656 lines
23 KiB
HTML
656 lines
23 KiB
HTML
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<HTML>
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<HEAD>
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<TITLE>
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Modifying The TIFF Library
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</TITLE>
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</HEAD>
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<H1>
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<IMG SRC=images/dave.gif WIDTH=107 HEIGHT=148 BORDER=2 ALIGN=left HSPACE=6>
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Modifying The TIFF Library
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</H1>
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<P>
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This chapter provides information about the internal structure of
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the library, how to control the configuration when building it, and
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how to add new support to the library.
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The following sections are found in this chapter:
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<UL>
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<LI><A HREF=#Config>Library Configuration</A>
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<LI><A HREF=#Portability>General Portability Comments</A>
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<LI><A HREF="#Types">Types and Portability</A>
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<LI><A HREF=#AddingTags>Adding New Tags</A>
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<LI><A HREF=#AddingCODECS>Adding New Builtin Codecs</A>
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<LI><A HREF=#AddingCODECTags>Adding New Codec-private Tags</A>
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<LI><A HREF=#Other>Other Comments</A>
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</UL>
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<A NAME="Config"><P><HR WIDTH=65% ALIGN=right><H3>Library Configuration</H3></A>
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Information on compiling the library is given
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<A HREF=build.html>elsewhere in this documentation</A>.
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This section describes the low-level mechanisms used to control
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the optional parts of the library that are configured at build
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time. Control is based on
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a collection of C defines that are specified either on the compiler
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command line or in a configuration file such as <TT>port.h</TT>
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(as generated by the <TT>configure</TT> script for UNIX systems)
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or <B>tiffconf.h</B>.
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<P>
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Configuration defines are split into three areas:
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<UL>
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<LI>those that control which compression schemes are
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configured as part of the builtin codecs,
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<LI>those that control support for groups of tags that
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are considered optional, and
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<LI>those that control operating system or machine-specific support.
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</UL>
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<P>
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If the define <TT>COMPRESSION_SUPPORT</TT> is <STRONG>not defined</STRONG>
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then a default set of compression schemes is automatically
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configured:
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<UL>
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<LI>CCITT Group 3 and 4 algorithms (compression codes 2, 3, 4, and 32771),
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<LI>the Macintosh PackBits algorithm (compression 32773),
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<LI>a Lempel-Ziv & Welch (LZW) algorithm (compression 5),
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<LI>a 4-bit run-length encoding scheme from ThunderScan (compression 32809),
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<LI>a 2-bit encoding scheme used by NeXT (compression 32766), and
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<LI>two experimental schemes intended for images with high dynamic range
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(compression 34676 and 34677).
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</UL>
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To override this behaviour define <TT>COMPRESSION_SUPPORT</TT>
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and then one or more additional defines to enable configuration of
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the appropriate codecs (see the table below); e.g.
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<UL><PRE>
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#define COMPRESSION_SUPPORT
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#define CCITT_SUPPORT
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#define PACKBITS_SUPPORT
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</PRE></UL>
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Several other compression schemes are configured separately from
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the default set because they depend on ancillary software
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packages that are not distributed with <TT>libtiff</TT>.
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<P>
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Support for JPEG compression is controlled by <TT>JPEG_SUPPORT</TT>.
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The JPEG codec that comes with <TT>libtiff</TT> is designed for
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use with release 5 or later of the Independent JPEG Group's freely
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available software distribution.
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This software can be retrieved from the directory
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<A HREF=ftp://ftp.uu.net/graphics/jpeg>ftp.uu.net:/graphics/jpeg/</A>.
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<P>
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<IMG SRC="images/info.gif" ALT="NOTE: " ALIGN=left HSPACE=8>
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<EM>Enabling JPEG support automatically enables support for
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the TIFF 6.0 colorimetry and YCbCr-related tags.</EM>
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<P>
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Experimental support for the deflate algorithm is controlled by
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<TT>DEFLATE_SUPPORT</TT>.
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The deflate codec that comes with <TT>libtiff</TT> is designed
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for use with version 0.99 or later of the freely available
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<TT>libz</TT> library written by Jean-loup Gailly and Mark Adler.
