|
|
if (color_type == PNG_COLOR_TYPE_PALETTE &&
|
|
bit_depth <= 8) png_set_expand(png_ptr);
|
|
|
|
if (color_type == PNG_COLOR_TYPE_GRAY &&
|
|
bit_depth < 8) png_set_expand(png_ptr);
|
|
|
|
if (png_get_valid(png_ptr, info_ptr,
|
|
PNG_INFO_tRNS)) png_set_expand(png_ptr);
|
|
|
|
PNG can have files with 16 bits per channel. If you only can handle
|
|
8 bits per channel, this will strip the pixels down to 8 bit.
|
|
|
|
if (bit_depth == 16)
|
|
png_set_strip_16(png_ptr);
|
|
|
|
The png_set_background() function tells libpng to composite images
|
|
with alpha or simple transparency against the supplied background
|
|
color. If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid),
|
|
you may use this color, or supply another color more suitable for
|
|
the current display (e.g., the background color from a web page). You
|
|
need to tell libpng whether the color is in the gamma space of the
|
|
display (PNG_BACKGROUND_GAMMA_SCREEN for colors you supply), the file
|
|
(PNG_BACKGROUND_GAMMA_FILE for colors from the bKGD chunk), or one
|
|
that is neither of these gammas (PNG_BACKGROUND_GAMMA_UNIQUE - I don't
|
|
know why anyone would use this, but it's here).
|
|
|
|
If, for some reason, you don't need the alpha channel on an image,
|
|
and you want to remove it rather than combining it with the background
|
|
(but the image author certainly had in mind that you *would* combine
|
|
it with the background, so that's what you should probably do):
|
|
|
|
if (color_type & PNG_COLOR_MASK_ALPHA)
|
|
png_set_strip_alpha(png_ptr);
|
|
|
|
PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
|
|
they can, resulting in, for example, 8 pixels per byte for 1 bit
|
|
files. This code expands to 1 pixel per byte without changing the
|
|
values of the pixels:
|
|
|
|
if (bit_depth < 8)
|
|
png_set_packing(png_ptr);
|
|
|
|
PNG files have possible bit depths of 1, 2, 4, 8, and 16. All pixels
|
|
stored in a PNG image have been "scaled" or "shifted" up to the next
|
|
higher possible bit depth (e.g. from 5 bits/sample in the range [0,31] to
|
|
8 bits/sample in the range [0, 255]). However, it is also possible to
|
|
convert the PNG pixel data back to the original bit depth of the image.
|
|
This call reduces the pixels back down to the original bit depth:
|
|
|
|
png_color_16p sig_bit;
|
|
|
|
if (png_get_sBIT(png_ptr, info_ptr, &sig_bit))
|
|
png_set_shift(png_ptr, sig_bit);
|
|
|
|
PNG files store 3-color pixels in red, green, blue order. This code
|
|
changes the storage of the pixels to blue, green, red:
|
|
|
|
if (color_type == PNG_COLOR_TYPE_RGB ||
|
|
color_type == PNG_COLOR_TYPE_RGB_ALPHA)
|
|
png_set_bgr(png_ptr);
|
|
|
|
PNG files store RGB pixels packed into 3 bytes. This code expands them
|
|
into 4 bytes for windowing systems that need them in this format:
|
|
|
|
if (bit_depth == 8 && color_type ==
|
|
PNG_COLOR_TYPE_RGB) png_set_filler(png_ptr,
|
|
filler, PNG_FILLER_BEFORE);
|
|
|
|
where "filler" is the number to fill with, and the location is
|
|
either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether
|
|
you want the filler before the RGB or after. This transformation
|
|
does not affect images that already have full alpha channels.
|
|
|
|
If you are reading an image with an alpha channel, and you need the
|
|
data as ARGB instead of the normal PNG format RGBA:
|
|
|
|
if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
|
|
png_set_swap_alpha(png_ptr);
|
|
|
|
For some uses, you may want a grayscale image to be represented as
|
|
RGB. This code will do that conversion:
|
|
|
|
if (color_type == PNG_COLOR_TYPE_GRAY ||
|
|
color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
|
|
png_set_gray_to_rgb(png_ptr);
|
|
|
|
If you have a grayscale and you are using png_set_expand() to change to
|
|
a higher bit-depth you must indicate if the supplied background gray
|
|
is supplied in the original file bit depth (need_expand = 1) or in the
|
|
expanded bit depth (need_expand = 0). Similarly, if you are reading
|
|
a paletted image, you must indicate if you have supplied the background
|
|
as a palette index that needs to be expanded (need_expand = 1). You can
|
|
also specify an RGB triplet that isn't in the palette when setting your
|
|
background for a paletted image.
|
|
|
|
png_color_16 my_background;
|
|
png_color_16p image_background;
|
|
|
|
if (png_get_bKGD(png_ptr, info_ptr,
|
|
&image_background))
|
|
png_set_background(png_ptr, image_background),
|
|
PNG_BACKGROUND_GAMMA_FILE, 1, 1.0);
|
|
else
|
|
png_set_background(png_ptr, &my_background,
|
|
PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0);
|
|
|
|
To properly display PNG images on any kind of system, the application needs
|
|
to know what the display gamma is. Ideally, the user will know this, and
|
|
the application will allow them to set it. One method of allowing the user
|
|
to set the display gamma separately for each system is to check for the
|
|
DISPLAY_GAMMA and VIEWING_GAMMA environment variables or for a SCREEN_GAMMA
|
|
environment variable, which will hopefully be correctly set.
|
|
|
|
Note that display_gamma is the gamma of your display, while screen_gamma is
|
|
the overall gamma correction required to produce pleasing results,
|
|
which depends on the lighting conditions in the surrounding environment.
|
|
Screen_gamma is display_gamma/viewing_gamma, where viewing_gamma is
|
|
the amount of additional gamma correction needed to compensate for
|
|
a (viewing_gamma=1.25) environment. In a dim or brightly lit room, no
|
|
compensation other than the display_gamma is needed (viewing_gamma=1.0).
|
|
|
|
if (/* We have a user-defined screen
|
|
gamma value */)
|
|
{
|
|
screen_gamma = user_defined_screen_gamma;
|
|
}
|
|
/* One way that applications can share the same
|
|
screen gamma value */
|
|
else if ((gamma_str = getenv("SCREEN_GAMMA"))
|
|
!= NULL)
|
|
{
|
|
screen_gamma = atof(gamma_str);
|
|
}
|
|
/* If we don't have another value */
|
|
else
|
|
{
|
|
screen_gamma = 2.2; /* A good guess for a
|
|
PC monitor in a bright office or a dim room */
|
|
screen_gamma = 2.0; /* A good guess for a
|
|
PC monitor in a dark room */
|
|
screen_gamma = 1.7 or 1.0; /* A good
|
|
guess for Mac systems */
|
|
}
|
|
|
|
The png_set_gamma() function handles gamma transformations of the data.
|
|
Pass both the file gamma and the current screen_gamma. If the file does
|
|
not have a gamma value, you can pass one anyway if you have an idea what
|
|
it is (usually 0.50 is a good guess for GIF images on PCs). Note
|
|
that file gammas are inverted from screen gammas. See the discussions
|
|
on gamma in the PNG specification for an excellent description of what
|
|
gamma is, and why all applications should support it. It is strongly
|
|
recommended that PNG viewers support gamma correction.
|
|
|
|
if (png_get_gAMA(png_ptr, info_ptr, &gamma))
|
|
png_set_gamma(png_ptr, screen_gamma, gamma);
|
|
else
|
|
png_set_gamma(png_ptr, screen_gamma, 0.50);
|
|
|
|
If you need to reduce an RGB file to a paletted file, or if a paletted
|
|
file has more entries then will fit on your screen, png_set_dither()
|
|
will do that. Note that this is a simple match dither that merely
|
|
finds the closest color available. This should work fairly well with
|
|
optimized palettes, and fairly badly with linear color cubes. If you
|
|
pass a palette that is larger then maximum_colors, the file will
|
|
reduce the number of colors in the palette so it will fit into
|
|
maximum_colors. If there is a histogram, it will use it to make
|
|
more intelligent choices when reducing the palette. If there is no
|
|
histogram, it may not do as good a job.
