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Modifying The TIFF Library

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Modifying The TIFF Library

This chapter provides information about the internal structure of

the library, how to control the configuration when building it, and

how to add new support to the library.

The following sections are found in this chapter:

Library Configuration

General Portability Comments

Types and Portability

Adding New Tags

Adding New Builtin Codecs

Adding New Codec-private Tags

Other Comments

Library Configuration

Information on compiling the library is given

elsewhere in this documentation.

This section describes the low-level mechanisms used to control

the optional parts of the library that are configured at build

time.   Control is based on

a collection of C defines that are specified either on the compiler

command line or in a configuration file such as <tt>port.h</tt>

(as generated by the <tt>configure</tt> script for UNIX systems)

or tiffconf.h.

Configuration defines are split into three areas:

those that control which compression schemes are

    configured as part of the builtin codecs,

those that control support for groups of tags that

    are considered optional, and

those that control operating system or machine-specific support.

If the define <tt>COMPRESSION_SUPPORT</tt> is not defined

then a default set of compression schemes is automatically

configured:

CCITT Group 3 and 4 algorithms (compression codes 2, 3, 4, and 32771),

the Macintosh PackBits algorithm (compression 32773),

a 4-bit run-length encoding scheme from ThunderScan (compression 32809),

a 2-bit encoding scheme used by NeXT (compression 32766), and

two experimental schemes intended for images with high dynamic range

(compression 34676 and 34677).

To override the default compression behaviour define

<tt>COMPRESSION_SUPPORT</tt> and then one or more additional defines

to enable configuration of the appropriate codecs (see the table

below); e.g.

#define	COMPRESSION_SUPPORT

#define	CCITT_SUPPORT

#define	PACKBITS_SUPPORT

Several other compression schemes are configured separately from

the default set because they depend on ancillary software

packages that are not distributed with <tt>libtiff</tt>.

Support for JPEG compression is controlled by <tt>JPEG_SUPPORT</tt>.

The JPEG codec that comes with <tt>libtiff</tt> is designed for

use with release 5 or later of the Independent JPEG Group's freely

available software distribution.

This software can be retrieved from the directory

ftp.uu.net:/graphics/jpeg/.

Enabling JPEG support automatically enables support for

the TIFF 6.0 colorimetry and YCbCr-related tags.

Experimental support for the deflate algorithm is controlled by

<tt>DEFLATE_SUPPORT</tt>.

The deflate codec that comes with <tt>libtiff</tt> is designed

for use with version 0.99 or later of the freely available

<tt>libz</tt> library written by Jean-loup Gailly and Mark Adler.

The data format used by this library is described

in the files

zlib-3.1.doc,

and

deflate-1.1.doc,

available in the directory

ftp.uu.net:/pub/archiving/zip/doc.

The library can be retried from the directory

ftp.uu.net:/pub/archiving/zip/zlib/

(or try quest.jpl.nasa.gov:/beta/zlib/).

The deflate algorithm is experimental.  Do not expect

to exchange files using this compression scheme;

it is included only because the similar, and more common,

LZW algorithm is claimed to be governed by licensing restrictions.

By default tiffconf.h defines

<tt>COLORIMETRY_SUPPORT</tt>, 

<tt>YCBCR_SUPPORT</tt>,

and 

<tt>CMYK_SUPPORT</tt>.

DefineDescription

<tt>CCITT_SUPPORT</tt>

CCITT Group 3 and 4 algorithms (compression codes 2, 3, 4,

    and 32771)

<tt>PACKBITS_SUPPORT</tt>

Macintosh PackBits algorithm (compression 32773)

<tt>LZW_SUPPORT</tt>

Lempel-Ziv & Welch (LZW) algorithm (compression 5)

<tt>THUNDER_SUPPORT</tt>

4-bit

run-length encoding scheme from ThunderScan (compression 32809)

<tt>NEXT_SUPPORT</tt>

2-bit encoding scheme used by NeXT (compression 32766)

<tt>OJPEG_SUPPORT</tt>

obsolete JPEG scheme defined in the 6.0 spec (compression 6)

<tt>JPEG_SUPPORT</tt>

current JPEG scheme defined in TTN2 (compression 7)

<tt>ZIP_SUPPORT</tt>

experimental Deflate scheme (compression 32946)

<tt>PIXARLOG_SUPPORT</tt>

Pixar's compression scheme for high-resolution color images (compression 32909)

<tt>SGILOG_SUPPORT</tt>

SGI's compression scheme for high-resolution color images (compression 34676 and 34677)

<tt>COLORIMETRY_SUPPORT</tt>

support for the TIFF 6.0 colorimetry tags

<tt>YCBCR_SUPPORT</tt>

support for the TIFF 6.0 YCbCr-related tags

<tt>CMYK_SUPPORT</tt>

support for the TIFF 6.0 CMYK-related tags

<tt>ICC_SUPPORT</tt>

support for the ICC Profile tag; see

The ICC Profile Format Specification,

Annex B.3 "Embedding ICC Profiles in TIFF Files";

available at

http://www.color.org

General Portability Comments

This software is developed on Silicon Graphics UNIX

systems (big-endian, MIPS CPU, 32-bit ints,

IEEE floating point). 

The <tt>configure</tt> shell script generates the appropriate

include files and make files for UNIX systems.

Makefiles exist for non-UNIX platforms that the

code runs on -- this work has mostly been done by other people.