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The data format used by this library is described
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in the files
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<A HREF=ftp://ftp.uu.net/pub/archiving/zip/doc/zlib-3.1.doc>zlib-3.1.doc</A>,
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and
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<A HREF=ftp://ftp.uu.net/pub/archiving/zip/doc/deflate-1.1.doc>deflate-1.1.doc</A>,
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available in the directory
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<A HREF=ftp://ftp.uu.net/pub/archiving/zip/doc>ftp.uu.net:/pub/archiving/zip/doc</A>.</EM>
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The library can be retried from the directory
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<A HREF=ftp://ftp.uu.net/pub/archiving/zip/zlib/>ftp.uu.net:/pub/archiving/zip/zlib/</A>
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(or try <A HREF=ftp://quest.jpl.nasa.gov/beta/zlib/>quest.jpl.nasa.gov:/beta/zlib/</A>).
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<P>
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<IMG SRC="images/warning.gif" ALT="NOTE: " ALIGN=left HSPACE=8 VSPACE=6>
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<EM>The deflate algorithm is experimental. Do not expect
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to exchange files using this compression scheme;
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it is included only because the similar, and more common,
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LZW algorithm is claimed to be governed by licensing restrictions.</EM>
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<P>
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By default <B>tiffconf.h</B> defines
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<TT>COLORIMETRY_SUPPORT</TT>,
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<TT>YCBCR_SUPPORT</TT>,
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and
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<TT>CMYK_SUPPORT</TT>.
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<P>
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<TABLE BORDER CELLPADDING=3>
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<TR><TH ALIGN=left>Define</TH><TH ALIGN=left>Description</TH></TR>
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<TR>
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<TD VALIGN=top><TT>CCITT_SUPPORT</TT></TD>
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<TD>CCITT Group 3 and 4 algorithms (compression codes 2, 3, 4,
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and 32771)</TD>
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</TR>
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<TR>
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<TD VALIGN=top><TT>PACKBITS_SUPPORT</TT></TD>
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<TD>Macintosh PackBits algorithm (compression 32773)</TD>
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</TR>
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<TR>
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<TD VALIGN=top><TT>LZW_SUPPORT</TT></TD>
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<TD>Lempel-Ziv & Welch (LZW) algorithm (compression 5)</TD>
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</TR>
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<TR>
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<TD VALIGN=top><TT>THUNDER_SUPPORT</TT></TD>
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<TD>4-bit
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run-length encoding scheme from ThunderScan (compression 32809)</TD>
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</TR>
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<TR>
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<TD VALIGN=top><TT>NEXT_SUPPORT</TT></TD>
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<TD>2-bit encoding scheme used by NeXT (compression 32766)</TD>
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</TR>
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<TR>
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<TD VALIGN=top><TT>OJPEG_SUPPORT</TT></TD>
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<TD>obsolete JPEG scheme defined in the 6.0 spec (compression 6)</TD>
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</TR>
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<TR>
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<TD VALIGN=top><TT>JPEG_SUPPORT</TT></TD>
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<TD>current JPEG scheme defined in TTN2 (compression 7)</TD>
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</TR>
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<TR>
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<TD VALIGN=top><TT>ZIP_SUPPORT</TT></TD>
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<TD>experimental Deflate scheme (compression 32946)</TD>
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</TR>
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<TR>
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<TD VALIGN=top><TT>PIXARLOG_SUPPORT</TT></TD>
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<TD>Pixar's compression scheme for high-resolution color images (compression 32909)</TD>
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</TR>
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<TR>
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<TD VALIGN=top><TT>SGILOG_SUPPORT</TT></TD>
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<TD>SGI's compression scheme for high-resolution color images (compression 34676 and 34677)</TD>
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</TR>
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<TR>
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<TD VALIGN=top><TT>COLORIMETRY_SUPPORT</TT></TD>
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<TD>support for the TIFF 6.0 colorimetry tags</TD>
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</TR>
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<TR>
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<TD VALIGN=top><TT>YCBCR_SUPPORT</TT></TD>
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<TD>support for the TIFF 6.0 YCbCr-related tags</TD>
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</TR>
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<TR>
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<TD VALIGN=top><TT>CMYK_SUPPORT</TT></TD>
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<TD>support for the TIFF 6.0 CMYK-related tags</TD>
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</TR>
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<TR>
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<TD VALIGN=top><TT>ICC_SUPPORT</TT></TD>
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<TD>support for the ICC Profile tag; see
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<I>The ICC Profile Format Specification</I>,
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Annex B.3 "Embedding ICC Profiles in TIFF Files";
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available at
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<A HREF=http://www.color.org>http://www.color.org</A>
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</TD>
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</TR>
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</TABLE>
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<A NAME="Portability"><P><HR WIDTH=65% ALIGN=right><H3>General Portability Comments</H3></A>
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This software is developed on Silicon Graphics UNIX
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systems (big-endian, MIPS CPU, 32-bit ints,
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IEEE floating point).