|
|
|
|
if (color_type & PNG_COLOR_MASK_COLOR)
|
|
{
|
|
if (png_get_valid(png_ptr, info_ptr,
|
|
PNG_INFO_PLTE))
|
|
{
|
|
png_color_16p histogram;
|
|
|
|
png_get_hIST(png_ptr, info_ptr,
|
|
&histogram);
|
|
png_set_dither(png_ptr, palette, num_palette,
|
|
max_screen_colors, histogram, 1);
|
|
}
|
|
else
|
|
{
|
|
png_color std_color_cube[MAX_SCREEN_COLORS] =
|
|
{ ... colors ... };
|
|
|
|
png_set_dither(png_ptr, std_color_cube,
|
|
MAX_SCREEN_COLORS, MAX_SCREEN_COLORS,
|
|
NULL,0);
|
|
}
|
|
}
|
|
|
|
PNG files describe monochrome as black being zero and white being one.
|
|
The following code will reverse this (make black be one and white be
|
|
zero):
|
|
|
|
if (bit_depth == 1 && color_type == PNG_COLOR_GRAY)
|
|
png_set_invert_mono(png_ptr);
|
|
|
|
PNG files store 16 bit pixels in network byte order (big-endian,
|
|
ie. most significant bits first). This code changes the storage to the
|
|
other way (little-endian, i.e. least significant bits first, the
|
|
way PCs store them):
|
|
|
|
if (bit_depth == 16)
|
|
png_set_swap(png_ptr);
|
|
|
|
If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
|
|
need to change the order the pixels are packed into bytes, you can use:
|
|
|
|
if (bit_depth < 8)
|
|
png_set_packswap(png_ptr);
|
|
|
|
The last thing to handle is interlacing; this is covered in detail below,
|
|
but you must call the function here if you want libpng to handle expansion
|
|
of the interlaced image.
|
|
|
|
number_of_passes = png_set_interlace_handling(png_ptr);
|
|
|
|
After setting the transformations, libpng can update your png_info
|
|
structure to reflect any transformations you've requested with this
|
|
call. This is most useful to update the info structure's rowbytes
|
|
field so you can use it to allocate your image memory. This function
|
|
will also update your palette with the correct screen_gamma and
|
|
background if these have been given with the calls above.
|
|
|
|
png_read_update_info(png_ptr, info_ptr);
|
|
|
|
After you call png_read_update_info(), you can allocate any
|
|
memory you need to hold the image. The row data is simply
|
|
raw byte data for all forms of images. As the actual allocation
|
|
varies among applications, no example will be given. If you
|
|
are allocating one large chunk, you will need to build an
|
|
array of pointers to each row, as it will be needed for some
|
|
of the functions below.
|
|
|
|
After you've allocated memory, you can read the image data.
|
|
The simplest way to do this is in one function call. If you are
|
|
allocating enough memory to hold the whole image, you can just
|
|
call png_read_image() and libpng will read in all the image data
|
|
and put it in the memory area supplied. You will need to pass in
|
|
an array of pointers to each row.
|
|
|
|
This function automatically handles interlacing, so you don't need
|
|
to call png_set_interlace_handling() or call this function multiple
|
|
times, or any of that other stuff necessary with png_read_rows().
|
|
|
|
png_read_image(png_ptr, row_pointers);
|
|
|
|
where row_pointers is:
|
|
|
|
png_bytep row_pointers[height];
|
|
|
|
You can point to void or char or whatever you use for pixels.
|
|
|
|
If you don't want to read in the whole image at once, you can
|
|
use png_read_rows() instead. If there is no interlacing (check
|
|
interlace_type == PNG_INTERLACE_NONE), this is simple:
|
|
|
|
png_read_rows(png_ptr, row_pointers, NULL,
|
|
number_of_rows);
|
|
|
|
where row_pointers is the same as in the png_read_image() call.
|
|
|
|
If you are doing this just one row at a time, you can do this with
|
|
row_pointers:
|
|
|
|
png_bytep row_pointers = row;
|
|
png_read_row(png_ptr, &row_pointers, NULL);
|
|
|
|
If the file is interlaced (info_ptr->interlace_type != 0), things get
|
|
somewhat harder. The only current (PNG Specification version 1.0)
|
|
interlacing type for PNG is (interlace_type == PNG_INTERLACE_ADAM7)
|
|
is a somewhat complicated 2D interlace scheme, known as Adam7, that
|
|
breaks down an image into seven smaller images of varying size, based
|
|
on an 8x8 grid.
|
|
|
|
libpng can fill out those images or it can give them to you "as is".
|
|
If you want them filled out, there are two ways to do that. The one
|
|
mentioned in the PNG specification is to expand each pixel to cover
|
|
those pixels that have not been read yet (the "rectangle" method).
|
|
This results in a blocky image for the first pass, which gradually
|
|
smooths out as more pixels are read. The other method is the "sparkle"
|
|
method, where pixels are drawn only in their final locations, with the
|
|
rest of the image remaining whatever colors they were initialized to
|
|
before the start of the read. The first method usually looks better,
|
|
but tends to be slower, as there are more pixels to put in the rows.
|
|
|
|
If you don't want libpng to handle the interlacing details, just call
|
|
png_read_rows() seven times to read in all seven images. Each of the
|
|
images is a valid image by itself, or they can all be combined on an
|
|
8x8 grid to form a single image (although if you intend to combine them
|
|
you would be far better off using the libpng interlace handling).
|
|
|
|
The first pass will return an image 1/8 as wide as the entire image
|
|
(every 8th column starting in column 0) and 1/8 as high as the original
|
|
(every 8th row starting in row 0), the second will be 1/8 as wide
|
|
(starting in column 4) and 1/8 as high (also starting in row 0). The
|
|
third pass will be 1/4 as wide (every 4th pixel starting in column 0) and
|
|
1/8 as high (every 8th row starting in row 4), and the fourth pass will
|
|
be 1/4 as wide and 1/4 as high (every 4th column starting in column 2,
|
|
and every 4th row starting in row 0). The fifth pass will return an
|
|
image 1/2 as wide, and 1/4 as high (starting at column 0 and row 2),
|
|
while the sixth pass will be 1/2 as wide and 1/2 as high as the original
|
|
(starting in column 1 and row 0). The seventh and final pass will be as
|
|
wide as the original, and 1/2 as high, containing all of the odd
|
|
numbered scanlines. Phew!
|
|
|
|
If you want libpng to expand the images, call this before calling
|
|
png_start_read_image() or png_read_update_info():
|
|
|
|
if (interlace_type == PNG_INTERLACE_ADAM7)
|
|
number_of_passes
|
|
= png_set_interlace_handling(png_ptr);
|
|
|
|
This will return the number of passes needed. Currently, this
|
|
is seven, but may change if another interlace type is added.
|
|
This function can be called even if the file is not interlaced,
|
|
where it will return one pass.
|
|
|
|
If you are not going to display the image after each pass, but are
|
|
going to wait until the entire image is read in, use the sparkle
|
|
effect. This effect is faster and the end result of either method
|
|
is exactly the same. If you are planning on displaying the image
|
|
after each pass, the "rectangle" effect is generally considered the
|
|
better looking one.
|
|
|
|
If you only want the "sparkle" effect, just call png_read_rows() as
|
|
normal, with the third parameter NULL. Make sure you make pass over
|
|
the image number_of_passes times, and you don't change the data in the
|
|
rows between calls. You can change the locations of the data, just
|
|
not the data. Each pass only writes the pixels appropriate for that
|
|
pass, and assumes the data from previous passes is still valid.
|
|
|
|
png_read_rows(png_ptr, row_pointers, NULL,
|
|
number_of_rows);
|
|
|
|
If you only want the first effect (the rectangles), do the same as
|
|
before except pass the row buffer in the third parameter, and leave
|
|
the second parameter NULL.
|
|
|
|
png_read_rows(png_ptr, NULL, row_pointers,
|
|
number_of_rows);
|
|
|
|
After you are finished reading the image, you can finish reading
|
|
the file. If you are interested in comments or time, which may be
|
|
stored either before or after the image data, you should pass the
|
|
separate png_info struct if you want to keep the comments from
|
|
before and after the image separate. If you are not interested, you
|
|
can pass NULL.
|
|
|
|
png_read_end(png_ptr, end_info);
|
|
|
|
When you are done, you can free all memory allocated by libpng like this:
|
|
|
|
png_destroy_read_struct(&png_ptr, &info_ptr,
|
|
&end_info);
|
|
|
|
For a more compact example of reading a PNG image, see the file example.c.
|
|
|
|
|
|
Reading PNG files progressively:
|
|
|
|
The progressive reader is slightly different then the non-progressive
|
|
reader. Instead of calling png_read_info(), png_read_rows(), and
|
|
png_read_end(), you make one call to png_process_data(), which calls
|
|
callbacks when it has the info, a row, or the end of the image. You
|
|
set up these callbacks with png_set_progressive_read_fn(). You don't
|
|
have to worry about the input/output functions of libpng, as you are
|
|
giving the library the data directly in png_process_data(). I will
|
|
assume that you have read the section on reading PNG files above,
|
|
so I will only highlight the differences (although I will show
|
|
all of the code).
