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:
Configuration defines are split into three areas:
If the define COMPRESSION_SUPPORT 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 Lempel-Ziv & Welch (LZW) algorithm (compression 5), a 4-bit run-length encoding scheme from ThunderScan (compression 32809), and a 2-bit encoding scheme used by NeXT (compression 32766). To override this behaviour define COMPRESSION_SUPPORT 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_SUPPORTSeveral other compression schemes are configured separately from the default set because they depend on ancillary software packages that are not distributed with libtiff.
Support for JPEG compression is controlled by JPEG_SUPPORT. The JPEG codec that comes with libtiff 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 DEFLATE_SUPPORT. The deflate codec that comes with libtiff is designed for use with version 0.92 or later of the freely available libz 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 COLORIMETRY_SUPPORT, YCBCR_SUPPORT, and CMYK_SUPPORT.
|CCITT_SUPPORT||CCITT Group 3 and 4 algorithms (compression codes 2, 3, 4, and 32771)|
|PACKBITS_SUPPORT||Macintosh PackBits algorithm (compression 32773)|
|LZW_SUPPORT||Lempel-Ziv & Welch (LZW) algorithm (compression 5)|
|THUNDER_SUPPORT||4-bit run-length encoding scheme from ThunderScan (compression 32809)|
|NEXT_SUPPORT||2-bit encoding scheme used by NeXT (compression 32766)|
|OJPEG_SUPPORT||obsolete JPEG scheme defined in the 6.0 spec (compression 6)|
|JPEG_SUPPORT||current JPEG scheme defined in TTN2 (compression 7)|
|ZIP_SUPPORT||experimental Deflate scheme (compression 32946)|
|COLORIMETRY_SUPPORT||support for the TIFF 6.0 colorimetry tags|
|YCBCR_SUPPORT||support for the TIFF 6.0 YCbCr-related tags|
|CMYK_SUPPORT||support for the TIFF 6.0 CMYK-related tags|
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, const, and <stdarg.h> then you will have to make modifications to the software. In the past I have tried to support compilers without const and systems without <stdarg.h>, 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. open, read, write, fstat, malloc, free).
Native CPU byte order is determined on the fly by the library and does not need to be specified. The HOST_FILLORDER and HOST_BIGENDIAN definitions are not currently used, but may be employed by codecs for optimization purposes.
The following defines control general portability:
|BSDTYPES||Define this if your system does NOT define the usual BSD typedefs: u_char, u_short, u_int, u_long.|
|HAVE_IEEEFP||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.|
|HAVE_MMAP||Define this if there is support for memory mapping files.|
|HOST_FILLORDER||Define the native CPU bit order: one of FILLORDER_MSB2LSB or FILLORDER_LSB2MSB|
|HOST_BIGENDIAN||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 HAVE_MMAP is defined through the running of
the configure script; otherwise support for memory-mapped
files is disabled.
Note that tiffcomp.h defines HAVE_IEEEFP to be
1 (BSDTYPES 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 dblparam_t, 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 tstrip_t, ttile_t, and tsize_t are constrained to be no more than 32-bit quantities by 32-bit fields they are stored in in the TIFF image. Likewise tsample_t is limited by the 16-bit field used to store the SamplesPerPixel tag. tdir_t constrains the maximum number of IFDs that may appear in an image and may be an arbitrary size (without penalty). ttag_t must be either int, unsigned int, pointer, or double because the library uses a varargs interface and ANSI C restricts the type of the parameter before an ellipsis to be a promoted type. toff_t is defined as int32 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 tidata_t 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.
If you want to maintain portability, beware of making assumptions
about data types. Use the typedefs (uint16, etc. when dealing with
data on disk and t*_t when stuff is in memory) and be careful about
passing items through printf or similar vararg interfaces.
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, above),
or do the following to add support directly to the core library:
A codec, say foo, 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 PlanarConfig=2
(separate planes). This is important for algorithms such as JPEG.
If PlanarConfig=1 (interleaved), then sample will always be 0.
Adding New Codec-private Tags
To add tags that are meaningful only when a particular compression
algorithm is used follow these steps:
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 */(Actually you may decide not to override the tif_printdir method, but rather just specify it).
_TIFFMergeFieldInfo(tif, fooFieldInfo, N(fooFieldInfo));(where N is a macro used liberaly throughout the distributed code).
The variables tif_rawcc, tif_rawdata, and tif_rawcp in a TIFF structure are associated with the raw data buffer. tif_rawcc must be non-zero for the library to automatically flush data. The variable tif_scanlinesize is the size a user's scanline buffer should be. The variable tif_tilesize 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 cc parameter to the tif_decode* and tif_encode* 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.