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The <TT>configure</TT> shell script generates the appropriate
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include files and make files for UNIX systems.
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Makefiles exist for non-UNIX platforms that the
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code runs on -- this work has mostly been done by other people.
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<P>
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In general, the code is guaranteed to work only on SGI machines.
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In practice it is highly portable to any 32-bit or 64-bit system and much
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work has been done to insure portability to 16-bit systems.
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If you encounter portability problems please return fixes so
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that future distributions can be improved.
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<P>
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The software is written to assume an ANSI C compilation environment.
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If your compiler does not support ANSI function prototypes, <TT>const</TT>,
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and <TT><stdarg.h></TT> then you will have to make modifications to the
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software. In the past I have tried to support compilers without <TT>const</TT>
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and systems without <TT><stdarg.h></TT>, but I am
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<EM>no longer interested in these
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antiquated environments</EM>. With the general availability of
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the freely available GCC compiler, I
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see no reason to incorporate modifications to the software for these
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purposes.
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<P>
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An effort has been made to isolate as many of the
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operating system-dependencies
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as possible in two files: <B>tiffcomp.h</B> and
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<B>libtiff/tif_<os>.c</B>. The latter file contains
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operating system-specific routines to do I/O and I/O-related operations.
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The UNIX (<B>tif_unix.c</B>),
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Macintosh (<B>tif_apple.c</B>),
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and VMS (<B>tif_vms.c</B>)
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code has had the most use;
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the MS/DOS support (<B>tif_msdos.c</B>) assumes
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some level of UNIX system call emulation (i.e.
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<TT>open</TT>,
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<TT>read</TT>,
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<TT>write</TT>,
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<TT>fstat</TT>,
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<TT>malloc</TT>,
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<TT>free</TT>).
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<P>
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Native CPU byte order is determined on the fly by
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the library and does not need to be specified.
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The <TT>HOST_FILLORDER</TT> and <TT>HOST_BIGENDIAN</TT>
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definitions are not currently used, but may be employed by
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codecs for optimization purposes.
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<P>
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The following defines control general portability:
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<P>
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<TABLE BORDER CELLPADDING=3 WIDTH=100%>
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<TR>
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<TD VALIGN=top><TT>BSDTYPES</TT></TD>
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<TD>Define this if your system does NOT define the
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usual BSD typedefs: <TT>u_char</TT>,
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<TT>u_short</TT>, <TT>u_int</TT>, <TT>u_long</TT>.</TD>
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</TR>
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<TR>
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<TD VALIGN=top><TT>HAVE_IEEEFP</TT></TD>
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<TD>Define this as 0 or 1 according to the floating point
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format suported by the machine. If your machine does
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not support IEEE floating point then you will need to
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add support to tif_machdep.c to convert between the
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native format and IEEE format.</TD>
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</TR>
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<TR>
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<TD VALIGN=top><TT>HAVE_MMAP</TT></TD>
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<TD>Define this if there is <I>mmap-style</I> support for
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mapping files into memory (used only to read data).</TD>
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</TR>
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<TR>
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<TD VALIGN=top><TT>HOST_FILLORDER</TT></TD>
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<TD>Define the native CPU bit order: one of <TT>FILLORDER_MSB2LSB</TT>
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or <TT>FILLORDER_LSB2MSB</TT></TD>
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</TR>
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<TR>
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<TD VALIGN=top><TT>HOST_BIGENDIAN</TT></TD>
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<TD>Define the native CPU byte order: 1 if big-endian (Motorola)
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or 0 if little-endian (Intel); this may be used
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in codecs to optimize code</TD>
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</TR>
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</TABLE>
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<P>
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On UNIX systems <TT>HAVE_MMAP</TT> is defined through the running of
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the <TT>configure</TT> script; otherwise support for memory-mapped
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files is disabled.