|
|
|
|
png_structp png_ptr;
|
|
png_infop info_ptr;
|
|
|
|
/* An example code fragment of how you would
|
|
initialize the progressive reader in your
|
|
application. */
|
|
int
|
|
initialize_png_reader()
|
|
{
|
|
png_ptr = png_create_read_struct
|
|
(PNG_LIBPNG_VER_STRING, (void *)user_error_ptr,
|
|
user_error_fn, user_warning_fn);
|
|
if (!png_ptr)
|
|
return -1;
|
|
info_ptr = png_create_info_struct(png_ptr);
|
|
if (!info_ptr)
|
|
{
|
|
png_destroy_read_struct(&png_ptr, (png_infopp)NULL,
|
|
(png_infopp)NULL);
|
|
return -1;
|
|
}
|
|
|
|
if (setjmp(png_ptr->jmpbuf))
|
|
{
|
|
png_destroy_read_struct(&png_ptr, &info_ptr,
|
|
(png_infopp)NULL);
|
|
return -1;
|
|
}
|
|
|
|
/* This one's new. You can provide functions
|
|
to be called when the header info is valid,
|
|
when each row is completed, and when the image
|
|
is finished. If you aren't using all functions,
|
|
you can specify a NULL parameter. You can use
|
|
any struct as the user_ptr (cast to a void pointer
|
|
for the function call), and retrieve the pointer
|
|
from inside the callbacks using the function
|
|
|
|
png_get_progressive_ptr(png_ptr);
|
|
|
|
which will return a void pointer, which you have
|
|
to cast appropriately.
|
|
*/
|
|
png_set_progressive_read_fn(png_ptr, (void *)user_ptr,
|
|
info_callback, row_callback, end_callback);
|
|
|
|
return 0;
|
|
}
|
|
|
|
/* A code fragment that you call as you receive blocks
|
|
of data */
|
|
int
|
|
process_data(png_bytep buffer, png_uint_32 length)
|
|
{
|
|
if (setjmp(png_ptr->jmpbuf))
|
|
{
|
|
png_destroy_read_struct(&png_ptr, &info_ptr,
|
|
(png_infopp)NULL);
|
|
return -1;
|
|
}
|
|
|
|
/* This one's new also. Simply give it a chunk
|
|
of data from the file stream (in order, of
|
|
course). On machines with segmented memory
|
|
models machines, don't give it any more than
|
|
64K. The library seems to run fine with sizes
|
|
of 4K. Although you can give it much less if
|
|
necessary (I assume you can give it chunks of
|
|
1 byte, I haven't tried less then 256 bytes
|
|
yet). When this function returns, you may
|
|
want to display any rows that were generated
|
|
in the row callback if you don't already do
|
|
so there.
|
|
*/
|
|
png_process_data(png_ptr, info_ptr, buffer, length);
|
|
return 0;
|
|
}
|
|
|
|
/* This function is called (as set by
|
|
png_set_progressive_fn() above) when enough data
|
|
has been supplied so all of the header has been
|
|
read.
|
|
*/
|
|
void
|
|
info_callback(png_structp png_ptr, png_infop info)
|
|
{
|
|
/* Do any setup here, including setting any of
|
|
the transformations mentioned in the Reading
|
|
PNG files section. For now, you _must_ call
|
|
either png_start_read_image() or
|
|
png_read_update_info() after all the
|
|
transformations are set (even if you don't set
|
|
any). You may start getting rows before
|
|
png_process_data() returns, so this is your
|
|
last chance to prepare for that.
|
|
*/
|
|
}
|
|
|
|
/* This function is called when each row of image
|
|
data is complete */
|
|
void
|
|
row_callback(png_structp png_ptr, png_bytep new_row,
|
|
png_uint_32 row_num, int pass)
|
|
{
|
|
/* If the image is interlaced, and you turned
|
|
on the interlace handler, this function will
|
|
be called for every row in every pass. Some
|
|
of these rows will not be changed from the
|
|
previous pass. When the row is not changed,
|
|
the new_row variable will be NULL. The rows
|
|
and passes are called in order, so you don't
|
|
really need the row_num and pass, but I'm
|
|
supplying them because it may make your life
|
|
easier.
|
|
|
|
For the non-NULL rows of interlaced images,
|
|
you must call png_progressive_combine_row()
|
|
passing in the row and the old row. You can
|
|
call this function for NULL rows (it will just
|
|
return) and for non-interlaced images (it just
|
|
does the memcpy for you) if it will make the
|
|
code easier. Thus, you can just do this for
|
|
all cases:
|
|
*/
|
|
|
|
png_progressive_combine_row(png_ptr, old_row,
|
|
new_row);
|
|
|
|
/* where old_row is what was displayed for
|
|
previous rows. Note that the first pass
|
|
(pass == 0, really) will completely cover
|
|
the old row, so the rows do not have to be
|
|
initialized. After the first pass (and only
|
|
for interlaced images), you will have to pass
|
|
the current row, and the function will combine
|
|
the old row and the new row.
|
|
*/
|
|
}
|
|
|
|
void
|
|
end_callback(png_structp png_ptr, png_infop info)
|
|
{
|
|
/* This function is called after the whole image
|
|
has been read, including any chunks after the
|
|
image (up to and including the IEND). You
|
|
will usually have the same info chunk as you
|
|
had in the header, although some data may have
|
|
been added to the comments and time fields.
|
|
|
|
Most people won't do much here, perhaps setting
|
|
a flag that marks the image as finished.
|
|
*/
|
|
}
|
|
|
|
.SH IV. Writing
|
|
|
|
Much of this is very similar to reading. However, everything of
|
|
importance is repeated here, so you won't have to constantly look
|
|
back up in the reading section to understand writing.
|
|
|
|
You will want to do the I/O initialization before you get into libpng,
|
|
so if it doesn't work, you don't have anything to undo. If you are not
|
|
using the standard I/O functions, you will need to replace them with
|
|
custom writing functions. See the discussion under Customizing libpng.
|
|
|
|
FILE *fp = fopen(file_name, "wb");
|
|
if (!fp)
|
|
{
|
|
return;
|
|
}
|
|
|
|
Next, png_struct and png_info need to be allocated and initialized.
|
|
As these can be both relatively large, you may not want to store these
|
|
on the stack, unless you have stack space to spare. Of course, you
|
|
will want to check if they return NULL. If you are also reading,
|
|
you won't want to name your read structure and your write structure
|
|
both "png_ptr"; you can call them anything you like, such as
|
|
"read_ptr" and "write_ptr". Look at pngtest.c, for example.
|
|
|
|
png_structp png_ptr = png_create_write_struct
|
|
(PNG_LIBPNG_VER_STRING, (void *)user_error_ptr,
|
|
user_error_fn, user_warning_fn);
|
|
if (!png_ptr)
|
|
return;
|
|
|
|
png_infop info_ptr = png_create_info_struct(png_ptr);
|
|
if (!info_ptr)
|
|
{
|
|
png_destroy_write_struct(&png_ptr,
|
|
(png_infopp)NULL);
|
|
return;
|
|
}
|
|
|
|
After you have these structures, you will need to set up the
|
|
error handling. When libpng encounters an error, it expects to
|
|
longjmp() back to your routine. Therefore, you will need to call
|
|
setjmp and pass the jmpbuf field of your png_struct. If you
|
|
write the file from different routines, you will need to update
|
|
the jmpbuf field every time you enter a new routine that will
|
|
call a png_ function. See your documentation of setjmp/longjmp
|
|
for your compiler for more information on setjmp/longjmp. See
|
|
the discussion on libpng error handling in the Customizing Libpng
|
|
section below for more information on the libpng error handling.
|
|
|
|
if (setjmp(png_ptr->jmpbuf))
|
|
{
|
|
png_destroy_write_struct(&png_ptr, &info_ptr);
|
|
fclose(fp);
|
|
return;
|
|
}
|
|
|
|
Now you need to set up the output code. The default for libpng is to
|
|
use the C function fwrite(). If you use this, you will need to pass a
|
|
valid FILE * in the function png_init_io(). Be sure that the file is
|
|
opened in binary mode. Again, if you wish to handle writing data in
|
|
another way, see the discussion on libpng I/O handling in the Customizing
|
|
Libpng section below.
|
|
|
|
png_init_io(png_ptr, fp);
|
|
|
|
At this point, you can set up a callback function that will be
|
|
called after each row has been written, which you can use to control
|
|
a progress meter or the like. It's demonstrated in pngtest.c.