In general, the code is guaranteed to work only on SGI machines.

In practice it is highly portable to any 32-bit or 64-bit system and much

work has been done to insure portability to 16-bit systems.

If you encounter portability problems please return fixes so

that future distributions can be improved.

The software is written to assume an ANSI C compilation environment.

If your compiler does not support ANSI function prototypes, <tt>const</tt>,

and <tt><stdarg.h></tt> then you will have to make modifications to the

software.  In the past I have tried to support compilers without <tt>const</tt>

and systems without <tt><stdarg.h></tt>, but I am

no longer interested in these

antiquated environments.  With the general availability of

the freely available GCC compiler, I

see no reason to incorporate modifications to the software for these

purposes.

An effort has been made to isolate as many of the

operating system-dependencies

as possible in two files: tiffcomp.h and

libtiff/tif_<os>.c.  The latter file contains

operating system-specific routines to do I/O and I/O-related operations.

The UNIX (tif_unix.c),

Macintosh (tif_apple.c),

and VMS (tif_vms.c)

code has had the most use;

the MS/DOS support (tif_msdos.c) assumes

some level of UNIX system call emulation (i.e.

<tt>open</tt>,

<tt>read</tt>,

<tt>write</tt>,

<tt>fstat</tt>,

<tt>malloc</tt>,

<tt>free</tt>).

Native CPU byte order is determined on the fly by

the library and does not need to be specified.

The <tt>HOST_FILLORDER</tt> and <tt>HOST_BIGENDIAN</tt>

definitions are not currently used, but may be employed by

codecs for optimization purposes.

The following defines control general portability:

<tt>BSDTYPES</tt>

Define this if your system does NOT define the

		usual BSD typedefs: <tt>u_char</tt>,

		<tt>u_short</tt>, <tt>u_int</tt>, <tt>u_long</tt>.

<tt>HAVE_IEEEFP</tt>

Define this as 0 or 1 according to the floating point

		format suported by the machine.  If your machine does

		not support IEEE floating point then you will need to

		add support to tif_machdep.c to convert between the

		native format and IEEE format.

<tt>HAVE_MMAP</tt>

Define this if there is mmap-style support for

mapping files into memory (used only to read data).

<tt>HOST_FILLORDER</tt>

Define the native CPU bit order: one of <tt>FILLORDER_MSB2LSB</tt>

 or <tt>FILLORDER_LSB2MSB</tt>

<tt>HOST_BIGENDIAN</tt>

Define the native CPU byte order: 1 if big-endian (Motorola)

 or 0 if little-endian (Intel); this may be used

 in codecs to optimize code

On UNIX systems <tt>HAVE_MMAP</tt> is defined through the running of

the <tt>configure</tt> script; otherwise support for memory-mapped

files is disabled.

Note that tiffcomp.h defines <tt>HAVE_IEEEFP</tt> to be

1 (<tt>BSDTYPES</tt> is not defined).

Types and Portability

The software makes extensive use of C typedefs to promote portability.

Two sets of typedefs are used, one for communication with clients

of the library and one for internal data structures and parsing of the

TIFF format.  There are interactions between these two to be careful

of, but for the most part you should be able to deal with portability

purely by fiddling with the following machine-dependent typedefs:

uint8

8-bit unsigned integer

tiff.h

int8

8-bit signed integer

tiff.h

uint16

16-bit unsigned integer

tiff.h

int16

16-bit signed integer

tiff.h

uint32

32-bit unsigned integer

tiff.h

int32

32-bit signed integer

tiff.h

dblparam_t

promoted type for floats

tiffcomp.h

(to clarify <tt>dblparam_t</tt>, it is the type that float parameters are

promoted to when passed by value in a function call.)

The following typedefs are used throughout the library and interfaces

to refer to certain objects whose size is dependent on the TIFF image

structure:

typedef unsigned int ttag_t;	directory tag

typedef uint16 tdir_t;		directory index

typedef uint16 tsample_t;	sample number

typedef uint32 tstrip_t;	strip number

typedef uint32 ttile_t;		tile number

typedef int32 tsize_t;		i/o size in bytes

typedef void* tdata_t;		image data ref

typedef void* thandle_t;	client data handle

typedef int32 toff_t;		file offset (should be off_t)

typedef unsigned char* tidata_t; internal image data

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.

General Comments

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).

Adding New Builtin Codecs

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 Adding New Tags), 

or do the following to add support directly to the core library:

Define the tag value in tiff.h.

Edit the file tif_codec.c to add an entry to the

   _TIFFBuiltinCODECS array (see how other algorithms are handled).

Add the appropriate function prototype declaration to

   tiffiop.h (close to the bottom).

Create a file with the compression scheme code, by convention files

   are named tif_*.c (except perhaps on some systems where the

   tif_ prefix pushes some filenames over 14 chars.

Edit Makefile.in (and any other Makefiles)

   to include the new source file.

A codec, say <tt>foo</tt>, can have many different entry points:

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 */

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.

Other Comments

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 tif_predict.c.

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.

In general, if you have a new compression algorithm to add, work from

the code for an existing routine.  In particular,

tif_dumpmode.c

has the trivial code for the "nil" compression scheme,

tif_packbits.c is a

simple byte-oriented scheme that has to watch out for buffer

boundaries, and tif_lzw.c 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.

Last updated: $Date: 2004/09/10 14:47:31 $