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Note that <B>tiffcomp.h</B> defines <TT>HAVE_IEEEFP</TT> to be
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1 (<TT>BSDTYPES</TT> is not defined).
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<A NAME="Types"><P><HR WIDTH=65% ALIGN=right><H3>Types and Portability</H3></A>
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The software makes extensive use of C typedefs to promote portability.
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Two sets of typedefs are used, one for communication with clients
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of the library and one for internal data structures and parsing of the
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TIFF format. There are interactions between these two to be careful
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of, but for the most part you should be able to deal with portability
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purely by fiddling with the following machine-dependent typedefs:
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<P>
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<TABLE BORDER CELLPADDING=3 WIDTH=100%>
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<TR>
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<TD>uint8</TD>
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<TD>8-bit unsigned integer</TD>
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<TD>tiff.h</TD>
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</TR>
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<TR>
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<TD>int8</TD>
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<TD>8-bit signed integer</TD>
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<TD>tiff.h</TD>
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</TR>
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<TR>
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<TD>uint16</TD>
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<TD>16-bit unsigned integer</TD>
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<TD>tiff.h</TD>
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</TR>
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<TR>
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<TD>int16</TD>
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<TD>16-bit signed integer</TD>
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<TD>tiff.h</TD>
|
||
|
|
</TR>
|
||
|
|
|
||
|
|
<TR>
|
||
|
|
<TD>uint32</TD>
|
||
|
|
<TD>32-bit unsigned integer</TD>
|
||
|
|
<TD>tiff.h</TD>
|
||
|
|
</TR>
|
||
|
|
|
||
|
|
<TR>
|
||
|
|
<TD>int32</TD>
|
||
|
|
<TD>32-bit signed integer</TD>
|
||
|
|
<TD>tiff.h</TD>
|
||
|
|
</TR>
|
||
|
|
|
||
|
|
<TR>
|
||
|
|
<TD>dblparam_t</TD>
|
||
|
|
<TD>promoted type for floats</TD>
|
||
|
|
<TD>tiffcomp.h</TD>
|
||
|
|
</TR>
|
||
|
|
|
||
|
|
</TABLE>
|
||
|
|
|
||
|
|
<P>
|
||
|
|
(to clarify <TT>dblparam_t</TT>, it is the type that float parameters are
|
||
|
|
promoted to when passed by value in a function call.)
|
||
|
|
|
||
|
|
<P>
|
||
|
|
The following typedefs are used throughout the library and interfaces
|
||
|
|
to refer to certain objects whose size is dependent on the TIFF image
|
||
|
|
structure:
|
||
|
|
|
||
|
|
|
||
|
|
<P>
|
||
|
|
<TABLE BORDER CELLPADDING=3 WIDTH=100%>
|
||
|
|
|
||
|
|
<TR>
|
||
|
|
<TD WIDTH=25%>typedef unsigned int ttag_t;</TD> <TD>directory tag</TD>
|
||
|
|
</TR>
|
||
|
|
|
||
|
|
<TR>
|
||
|
|
<TD>typedef uint16 tdir_t;</TD> <TD>directory index</TD>