|
|
You must supply a function
|
|
|
|
void write_row_callback(png_ptr, png_uint_32 row, int pass);
|
|
{
|
|
/* put your code here */
|
|
}
|
|
|
|
(You can give it another name that you like instead of "write_row_callback")
|
|
|
|
To inform libpng about your function, use
|
|
|
|
png_set_write_status_fn(png_ptr, write_row_callback);
|
|
|
|
You now have the option of modifying how the compression library will
|
|
run. The following functions are mainly for testing, but may be useful
|
|
in some cases, like if you need to write PNG files extremely fast and
|
|
are willing to give up some compression, or if you want to get the
|
|
maximum possible compression at the expense of slower writing. If you
|
|
have no special needs in this area, let the library do what it wants by
|
|
not calling this function at all, as it has been tuned to deliver a good
|
|
speed/compression ratio. The second parameter to png_set_filter() is
|
|
the filter method, for which the only valid value is '0' (as of the
|
|
October 1996 PNG specification, version 1.0). The third parameter is a
|
|
flag that indicates
|
|
which filter type(s) are to be tested for each scanline. See the
|
|
Compression Library for details on the specific filter types.
|
|
|
|
|
|
/* turn on or off filtering, and/or choose
|
|
specific filters */
|
|
png_set_filter(png_ptr, 0,
|
|
PNG_FILTER_NONE | PNG_FILTER_SUB |
|
|
PNG_FILTER_PAETH);
|
|
|
|
The png_set_compression_???() functions interface to the zlib compression
|
|
library, and should mostly be ignored unless you really know what you are
|
|
doing. The only generally useful call is png_set_compression_level()
|
|
which changes how much time zlib spends on trying to compress the image
|
|
data. See the Compression Library for details on the compression levels.
|
|
|
|
/* set the zlib compression level */
|
|
png_set_compression_level(png_ptr,
|
|
Z_BEST_COMPRESSION);
|
|
|
|
/* set other zlib parameters */
|
|
png_set_compression_mem_level(png_ptr, 8);
|
|
png_set_compression_strategy(png_ptr,
|
|
Z_DEFAULT_STRATEGY);
|
|
png_set_compression_window_bits(png_ptr, 15);
|
|
png_set_compression_method(png_ptr, 8);
|
|
|
|
You now need to fill in the png_info structure with all the data you
|
|
wish to write before the actual image. Note that the only thing you
|
|
are allowed to write after the image is the text chunks and the time
|
|
chunk (as of PNG Specification 1.0, anyway). See png_write_end() and
|
|
the latest PNG specification for more information on that. If you
|
|
wish to write them before the image, fill them in now, and flag that
|
|
data as being valid. If you want to wait until after the data, don't
|
|
fill them until png_write_end(). For all the fields in png_info and
|
|
their data types, see png.h. For explanations of what the fields
|
|
contain, see the PNG specification.
|
|
|
|
Some of the more important parts of the png_info are:
|
|
|
|
png_set_IHDR(png_ptr, info_ptr, width, height,
|
|
bit_depth, color_type, interlace_type,
|
|
compression_type, filter_type)
|
|
width - holds the width of the image
|
|
in pixels (up to 2^31).
|
|
height - holds the height of the image
|
|
in pixels (up to 2^31).
|
|
bit_depth - holds the bit depth of one of the
|
|
image channels.
|
|
(valid values are 1, 2, 4, 8, 16
|
|
and depend also on the
|
|
color_type. See also significant
|
|
bits (sBIT) below).
|
|
color_type - describes which color/alpha
|
|
channels are present.
|
|
PNG_COLOR_TYPE_GRAY
|
|
(bit depths 1, 2, 4, 8, 16)
|
|
PNG_COLOR_TYPE_GRAY_ALPHA
|
|
(bit depths 8, 16)
|
|
PNG_COLOR_TYPE_PALETTE
|
|
(bit depths 1, 2, 4, 8)
|
|
PNG_COLOR_TYPE_RGB
|
|
(bit_depths 8, 16)
|
|
PNG_COLOR_TYPE_RGB_ALPHA
|
|
(bit_depths 8, 16)
|
|
|
|
PNG_COLOR_MASK_PALETTE
|
|
PNG_COLOR_MASK_COLOR
|
|
PNG_COLOR_MASK_ALPHA
|
|
|
|
interlace_type - PNG_INTERLACE_NONE or
|
|
PNG_INTERLACE_ADAM7
|
|
compression_type - (must be
|
|
PNG_COMPRESSION_TYPE_DEFAULT)
|
|
filter_type - (must be PNG_FILTER_TYPE_DEFAULT)
|
|
|
|
png_set_PLTE(png_ptr, info_ptr, palette,
|
|
num_palette);
|
|
palette - the palette for the file
|
|
(array of png_color)
|
|
num_palette - number of entries in the palette
|
|
|
|
png_set_gAMA(png_ptr, info_ptr, gamma);
|
|
gamma - the gamma the image was created
|
|
at (PNG_INFO_gAMA)
|
|
|
|
png_set_sRGB(png_ptr, info_ptr, srgb_intent);
|
|
srgb_intent - the rendering intent
|
|
(PNG_INFO_sRGB) The presence of
|
|
the sRGB chunk means that the pixel
|
|
data is in the sRGB color space.
|
|
This chunk also implies specific
|
|
values of gAMA and cHRM. Rendering
|
|
intent is the CSS-1 property that
|
|
has been defined by the International
|
|
Color Consortium
|
|
(http://www.color.org).
|
|
It can be one of
|
|
PNG_SRGB_INTENT_SATURATION,
|
|
PNG_SRGB_INTENT_PERCEPTUAL,
|
|
PNG_SRGB_INTENT_ABSOLUTE, or
|
|
PNG_SRGB_INTENT_RELATIVE.
|
|
|
|
|
|
png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr,
|
|
srgb_intent);
|
|
srgb_intent - the rendering intent
|
|
(PNG_INFO_sRGB) The presence of the
|
|
sRGB chunk means that the pixel
|
|
data is in the sRGB color space.
|
|
This function also causes gAMA and
|
|
cHRM chunks with the specific values
|
|
that are consistent with sRGB to be
|
|
written.
|
|
|
|
png_set_sBIT(png_ptr, info_ptr, sig_bit);
|
|
sig_bit - the number of significant bits for
|
|
(PNG_INFO_sBIT) each of the gray, red,
|
|
green, and blue channels, whichever are
|
|
appropriate for the given color type
|
|
(png_color_16)
|
|
|
|
png_set_tRNS(png_ptr, info_ptr, trans, num_trans,
|
|
trans_values);
|
|
trans - array of transparent entries for
|
|
palette (PNG_INFO_tRNS)
|
|
trans_values - transparent pixel for non-paletted
|
|
images (PNG_INFO_tRNS)
|
|
num_trans - number of transparent entries
|
|
(PNG_INFO_tRNS)
|
|
|
|
png_set_hIST(png_ptr, info_ptr, hist);
|
|
(PNG_INFO_hIST)
|
|
hist - histogram of palette (array of
|
|
png_color_16)
|
|
|
|
png_set_tIME(png_ptr, info_ptr, mod_time);
|
|
mod_time - time image was last modified
|
|
(PNG_VALID_tIME)
|
|
|
|
png_set_bKGD(png_ptr, info_ptr, background);
|
|
background - background color (PNG_VALID_bKGD)
|
|
|
|
png_set_text(png_ptr, info_ptr, text_ptr, num_text);
|
|
text_ptr - array of png_text holding image
|
|
comments
|
|
text_ptr[i]->key - keyword for comment.
|
|
text_ptr[i]->text - text comments for current
|
|
keyword.
|
|
text_ptr[i]->compression - type of compression used
|
|
on "text" PNG_TEXT_COMPRESSION_NONE or
|
|
PNG_TEXT_COMPRESSION_zTXt
|
|
num_text - number of comments in text_ptr
|
|
|
|
png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y,
|
|
unit_type);
|
|
offset_x - positive offset from the left
|
|
edge of the screen
|
|
offset_y - positive offset from the top
|
|
edge of the screen
|
|
unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
|
|
|
|
png_set_pHYs(png_ptr, info_ptr, res_x, res_y,
|
|
unit_type);
|
|
res_x - pixels/unit physical resolution
|
|
in x direction
|
|
res_y - pixels/unit physical resolution
|
|
in y direction
|
|
unit_type - PNG_RESOLUTION_UNKNOWN,
|
|
PNG_RESOLUTION_METER
|
|
|
|
In PNG files, the alpha channel in an image is the level of opacity.
