cheng/libpng
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

[devel] Minor revision of commentary about png_set_alpha_mode().

Browse Source
This commit is contained in:
Glenn Randers-Pehrson 2011-05-08 23:26:19 -05:00
parent 44e26d4149
commit d768d103cf
3 changed files with 11 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
ANNOUNCE
View File

@ -102,7 +102,7 @@ Version 1.5.3beta06 [May 8, 2011]
Implemented premultiplied alpha support: png_set_alpha_mode API
Version 1.5.3beta07 [May 9, 2011]
Added expand_16 suppport to the high level interface.
Added expand_16 support to the high level interface.
Send comments/corrections/commendations to png-mng-implement at lists.sf.net:
(subscription required; visit

2
CHANGES
View File

@ -3361,7 +3361,7 @@ Version 1.5.3beta06 [May 8, 2011]
Implemented premultiplied alpha support: png_set_alpha_mode API
Version 1.5.3beta07 [May 9, 2011]
Added expand_16 suppport to the high level interface.
Added expand_16 support to the high level interface.
Send comments/corrections/commendations to png-mng-implement at lists.sf.net
(subscription required; visit

18
png.h
View File

@ -1084,7 +1084,7 @@ PNG_EXPORT(29, void, png_set_tRNS_to_alpha, (png_structp png_ptr));
#endif
#ifdef PNG_READ_EXPAND_16_SUPPORTED
/* Expand to 16 bit channels, forces conversion of palette to RGB and expansion
/* Expand to 16-bit channels, forces conversion of palette to RGB and expansion
* of a tRNS chunk if present.
*/
PNG_EXPORT(221, void, png_set_expand_16, (png_structp png_ptr));
@ -1136,11 +1136,11 @@ PNG_EXPORT(35, void, png_build_grayscale_palette, (int bit_depth,
* storing color channel values that have been scaled by the alpha. The
* advantage is that the color channels can be resampled (the image can be
* scaled) in this form. The disadvantage is that normal practice is to store
* linear, not (gamma) encoded, values and this requires 16 bit channels for
* still images rather than the 8 bit channels that are just about sufficient if
* linear, not (gamma) encoded, values and this requires 16-bit channels for
* still images rather than the 8-bit channels that are just about sufficient if
* gamma encoding is used. In addition all non-transparent pixel values,
* including completely opaque ones, must be gamma encoded to produce the final
* image. This is the 'STANDARD', 'ASSOCIATED' or 'PREMULTIPLIED' format (the
* image. This is the 'STANDARD', 'ASSOCIATED' or 'PREMULTIPLIED' mode (the
* latter being the two common names for associated alpha color channels.)
*
* Since it is not necessary to perform arithmetic on opaque color values so
@ -1149,10 +1149,10 @@ PNG_EXPORT(35, void, png_build_grayscale_palette, (int bit_depth,
* the PNG format (adjusted for the output color space) while storing partially
* opaque pixels in the standard, linear, format. The accuracy required for
* standard alpha composition is relatively low, because the pixels are
* isolated, therefore typically the accuracy loss in storing 8 bit linear
* isolated, therefore typically the accuracy loss in storing 8-bit linear
* values is acceptable. (This is not true if the alpha channel is used to
* simulate transparency over large areas - use 16 bits or the PNG format in
* this case!) This is the 'OPTIMIZED' format. For this format a pixel is
* simulate transparency over large areas - use 16 bits or the PNG mode in
* this case!) This is the 'OPTIMIZED' mode. For this mode a pixel is
* treated as opaque only if the alpha value is equal to the maximum value.
*
* The final choice is to gamma encode the alpha channel as well. This is
@ -1163,7 +1163,7 @@ PNG_EXPORT(35, void, png_build_grayscale_palette, (int bit_depth,
* opaque areas of the composited PNG image because of arithmetic overflow.
*
* The API function png_set_alpha_mode specifies which of these choices to use
* with a enumerated 'mode' value and the gamma of the required output:
* with an enumerated 'mode' value and the gamma of the required output:
*/
#define PNG_ALPHA_PNG 0 /* according to the PNG standard */
#define PNG_ALPHA_STANDARD 1 /* according to Porter/Duff */
@ -1277,7 +1277,7 @@ PNG_FIXED_EXPORT(228, void, png_set_alpha_mode_fixed, (png_structp png_ptr,
* setting. In this case you just copy completely opaque pixels to the
* output. For pixels that are not completely transparent (you just skip
* those) you do the composition math using png_composite or png_composite_16
* below then encode the resultant 8 or 16 bit values to match the output
* below then encode the resultant 8-bit or 16-bit values to match the output
* encoding.
*
* Other cases