|
||
|
|
</TR>
|
||
|
|
|
||
|
|
<TR>
|
||
|
|
<TD>typedef uint16 tsample_t;</TD> <TD>sample number</TD>
|
||
|
|
</TR>
|
||
|
|
|
||
|
|
<TR>
|
||
|
|
<TD>typedef uint32 tstrip_t;</TD> <TD>strip number</TD>
|
||
|
|
</TR>
|
||
|
|
|
||
|
|
<TR>
|
||
|
|
<TD>typedef uint32 ttile_t;</TD> <TD>tile number</TD>
|
||
|
|
</TR>
|
||
|
|
|
||
|
|
<TR>
|
||
|
|
<TD>typedef int32 tsize_t;</TD> <TD>i/o size in bytes</TD>
|
||
|
|
</TR>
|
||
|
|
|
||
|
|
<TR>
|
||
|
|
<TD>typedef void* tdata_t;</TD> <TD>image data ref</TD>
|
||
|
|
</TR>
|
||
|
|
|
||
|
|
<TR>
|
||
|
|
<TD>typedef void* thandle_t;</TD> <TD>client data handle</TD>
|
||
|
|
</TR>
|
||
|
|
|
||
|
|
<TR>
|
||
|
|
<TD>typedef int32 toff_t;</TD> <TD>file offset (should be off_t)</TD>
|
||
|
|
</TR>
|
||
|
|
|
||
|
|
<TR>
|
||
|
|
<TD>typedef unsigned char* tidata_t;</TD> <TD>internal image data</TD>
|
||
|
|
</TR>
|
||
|
|
|
||
|
|
</TABLE>
|
||
|
|
|
||
|
|
<P>
|
||
|
|
Note that <TT>tstrip_t</TT>, <TT>ttile_t</TT>, and <TT>tsize_t</TT>
|
||
|
|
are constrained to be
|
||
|
|
no more than 32-bit quantities by 32-bit fields they are stored
|
||
|
|
in in the TIFF image. Likewise <TT>tsample_t</TT> is limited by the 16-bit
|
||
|
|
field used to store the <TT>SamplesPerPixel</TT> tag. <TT>tdir_t</TT>
|
||
|
|
constrains
|
||
|
|
the maximum number of IFDs that may appear in an image and may
|
||
|
|
be an arbitrary size (without penalty). <TT>ttag_t</TT> must be either
|
||
|
|
<TT>int</TT>, <TT>unsigned int</TT>, pointer, or <TT>double</TT>
|
||
|
|
because the library uses a varargs
|
||
|
|
interface and ANSI C restricts the type of the parameter before an
|
||
|
|
ellipsis to be a promoted type. <TT>toff_t</TT> is defined as
|
||
|
|
<TT>int32</TT> because
|
||
|
|
TIFF file offsets are (unsigned) 32-bit quantities. A signed
|
||
|
|
value is used because some interfaces return -1 on error (sigh).
|
||
|
|
Finally, note that <TT>tidata_t</TT> is used internally to the library to
|
||
|
|
manipulate internal data. User-specified data references are
|
||
|
|
passed as opaque handles and only cast at the lowest layers where
|
||
|
|
their type is presumed.
|
||
|
|
|
||
|
|
|
||
|
|
<P><HR WIDTH=65% ALIGN=right><H3>General Comments</H3></A>
|
||
|
|
|
||
|
|
The library is designed to hide as much of the details of TIFF from
|
||
|
|
applications as
|
||
|
|
possible. In particular, TIFF directories are read in their entirety
|
||
|
|
into an internal format. Only the tags known by the library are
|
||
|
|
available to a user and certain tag data may be maintained that a user
|
||
|
|
does not care about (e.g. transfer function tables).
|
||
|
|
|
||
|
|
<A NAME=AddingTags><P><HR WIDTH=65% ALIGN=right><H3>Adding New Tags</H3></A>
|
||
|
|
|
||
|
|
To add support for a new directory tag you have three options. If your
|
||
|
|
tag is specific to a compression algorithm, see below. If you have a lot
|
||
|
|
of tags you may want to try using Niles Ritter's runtime tag-extension
|
||
|
|
scheme in the "contrib/tags" directory, which makes the changes
|
||
|
|
orthogonal to the main libtiff code. Otherwise use
|
||
|
|
the following guidelines to add support to the ``core library''.
|
||
|
|
|
||
|
|
<OL>
|
||
|
|
<LI>Define the tag in <B>tiff.h</B>.
|
||
|
|
<LI>Add a field to the directory structure in <B>tif_dir.h</B>
|
||
|
|
and define a <TT>FIELD_*</TT> bit (also update the definition of
|
||
|
|
<TT>FIELD_CODEC</TT> to reflect your addition).
|
||
|
|
<LI>Add an entry in the <TT>TIFFFieldInfo</TT> array defined at the top of
|
||
|
|
<B>tif_dirinfo.c</B>.