|
|
If your data is supplied as a level of transparency, you can invert the
|
|
alpha channel before you write it, so that 0 is fully transparent and 255
|
|
(in 8-bit or paletted images) or 65535 (in 16-bit images) is fully opaque,
|
|
with
|
|
|
|
png_set_invert_alpha(png_ptr);
|
|
|
|
This must appear here instead of later with the other transformations
|
|
because in the case of paletted images the tRNS chunk data has to
|
|
be inverted before the tRNS chunk is written. If your image is not a
|
|
paletted image, the tRNS data (which in such cases represents a single
|
|
color to be rendered as transparent) won't be changed.
|
|
|
|
A quick word about text and num_text. text is an array of png_text
|
|
structures. num_text is the number of valid structures in the array.
|
|
If you want, you can use max_text to hold the size of the array, but
|
|
libpng ignores it for writing (it does use it for reading). Each
|
|
png_text structure holds a keyword-text value, and a compression type.
|
|
The compression types have the same valid numbers as the compression
|
|
types of the image data. Currently, the only valid number is zero.
|
|
However, you can store text either compressed or uncompressed, unlike
|
|
images which always have to be compressed. So if you don't want the
|
|
text compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE.
|
|
Until text gets around 1000 bytes, it is not worth compressing it.
|
|
After the text has been written out to the file, the compression type
|
|
is set to PNG_TEXT_COMPRESSION_NONE_WR or PNG_TEXT_COMPRESSION_zTXt_WR,
|
|
so that it isn't written out again at the end (in case you are calling
|
|
png_write_end() with the same struct.
|
|
|
|
The keywords that are given in the PNG Specification are:
|
|
|
|
Title Short (one line) title or
|
|
caption for image
|
|
Author Name of image's creator
|
|
Description Description of image (possibly long)
|
|
Copyright Copyright notice
|
|
Creation Time Time of original image creation
|
|
(usually RFC 1123 format, see below)
|
|
Software Software used to create the image
|
|
Disclaimer Legal disclaimer
|
|
Warning Warning of nature of content
|
|
Source Device used to create the image
|
|
Comment Miscellaneous comment; conversion
|
|
from other image format
|
|
|
|
The keyword-text pairs work like this. Keywords should be short
|
|
simple descriptions of what the comment is about. Some typical
|
|
keywords are found in the PNG specification, as is some recommendations
|
|
on keywords. You can repeat keywords in a file. You can even write
|
|
some text before the image and some after. For example, you may want
|
|
to put a description of the image before the image, but leave the
|
|
disclaimer until after, so viewers working over modem connections
|
|
don't have to wait for the disclaimer to go over the modem before
|
|
they start seeing the image. Finally, keywords should be full
|
|
words, not abbreviations. Keywords and text are in the ISO 8859-1
|
|
(Latin-1) character set (a superset of regular ASCII) and can not
|
|
contain NUL characters, and should not contain control or other
|
|
unprintable characters. To make the comments widely readable, stick
|
|
with basic ASCII, and avoid machine specific character set extensions
|
|
like the IBM-PC character set. The keyword must be present, but
|
|
you can leave off the text string on non-compressed pairs.
|
|
Compressed pairs must have a text string, as only the text string
|
|
is compressed anyway, so the compression would be meaningless.
|
|
|
|
PNG supports modification time via the png_time structure. Two
|
|
conversion routines are proved, png_convert_from_time_t() for
|
|
time_t and png_convert_from_struct_tm() for struct tm. The
|
|
time_t routine uses gmtime(). You don't have to use either of
|
|
these, but if you wish to fill in the png_time structure directly,
|
|
you should provide the time in universal time (GMT) if possible
|
|
instead of your local time. Note that the year number is the full
|
|
year (e.g. 1998, rather than 98 - PNG is year 2000 compliant!), and
|
|
that months start with 1.
|
|
|
|
If you want to store the time of the original image creation, you should
|
|
use a plain tEXt chunk with the "Creation Time" keyword. This is
|
|
necessary because the "creation time" of a PNG image is somewhat vague,
|
|
depending on whether you mean the PNG file, the time the image was
|
|
created in a non-PNG format, a still photo from which the image was
|
|
scanned, or possibly the subject matter itself. In order to facilitate
|
|
machine-readable dates, it is recommended that the "Creation Time"
|
|
tEXt chunk use RFC 1123 format dates (e.g. 22 May 1997 18:07:10 GMT"),
|
|
although this isn't a requirement. Unlike the tIME chunk, the
|
|
"Creation Time" tEXt chunk is not expected to be automatically changed
|
|
by the software. To facilitate the use of RFC 1123 dates, a function
|
|
png_convert_to_rfc1123(png_timep) is provided to convert from PNG
|
|
time to an RFC 1123 format string.
|
|
|
|
You are now ready to write all the file information up to the actual
|
|
image data. You do this with a call to png_write_info().
|
|
|
|
png_write_info(png_ptr, info_ptr);
|
|
|
|
After you've written the file information, you can set up the library
|
|
to handle any special transformations of the image data. The various
|
|
ways to transform the data will be described in the order that they
|
|
should occur. This is important, as some of these change the color
|
|
type and/or bit depth of the data, and some others only work on
|
|
certain color types and bit depths. Even though each transformation
|
|
checks to see if it has data that it can do something with, you should
|
|
make sure to only enable a transformation if it will be valid for the
|
|
data. For example, don't swap red and blue on grayscale data.
|
|
|
|
PNG files store RGB pixels packed into 3 bytes. This code tells
|
|
the library to expect input data with 4 bytes per pixel
|
|
|
|
png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);
|
|
|
|
where the 0 is the value that will be put in the 4th byte, and the
|
|
location is either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending
|
|
upon whether the filler byte is stored XRGB or RGBX.
|
|
|
|
PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
|
|
they can, resulting in, for example, 8 pixels per byte for 1 bit files.
|
|
If the data is supplied at 1 pixel per byte, use this code, which will
|
|
correctly pack the pixels into a single byte:
|
|
|
|
png_set_packing(png_ptr);
|
|
|
|
PNG files reduce possible bit depths to 1, 2, 4, 8, and 16. If your
|
|
data is of another bit depth, you can write an sBIT chunk into the
|
|
file so that decoders can get the original data if desired.
|
|
|
|
/* Set the true bit depth of the image data */
|
|
if (color_type & PNG_COLOR_MASK_COLOR)
|
|
{
|
|
sig_bit.red = true_bit_depth;
|
|
sig_bit.green = true_bit_depth;
|
|
sig_bit.blue = true_bit_depth;
|
|
}
|
|
else
|
|
{
|
|
sig_bit.gray = true_bit_depth;
|
|
}
|
|
if (color_type & PNG_COLOR_MASK_ALPHA)
|
|
{
|
|
sig_bit.alpha = true_bit_depth;
|
|
}
|
|
|
|
png_set_sBIT(png_ptr, info_ptr, &sig_bit);
|
|
|
|
If the data is stored in the row buffer in a bit depth other than
|
|
one supported by PNG (e.g. 3 bit data in the range 0-7 for a 4-bit PNG),
|
|
this will scale the values to appear to be the correct bit depth as
|
|
is required by PNG.
|
|
|
|
png_set_shift(png_ptr, &sig_bit);
|
|
|
|
PNG files store 16 bit pixels in network byte order (big-endian,
|
|
ie. most significant bits first). This code would be used if they are
|
|
supplied the other way (little-endian, i.e. least significant bits
|
|
first, the way PCs store them):
|
|
|
|
if (bit_depth > 8)
|
|
png_set_swap(png_ptr);
|
|
|
|
If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
|
|
need to change the order the pixels are packed into bytes, you can use:
|
|
|
|
if (bit_depth < 8)
|
|
png_set_packswap(png_ptr);
|
|
|
|
PNG files store 3 color pixels in red, green, blue order. This code
|
|
would be used if they are supplied as blue, green, red:
|
|
|
|
png_set_bgr(png_ptr);
|
|
|
|
PNG files describe monochrome as black being zero and white being
|
|
one. This code would be used if the pixels are supplied with this reversed
|
|
(black being one and white being zero):
|
|
|
|
png_set_invert_mono(png_ptr);
|
|
|
|
Finally, you can write your own transformation function if none of
|
|
the existing ones meets your needs. This is done by setting a callback
|
|
with
|
|
|
|
png_set_write_user_transform_fn(png_ptr,
|
|
write_transform_fn);
|
|
|
|
You must supply the function
|
|
|
|
void write_transform_fn(png_ptr ptr, row_info_ptr
|
|
row_info, png_bytep data)
|
|
|
|
See pngtest.c for a working example. Your function will be called
|
|
before any of the other transformations have been processed.