|
||
|
|
Note that you must keep this array sorted by tag
|
||
|
|
number and that the widest variant entry for a tag should come
|
||
|
|
first (e.g. <TT>LONG</TT> before <TT>SHORT</TT>).
|
||
|
|
<LI>Add entries in <TT>_TIFFVSetField()</TT> and <TT>_TIFFVGetField()</TT>
|
||
|
|
for the new tag.
|
||
|
|
<LI>(<I>optional</I>) If the value associated with the tag is not a scalar value
|
||
|
|
(e.g. the array for <TT>TransferFunction</TT>) and requires
|
||
|
|
special processing,
|
||
|
|
then add the appropriate code to <TT>TIFFReadDirectory()</TT> and
|
||
|
|
<TT>TIFFWriteDirectory()</TT>. You're best off finding a similar tag and
|
||
|
|
cribbing code.
|
||
|
|
<LI>Add support to <TT>TIFFPrintDirectory()</TT> in <B>tif_print.c</B>
|
||
|
|
to print the tag's value.
|
||
|
|
</OL>
|
||
|
|
|
||
|
|
<P>
|
||
|
|
If you want to maintain portability, beware of making assumptions
|
||
|
|
about data types. Use the typedefs (<TT>uint16</TT>, etc. when dealing with
|
||
|
|
data on disk and <TT>t*_t</TT> when stuff is in memory) and be careful about
|
||
|
|
passing items through printf or similar vararg interfaces.
|
||
|
|
|
||
|
|
<A NAME=AddingCODECS><P><HR WIDTH=65% ALIGN=right><H3>Adding New Builtin Codecs</H3></A>
|
||
|
|
|
||
|
|
To add builtin support for a new compression algorithm, you can either
|
||
|
|
use the "tag-extension" trick to override the handling of the
|
||
|
|
TIFF Compression tag (see <A HREF=#AddingTags>Adding New Tags</A>, above),
|
||
|
|
or do the following to add support directly to the core library:
|
||
|
|
|
||
|
|
<OL>
|
||
|
|
<LI>Define the tag value in <B>tiff.h</B>.
|
||
|
|
<LI>Edit the file <B>tif_codec.c</B> to add an entry to the
|
||
|
|
_TIFFBuiltinCODECS array (see how other algorithms are handled).
|
||
|
|
<LI>Add the appropriate function prototype declaration to
|
||
|
|
<B>tiffiop.h</B> (close to the bottom).
|
||
|
|
<LI>Create a file with the compression scheme code, by convention files
|
||
|
|
are named <B>tif_*.c</B> (except perhaps on some systems where the
|
||
|
|
tif_ prefix pushes some filenames over 14 chars.
|
||
|
|
<LI>Edit <B>Makefile.in</B> (and any other Makefiles)
|
||
|
|
to include the new source file.
|
||
|
|
</OL>
|
||
|
|
|
||
|
|
<P>
|
||
|
|
A codec, say <TT>foo</TT>, can have many different entry points:
|
||
|
|
|
||
|
|
<PRE>
|
||
|
|
TIFFInitfoo(tif, scheme)/* initialize scheme and setup entry points in tif */
|
||
|
|
fooSetupDecode(tif) /* called once per IFD after tags has been frozen */
|
||
|
|
fooPreDecode(tif, sample)/* called once per strip/tile, after data is read,
|
||
|
|
but before the first row is decoded */
|
||
|
|
fooDecode*(tif, bp, cc, sample)/* decode cc bytes of data into the buffer */
|
||
|
|
fooDecodeRow(...) /* called to decode a single scanline */
|
||
|
|
fooDecodeStrip(...) /* called to decode an entire strip */
|
||
|
|
fooDecodeTile(...) /* called to decode an entire tile */
|
||
|
|
fooSetupEncode(tif) /* called once per IFD after tags has been frozen */
|
||
|
|
fooPreEncode(tif, sample)/* called once per strip/tile, before the first row in
|
||
|
|
a strip/tile is encoded */
|
||
|
|
fooEncode*(tif, bp, cc, sample)/* encode cc bytes of user data (bp) */
|
||
|
|
fooEncodeRow(...) /* called to decode a single scanline */
|
||
|
|
fooEncodeStrip(...) /* called to decode an entire strip */
|
||
|
|
fooEncodeTile(...) /* called to decode an entire tile */
|
||
|
|
fooPostEncode(tif) /* called once per strip/tile, just before data is written */
|
||
|
|
fooSeek(tif, row) /* seek forwards row scanlines from the beginning
|
||
|
|
of a strip (row will always be >0 and <rows/strip */
|
||
|
|
fooCleanup(tif) /* called when compression scheme is replaced by user */
|
||
|
|
</PRE>
|
||
|
|
|
||
|
|
<P>
|
||
|
|
Note that the encoding and decoding variants are only needed when
|
||
|
|
a compression algorithm is dependent on the structure of the data.
|
||
|
|
For example, Group 3 2D encoding and decoding maintains a reference
|
||
|
|
scanline. The sample parameter identifies which sample is to be
|
||
|
|
encoded or decoded if the image is organized with <TT>PlanarConfig</TT>=2
|
||
|
|
(separate planes). This is important for algorithms such as JPEG.
|
||
|
|
If <TT>PlanarConfig</TT>=1 (interleaved), then sample will always be 0.
|
||
|
|
|
||
|
|
|
||
|
|
<A NAME=AddingCODECTags><P><HR WIDTH=65% ALIGN=right><H3>Adding New Codec-private Tags</H3></A>
|
||
|
|
|
||
|
|
To add tags that are meaningful <EM>only when a particular compression
|
||
|
|
algorithm is used</EM> follow these steps:
|
||
|
|
|
||
|
|
<OL>
|
||
|
|
<LI>Define the tag in <B>tiff.h</B>.
|
||
|
|
<LI>Allocate storage for the tag values in the private state block of
|
||
|
|
the codec.
|
||
|
|
<LI>Insure the state block is created when the codec is initialized.
|
||
|
|
<LI>At <TT>TIFFInitfoo</TT> time override the method pointers in the
|
||
|
|
TIFF structure
|
||
|
|
for getting, setting and printing tag values. For example,
|
||
|
|
<PRE>
|
||
|
|
sp->vgetparent = tif->tif_vgetfield;
|
||
|
|
tif->tif_vgetfield = fooVGetField; /* hook for codec tags */
|
||
|
|
sp->vsetparent = tif->tif_vsetfield;
|
||
|
|
tif->tif_vsetfield = fooVSetField; /* hook for codec tags */
|
||
|
|
tif->tif_printdir = fooPrintDir; /* hook for codec tags */
|
||
|
|
</PRE>
|
||
|
|
(Actually you may decide not to override the
|
||
|
|
<TT>tif_printdir</TT> method, but rather just specify it).
|
||
|
|
<LI>Create a private <TT>TIFFFieldInfo</TT> array for your tags and
|
||
|
|
merge them into the core tags at initialization time using
|
||
|
|
<TT>_TIFFMergeFieldInfo</TT>; e.g.
|
||
|
|
<PRE>
|
||
|
|
_TIFFMergeFieldInfo(tif, fooFieldInfo, N(fooFieldInfo));
|
||
|
|
</PRE>
|
||
|
|
(where <TT>N</TT> is a macro used liberaly throughout the distributed code).
|
||
|
|
<LI>Fill in the get and set routines. Be sure to call the parent method
|
||
|
|
for tags that you are not handled directly. Also be sure to set the
|
||
|
|
<TT>FIELD_*</TT> bits for tags that are to be written to the file. Note that
|
||
|
|
you can create ``pseudo-tags'' by defining tags that are processed
|
||
|
|
exclusively in the get/set routines and never written to file (see
|
||
|
|
the handling of <TT>TIFFTAG_FAXMODE</TT> in <B>tif_fax3.c</B>
|
||
|
|
for an example of this).
|
||
|
|
<LI>Fill in the print routine, if appropriate.
|
||
|
|
</OL>
|
||
|
|
|
||
|
|
Note that space has been allocated in the <TT>FIELD_*</TT> bit space for
|
||
|
|
codec-private tags. Define your bits as <TT>FIELD_CODEC+<offset></TT> to
|
||
|
|
keep them away from the core tags. If you need more tags than there
|
||
|
|
is room for, just increase <TT>FIELD_SETLONGS</TT> at the top of
|
||
|
|
<B>tiffiop.h</B>.