|
|
|
|
It is possible to have libpng flush any pending output, either manually,
|
|
or automatically after a certain number of lines have been written. To
|
|
flush the output stream a single time call:
|
|
|
|
png_write_flush(png_ptr);
|
|
|
|
and to have libpng flush the output stream periodically after a certain
|
|
number of scanlines have been written, call:
|
|
|
|
png_set_flush(png_ptr, nrows);
|
|
|
|
Note that the distance between rows is from the last time png_write_flush()
|
|
was called, or the first row of the image if it has never been called.
|
|
So if you write 50 lines, and then png_set_flush 25, it will flush the
|
|
output on the next scanline, and every 25 lines thereafter, unless
|
|
png_write_flush() is called before 25 more lines have been written.
|
|
If nrows is too small (less than about 10 lines for a 640 pixel wide
|
|
RGB image) the image compression may decrease noticeably (although this
|
|
may be acceptable for real-time applications). Infrequent flushing will
|
|
only degrade the compression performance by a few percent over images
|
|
that do not use flushing.
|
|
|
|
That's it for the transformations. Now you can write the image data.
|
|
The simplest way to do this is in one function call. If have the
|
|
whole image in memory, you can just call png_write_image() and libpng
|
|
will write the image. You will need to pass in an array of pointers to
|
|
each row. This function automatically handles interlacing, so you don't
|
|
need to call png_set_interlace_handling() or call this function multiple
|
|
times, or any of that other stuff necessary with png_write_rows().
|
|
|
|
png_write_image(png_ptr, row_pointers);
|
|
|
|
where row_pointers is:
|
|
|
|
png_bytef *row_pointers[height];
|
|
|
|
You can point to void or char or whatever you use for pixels.
|
|
|
|
If you can't want to write the whole image at once, you can
|
|
use png_write_rows() instead. If the file is not interlaced,
|
|
this is simple:
|
|
|
|
png_write_rows(png_ptr, row_pointers,
|
|
number_of_rows);
|
|
|
|
row_pointers is the same as in the png_write_image() call.
|
|
|
|
If you are just writing one row at a time, you can do this with
|
|
row_pointers:
|
|
|
|
png_bytep row_pointer = row;
|
|
|
|
png_write_row(png_ptr, &row_pointer);
|
|
|
|
When the file is interlaced, things can get a good deal more
|
|
complicated. The only currently (as of February 1998 -- PNG Specification
|
|
version 1.0, dated October 1996) defined interlacing scheme for PNG files
|
|
is the "Adam7" interlace scheme, that breaks down an
|
|
image into seven smaller images of varying size. libpng will build
|
|
these images for you, or you can do them yourself. If you want to
|
|
build them yourself, see the PNG specification for details of which
|
|
pixels to write when.
|
|
|
|
If you don't want libpng to handle the interlacing details, just
|
|
use png_set_interlace_handling() and call png_write_rows() the
|
|
correct number of times to write all seven sub-images.
|
|
|
|
If you want libpng to build the sub-images, call this before you start
|
|
writing any rows:
|
|
|
|
number_of_passes =
|
|
png_set_interlace_handling(png_ptr);
|
|
|
|
This will return the number of passes needed. Currently, this
|
|
is seven, but may change if another interlace type is added.
|
|
|
|
Then write the complete image number_of_passes times.
|
|
|
|
png_write_rows(png_ptr, row_pointers,
|
|
number_of_rows);
|
|
|
|
As some of these rows are not used, and thus return immediately,
|
|
you may want to read about interlacing in the PNG specification,
|
|
and only update the rows that are actually used.
|
|
|
|
After you are finished writing the image, you should finish writing
|
|
the file. If you are interested in writing comments or time, you should
|
|
pass an appropriately filled png_info pointer. If you are not interested,
|
|
you can pass NULL.
|
|
|
|
png_write_end(png_ptr, info_ptr);
|
|
|
|
When you are done, you can free all memory used by libpng like this:
|
|
|
|
png_destroy_write_struct(&png_ptr, &info_ptr);
|
|
|
|
You must free any data you allocated for info_ptr, such as comments,
|
|
palette, or histogram, before the call to png_destroy_write_struct();
|
|
|
|
For a more compact example of writing a PNG image, see the file example.c.
|
|
|
|
|
|
.SH V. Modifying/Customizing libpng:
|
|
|
|
There are two issues here. The first is changing how libpng does
|
|
standard things like memory allocation, input/output, and error handling.
|
|
The second deals with more complicated things like adding new chunks,
|
|
adding new transformations, and generally changing how libpng works.
|
|
|
|
All of the memory allocation, input/output, and error handling in libpng
|
|
goes through callbacks which are user settable. The default routines are
|
|
in pngmem.c, pngrio.c, pngwio.c, and pngerror.c respectively. To change
|
|
these functions, call the appropriate png_set_???_fn() function.
|
|
|
|
Memory allocation is done through the functions png_large_malloc(),
|
|
png_malloc(), png_realloc(), png_large_free(), and png_free(). These
|
|
currently just call the standard C functions. The large functions must
|
|
handle exactly 64K, but they don't have to handle more than that. If
|
|
your pointers can't access more then 64K at a time, you will want to set
|
|
MAXSEG_64K in zlib.h. Since it is unlikely that the method of handling
|
|
memory allocation on a platform will change between applications, these
|
|
functions must be modified in the library at compile time.
|
|
|
|
Input/Output in libpng is done through png_read() and png_write(),
|
|
which currently just call fread() and fwrite(). The FILE * is stored in
|
|
png_struct and is initialized via png_init_io(). If you wish to change
|
|
the method of I/O, the library supplies callbacks that you can set
|
|
through the function png_set_read_fn() and png_set_write_fn() at run
|
|
time, instead of calling the png_init_io() function. These functions
|
|
also provide a void pointer that can be retrieved via the function
|
|
png_get_io_ptr(). For example:
|
|
|
|
png_set_read_fn(png_structp png_ptr,
|
|
voidp io_ptr, png_rw_ptr read_data_fn)
|
|
|
|
png_set_write_fn(png_structp png_ptr,
|
|
voidp io_ptr, png_rw_ptr write_data_fn,
|
|
png_flush_ptr output_flush_fn);
|
|
|
|
voidp io_ptr = png_get_io_ptr(png_ptr);
|
|
|
|
The replacement I/O functions should have prototypes as follows:
|
|
|
|
void user_read_data(png_structp png_ptr,
|
|
png_bytep data, png_uint_32 length);
|
|
void user_write_data(png_structp png_ptr,
|
|
png_bytep data, png_uint_32 length);
|
|
void user_flush_data(png_structp png_ptr);
|
|
|
|
Supplying NULL for the read, write, or flush functions sets them back
|
|
to using the default C stream functions. It is an error to read from
|
|
a write stream, and vice versa.
|
|
|
|
Error handling in libpng is done through png_error() and png_warning().
|
|
Errors handled through png_error() are fatal, meaning that png_error()
|
|
should never return to its caller. Currently, this is handled via
|
|
setjmp() and longjmp(), but you could change this to do things like
|
|
exit() if you should wish. On non-fatal errors, png_warning() is called
|
|
to print a warning message, and then control returns to the calling code.
|
|
By default png_error() and png_warning() print a message on stderr via
|
|
fprintf() unless the library is compiled with PNG_NO_STDIO defined. If
|
|
you wish to change the behavior of the error functions, you will need to
|
|
set up your own message callbacks. These functions are normally supplied
|
|
at the time that the png_struct is created. It is also possible to change
|
|
these functions after png_create_???_struct() has been called by calling:
|
|
|
|
png_set_error_fn(png_structp png_ptr,
|
|
png_voidp error_ptr, png_error_ptr error_fn,
|
|
png_error_ptr warning_fn);
|
|
|
|
png_voidp error_ptr = png_get_error_ptr(png_ptr);
|
|
|
|
If NULL is supplied for either error_fn or warning_fn, then the libpng
|
|
default function will be used, calling fprintf() and/or longjmp() if a
|
|
problem is encountered. The replacement error functions should have
|
|
parameters as follows:
|
|
|
|
void user_error_fn(png_structp png_ptr,
|
|
png_const_charp error_msg);
|
|
void user_warning_fn(png_structp png_ptr,
|
|
png_const_charp warning_msg);
|
|
|
|
The motivation behind using setjmp() and longjmp() is the C++ throw and
|
|
catch exception handling methods. This makes the code much easier to write,
|
|
as there is no need to check every return code of every function call.
|
|
However, there are some uncertainties about the status of local variables
|
|
after a longjmp, so the user may want to be careful about doing anything after
|
|
setjmp returns non-zero besides returning itself. Consult your compiler
|
|
documentation for more details.
|
|
|
|
If you need to read or write custom chunks, you will need to get deeper
|
|
into the libpng code, as a mechanism has not yet been supplied for user
|
|
callbacks with custom chunks. First, read the PNG specification, and have
|
|
a first level of understanding of how it works. Pay particular attention
|
|
to the sections that describe chunk names, and look at how other chunks
|
|
were designed, so you can do things similarly. Second, check out the
|
|
sections of libpng that read and write chunks. Try to find a chunk that
|
|
is similar to yours and copy off of it. More details can be found in the
|
|
comments inside the code. A way of handling unknown chunks in a generic
|
|
method, potentially via callback functions, would be best.
|
|
|
|
If you wish to write your own transformation for the data, look through
|
|
the part of the code that does the transformations, and check out some of
|
|
the simpler ones to get an idea of how they work. Try to find a similar
|
|
transformation to the one you want to add and copy off of it. More details
|
|
can be found in the comments inside the code itself.
|
|
|
|
Configuring for 16 bit platforms:
|
|
|
|
You may need to change the png_large_malloc() and png_large_free()
|
|
routines in pngmem.c, as these are required to allocate 64K, although
|
|
there is already support for many of the common DOS compilers. Also,
|
|
you will want to look into zconf.h to tell zlib (and thus libpng) that
|
|
it cannot allocate more then 64K at a time. Even if you can, the memory
|
|
won't be accessible. So limit zlib and libpng to 64K by defining MAXSEG_64K.
|
|
|
|
Configuring for DOS:
|
|
|
|
For DOS users which only have access to the lower 640K, you will
|
|
have to limit zlib's memory usage via a png_set_compression_mem_level()
|
|
call. See zlib.h or zconf.h in the zlib library for more information.
|
|
|
|
Configuring for Medium Model:
|
|
|
|
Libpng's support for medium model has been tested on most of the popular
|
|
compilers. Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets
|
|
defined, and FAR gets defined to far in pngconf.h, and you should be
|
|
all set. Everything in the library (except for zlib's structure) is
|
|
expecting far data. You must use the typedefs with the p or pp on
|
|
the end for pointers (or at least look at them and be careful). Make
|
|
note that the row's of data are defined as png_bytepp which is a
|
|
unsigned char far * far *.
|
|
|
|
Configuring for gui/windowing platforms:
|
|
|
|
You will need to write new error and warning functions that use the GUI
|
|
interface, as described previously, and set them to be the error and
|
|
warning functions at the time that png_create_???_struct() is called,
|
|
in order to have them available during the structure initialization.
|
|
They can be changed later via png_set_error_fn(). On some compilers,
|
|
you may also have to change the memory allocators (png_malloc, etc.).
|
|
|
|
Configuring for compiler xxx:
|
|
|
|
All includes for libpng are in pngconf.h. If you need to add/change/delete
|
|
an include, this is the place to do it. The includes that are not
|
|
needed outside libpng are protected by the PNG_INTERNAL definition,
|
|
which is only defined for those routines inside libpng itself. The
|
|
files in libpng proper only include png.h, which includes pngconf.h.
|
|
|
|
Configuring zlib:
|
|
|
|
There are special functions to configure the compression. Perhaps the
|
|
most useful one changes the compression level, which currently uses
|
|
input compression values in the range 0 - 9. The library normally
|
|
uses the default compression level (Z_DEFAULT_COMPRESSION = 6). Tests
|
|
have shown that for a large majority of images, compression values in
|
|
the range 3-6 compress nearly as well as higher levels, and do so much
|
|
faster. For online applications it may be desirable to have maximum speed
|
|
(Z_BEST_SPEED = 1). With versions of zlib after v0.99, you can also
|
|
specify no compression (Z_NO_COMPRESSION = 0), but this would create
|
|
files larger than just storing the raw bitmap. You can specify the
|
|
compression level by calling:
|
|
|
|
png_set_compression_level(png_ptr, level);
|
|
|
|
Another useful one is to reduce the memory level used by the library.
|
|
The memory level defaults to 8, but it can be lowered if you are
|
|
short on memory (running DOS, for example, where you only have 640K).
|
|
|
|
png_set_compression_mem_level(png_ptr, level);
|
|
|
|
The other functions are for configuring zlib. They are not recommended
|
|
for normal use and may result in writing an invalid PNG file. See
|
|
zlib.h for more information on what these mean.
|
|
|
|
png_set_compression_strategy(png_ptr,
|
|
strategy);
|
|
png_set_compression_window_bits(png_ptr,
|
|
window_bits);
|
|
png_set_compression_method(png_ptr, method);
|
|
|
|
Controlling row filtering:
|
|
|
|
If you want to control whether libpng uses filtering or not, which
|
|
filters are used, and how it goes about picking row filters, you
|
|
can call one of these functions. The selection and configuration
|
|
of row filters can have a significant impact on the size and
|
|
encoding speed and a somewhat lesser impact on the decoding speed
|
|
of an image. Filtering is enabled by default for RGB and grayscale
|
|
images (with and without alpha), and for 8-bit paletted images, but
|
|
not for paletted images with bit depths less than 8 bits/pixel.
|
|
|
|
The 'method' parameter sets the main filtering method, which is
|
|
currently only '0' in the PNG 1.0 specification. The 'filters'
|
|
parameter sets which filter(s), if any, should be used for each
|
|
scanline. Possible values are PNG_ALL_FILTERS and PNG_NO_FILTERS
|
|
to turn filtering on and off, respectively.
|
|
|
|
Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB,
|
|
PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can be bitwise
|
|
ORed together '|' to specify one or more filters to use. These
|
|
filters are described in more detail in the PNG specification. If
|
|
you intend to change the filter type during the course of writing
|
|
the image, you should start with flags set for all of the filters
|
|
you intend to use so that libpng can initialize its internal
|
|
structures appropriately for all of the filter types.
|
|
|
|
filters = PNG_FILTER_NONE | PNG_FILTER_SUB
|
|
| PNG_FILTER_UP;
|
|
png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE,
|
|
filters);
|
|
|
|
It is also possible to influence how libpng chooses from among the
|
|
available filters. This is done in two ways - by telling it how
|
|
important it is to keep the same filter for successive rows, and
|
|
by telling it the relative computational costs of the filters.
|
|
|
|
double weights[3] = {1.5, 1.3, 1.1},
|
|
costs[PNG_FILTER_VALUE_LAST] =
|
|
{1.0, 1.3, 1.3, 1.5, 1.7};
|
|
|
|
png_set_filter_selection(png_ptr,
|
|
PNG_FILTER_SELECTION_WEIGHTED, 3,
|
|
weights, costs);
|
|
|
|
The weights are multiplying factors which indicate to libpng that row
|
|
should be the same for successive rows unless another row filter is that
|
|
many times better than the previous filter. In the above example, if
|
|
the previous 3 filters were SUB, SUB, NONE, the SUB filter could have a
|
|
"sum of absolute differences" 1.5 x 1.3 times higher than other filters
|
|
and still be chosen, while the NONE filter could have a sum 1.1 times
|
|
higher than other filters and still be chosen. Unspecified weights are
|
|
taken to be 1.0, and the specified weights should probably be declining
|
|
like those above in order to emphasize recent filters over older filters.
|
|
|
|
The filter costs specify for each filter type a relative decoding cost
|
|
to be considered when selecting row filters. This means that filters
|
|
with higher costs are less likely to be chosen over filters with lower
|
|
costs, unless their "sum of absolute differences" is that much smaller.
|
|
The costs do not necessarily reflect the exact computational speeds of
|
|
the various filters, since this would unduly influence the final image
|
|
size.
|
|
|
|
Note that the numbers above were invented purely for this example and
|
|
are given only to help explain the function usage. Little testing has
|
|
been done to find optimum values for either the costs or the weights.
|
|
|
|
Removing unwanted object code:
|
|
|
|
There are a bunch of #define's in pngconf.h that control what parts of
|
|
libpng are compiled. All the defines end in _SUPPORTED. If you are
|
|
never going to use an ability, you can change the #define to #undef
|
|
before recompiling libpng and save yourself code and data space.