|
||
|
|
|
||
|
|
|
||
|
|
<A NAME=Other><P><HR WIDTH=65% ALIGN=right><H3>Other Comments</H3></A>
|
||
|
|
|
||
|
|
The library handles most I/O buffering. There are two data buffers
|
||
|
|
when decoding data: a raw data buffer that holds all the data in a
|
||
|
|
strip, and a user-supplied scanline buffer that compression schemes
|
||
|
|
place decoded data into. When encoding data the data in the
|
||
|
|
user-supplied scanline buffer is encoded into the raw data buffer (from
|
||
|
|
where it is written). Decoding routines should never have to explicitly
|
||
|
|
read data -- a full strip/tile's worth of raw data is read and scanlines
|
||
|
|
never cross strip boundaries. Encoding routines must be cognizant of
|
||
|
|
the raw data buffer size and call <TT>TIFFFlushData1()</TT> when necessary.
|
||
|
|
Note that any pending data is automatically flushed when a new strip/tile is
|
||
|
|
started, so there's no need do that in the tif_postencode routine (if
|
||
|
|
one exists). Bit order is automatically handled by the library when
|
||
|
|
a raw strip or tile is filled. If the decoded samples are interpreted
|
||
|
|
by the decoding routine before they are passed back to the user, then
|
||
|
|
the decoding logic must handle byte-swapping by overriding the
|
||
|
|
<TT>tif_postdecode</TT>
|
||
|
|
routine (set it to <TT>TIFFNoPostDecode</TT>) and doing the required work
|
||
|
|
internally. For an example of doing this look at the horizontal
|
||
|
|
differencing code in the routines in <B>tif_predict.c</TT>.
|
||
|
|
|
||
|
|
<P>
|
||
|
|
The variables <TT>tif_rawcc</TT>, <TT>tif_rawdata</TT>, and
|
||
|
|
<TT>tif_rawcp</TT> in a <TT>TIFF</TT> structure
|
||
|
|
are associated with the raw data buffer. <TT>tif_rawcc</TT> must be non-zero
|
||
|
|
for the library to automatically flush data. The variable
|
||
|
|
<TT>tif_scanlinesize</TT> is the size a user's scanline buffer should be. The
|
||
|
|
variable <TT>tif_tilesize</TT> is the size of a tile for tiled images. This
|
||
|
|
should not normally be used by compression routines, except where it
|
||
|
|
relates to the compression algorithm. That is, the <TT>cc</TT> parameter to the
|
||
|
|
<TT>tif_decode*</TT> and <TT>tif_encode*</TT>
|
||
|
|
routines should be used in terminating
|
||
|
|
decompression/compression. This ensures these routines can be used,
|
||
|
|
for example, to decode/encode entire strips of data.
|
||
|
|
|
||
|
|
<P>
|
||
|
|
In general, if you have a new compression algorithm to add, work from
|
||
|
|
the code for an existing routine. In particular,
|
||
|
|
<B>tif_dumpmode.c</B>
|
||
|
|
has the trivial code for the "nil" compression scheme,
|
||
|
|
<B>tif_packbits.c</B> is a
|
||
|
|
simple byte-oriented scheme that has to watch out for buffer
|
||
|
|
boundaries, and <B>tif_lzw.c</B> has the LZW scheme that has the most
|
||
|
|
complexity -- it tracks the buffer boundary at a bit level.
|
||
|
|
Of course, using a private compression scheme (or private tags) limits
|
||
|
|
the portability of your TIFF files.
|
||
|
|
|
||
|
|
<P>
|
||
|
|
<HR>
|
||
|
|
|
||
|
|
<ADDRESS>
|
||
|
|
<A HREF="sam.html">Sam Leffler</A> / <A HREF="mailto:sam@engr.sgi.com">sam@engr.sgi.com</A>.
|
||
|
|
Last updated: $Date: 1999-07-27 21:50:27 $
|
||
|
|
</ADDRESS>
|
||
|
|
|
||
|
|
</BODY>
|
||
|
|
</HTML>
|