|
|
You can also turn a number of them off en masse with a compiler directive
|
|
that defines PNG_READ[or WRITE]_TRANSFORMS_NOT_SUPPORTED, or
|
|
PNG_READ[or WRITE]_ANCILLARY_CHUNKS_NOT_SUPPORTED, or all four,
|
|
along with directives to turn on any of the capabilities that you do
|
|
want. The PNG_READ[or WRITE]_TRANSFORMS_NOT_SUPPORTED directives disable
|
|
the extra transformations but still leave the library fully capable of reading
|
|
and writing PNG files with all known public chunks [except for sPLT].
|
|
Use of the PNG_READ[or WRITE]_ANCILLARY_CHUNKS_NOT_SUPPORTED directive
|
|
produces a library that is incapable of reading or writing ancillary chunks.
|
|
If you are not using the progressive reading capability, you can
|
|
turn that off with PNG_PROGRESSIVE_READ_NOT_SUPPORTED (don't confuse
|
|
this with the INTERLACING capability, which you'll still have).
|
|
|
|
All the reading and writing specific code are in separate files, so the
|
|
linker should only grab the files it needs. However, if you want to
|
|
make sure, or if you are building a stand alone library, all the
|
|
reading files start with pngr and all the writing files start with
|
|
pngw. The files that don't match either (like png.c, pngtrans.c, etc.)
|
|
are used for both reading and writing, and always need to be included.
|
|
The progressive reader is in pngpread.c
|
|
|
|
If you are creating or distributing a dynamically linked library (a .so
|
|
or DLL file), you should not remove or disable any parts of the library,
|
|
as this will cause applications linked with different versions of the
|
|
library to fail if they call functions not available in your library.
|
|
The size of the library itself should not be an issue, because only
|
|
those sections which are actually used will be loaded into memory.
|
|
|
|
|
|
Changes to Libpng from version 0.88
|
|
|
|
It should be noted that versions of libpng later than 0.96 are not
|
|
distributed by the original libpng author, Guy Schalnat, nor by
|
|
Andreas Dilger, who had taken over from Guy during 1996 and 1997, and
|
|
distributed versions 0.89 through 0.96, but rather by another member
|
|
of the original PNG Group, Glenn Randers-Pehrson. Guy and Andreas are
|
|
still alive and well, but they have moved on to other things.
|
|
|
|
The old libpng functions png_read_init(), png_write_init(),
|
|
png_info_init(), png_read_destroy(), and png_write_destory() have been
|
|
moved to PNG_INTERNAL in version 0.95 to discourage their use. The
|
|
preferred method of creating and initializing the libpng structures is
|
|
via the png_create_read_struct(), png_create_write_struct(), and
|
|
png_create_info_struct() because they isolate the size of the structures
|
|
from the application, allow version error checking, and also allow the
|
|
use of custom error handling routines during the initialization, which
|
|
the old functions do not. The functions png_read_destroy() and
|
|
png_write_destroy() do not actually free the memory that libpng
|
|
allocated for these structs, but just reset the data structures, so they
|
|
can be used instead of png_destroy_read_struct() and
|
|
png_destroy_write_struct() if you feel there is too much system overhead
|
|
allocating and freeing the png_struct for each image read.
|
|
|
|
Setting the error callbacks via png_set_message_fn() before
|
|
png_read_init() as was suggested in libpng-0.88 is no longer supported
|
|
because this caused applications which do not use custom error functions
|
|
to fail if the png_ptr was not initialized to zero. It is still possible
|
|
to set the error callbacks AFTER png_read_init(), or to change them with
|
|
png_set_error_fn(), which is essentially the same function, but with a
|
|
new name to force compilation errors with applications that try to use
|
|
the old method.
|
|
|
|
.SH NOTE
|
|
|
|
Note about libpng version numbers:
|
|
|
|
Due to various miscommunications, unforeseen code incompatibilities
|
|
and occasional factors outside the authors' control, version numbering
|
|
on the library has not always been consistent and straightforward.
|
|
The following table summarizes matters since version 0.89c, which was
|
|
the first widely used release:
|
|
|
|
source png.h shared-lib
|
|
version string version
|
|
------- ------ ----------
|
|
0.89c ("1.0 beta 3") 0.89 1.0.89
|
|
0.90 ("1.0 beta 4") 0.90 0.90
|
|
[should have been 2.0.90]
|
|
0.95 ("1.0 beta 5") 0.95 0.95
|
|
[should have been 2.0.95]
|
|
0.96 ("1.0 beta 6") 0.96 0.96
|
|
[should have been 2.0.96]
|
|
0.97b ("1.00.97 beta 7") 1.00.97 1.0.1
|
|
[should have been 2.0.97]
|
|
0.97c 0.97 2.0.97
|
|
0.98 0.98 2.0.98
|
|
0.99 0.99 2.0.99
|
|
0.99a-m 0.99 2.0.99
|
|
1.0.1 1.00 2.1.0
|
|
|
|
Henceforth the source version will match the shared-library minor
|
|
and patch numbers; the shared-library major version number will be
|
|
used for changes in backward compatibility, as it is intended.
|
|
|
|
.SH "SEE ALSO"
|
|
libpngpf(3), png(5)
|
|
.LP
|
|
.IR libpng :
|
|
.IP
|
|
ftp://ftp.uu.net/graphics/png
|
|
http://www.cdrom.com/pub/png
|
|
|
|
.LP
|
|
.IR zlib :
|
|
.IP
|
|
(generally) at the same location as
|
|
.I libpng
|
|
or at
|
|
.br
|
|
ftp://ftp.uu.net/pub/archiving/zip/zlib
|
|
.br
|
|
http://www.cdrom.com/pub/infozip/zlib
|
|
|
|
.LP
|
|
.IR PNG specification: RFC 2083
|
|
.IP
|
|
(generally) at the same location as
|
|
.I libpng
|
|
or at
|
|
.br
|
|
ftp://ds.internic.net/rfc/rfc2083.txt
|
|
.br
|
|
or (as a W3C Recommendation) at
|
|
.br
|
|
http://www.w3.org/TR/REC-png.html
|
|
|
|
.LP
|
|
In the case of any inconsistency between the PNG specification
|
|
and this library, the specification takes precedence.
|
|
|
|
.SH AUTHORS
|
|
This man page: Glenn Randers-Pehrson
|
|
<randeg@alumni.rpi.edu>
|
|
|
|
Contributing Authors: John Bowler, Kevin Bracey, Sam Bushell, Andreas Dilger,
|
|
Magnus Holmgren, Tom Lane, Dave Martindale, Glenn Randers-Pehrson,
|
|
Greg Roelofs, Guy Eric Schalnat, Paul Schmidt, Tom Tanner, Willem van
|
|
Schaik, Tim Wegner.
|
|
<png-implement@dworkin.wustl.edu>
|
|
|
|
The contributing authors would like to thank all those who helped
|
|
with testing, bug fixes, and patience. This wouldn't have been
|
|
possible without all of you.
|
|
|
|
Thanks to Frank J. T. Wojcik for helping with the documentation.
|
|
|
|
Libpng version 1.0.1 (March 9, 1998):
|
|
Initially created in 1995 by Guy Eric Schalnat, then of Group 42, Inc.
|
|
Currently maintained by Glenn Randers-Pehrson (randeg@alumni.rpi.edu).
|
|
|
|
Supported by the PNG development group
|
|
.br
|
|
(png-implement@dworkin.wustl.edu).
|
|
|
|
.SH COPYRIGHT NOTICE:
|
|
|
|
The PNG Reference Library (libpng) is supplied "AS IS". The Contributing
|
|
Authors and Group 42, Inc. disclaim all warranties, expressed or implied,
|
|
including, without limitation, the warranties of merchantability and of
|
|
fitness for any purpose. The Contributing Authors and Group 42, Inc.
|
|
assume no liability for direct, indirect, incidental, special, exemplary,
|
|
or consequential damages, which may result from the use of the PNG
|
|
Reference Library, even if advised of the possibility of such damage.
|
|
|
|
Permission is hereby granted to use, copy, modify, and distribute this
|
|
source code, or portions hereof, for any purpose, without fee, subject
|
|
to the following restrictions:
|
|
|
|
1. The origin of this source code must not be
|
|
misrepresented.
|
|
|
|
2. Altered versions must be plainly marked as such
|
|
and must not be misrepresented as being the
|
|
original source.
|
|
|
|
3. This Copyright notice may not be removed or
|
|
altered from any source or altered source
|
|
distribution.
|
|
|
|
The Contributing Authors and Group 42, Inc. specifically permit, without
|
|
fee, and encourage the use of this source code as a component to
|
|
supporting the PNG file format in commercial products. If you use this
|
|
source code in a product, acknowledgment is not required but would be
|
|
appreciated.
|
|
|
|
.\" end of man page
|
|
|