Using The TIFF Library
++ libtiff is a set of C functions (a library) that support + the manipulation of TIFF image files. + The library requires an ANSI C compilation environment for building + and presumes an ANSI C environment for use. +
+ |
+
+ Using The TIFF Library++ libtiff is a set of C functions (a library) that support + the manipulation of TIFF image files. + The library requires an ANSI C compilation environment for building + and presumes an ANSI C environment for use. + + |
+
+ libtiff + provides interfaces to image data at several layers of abstraction (and cost). + At the highest level image data can be read into an 8-bit/sample, + ABGR pixel raster format without regard for the underlying data organization, + colorspace, or compression scheme. Below this high-level interface + the library provides scanline-, strip-, and tile-oriented interfaces that + return data decompressed but otherwise untransformed. These interfaces + require that the application first identify the organization of stored + data and select either a strip-based or tile-based API for manipulating + data. At the lowest level the library + provides access to the raw uncompressed strips or tiles, + returning the data exactly as it appears in the file. +
++ The material presented in this chapter is a basic introduction + to the capabilities of the library; it is not an attempt to describe + everything a developer needs to know about the library or about TIFF. + Detailed information on the interfaces to the library are given in + the UNIX + manual pages that accompany this software. +
++ Michael Still has also written a useful introduction to libtiff for the + IBM DeveloperWorks site available at + http://www.ibm.com/developerworks/linux/library/l-libtiff. +
++ The following sections are found in this chapter: +
++ The software version can be found by looking at the file named + VERSION + that is located at the top of the source tree; the precise alpha number + is given in the file dist/tiff.alpha. + If you have need to refer to this + specific software, you should identify it as: +
++ TIFF <version> <alpha> +
++ where <version> is whatever you get from + "cat VERSION" and <alpha> is + what you get from "cat dist/tiff.alpha". +
++ Within an application that uses libtiff the TIFFGetVersion + routine will return a pointer to a string that contains software version + information. + The library include file <tiffio.h> contains a C pre-processor + define TIFFLIB_VERSION that can be used to check library + version compatiblity at compile time. +
++ libtiff defines a portable programming interface through the + use of a set of C type definitions. + These definitions, defined in in the files tiff.h and + tiffio.h, + isolate the libtiff API from the characteristics + of the underlying machine. + To insure portable code and correct operation, applications that use + libtiff should use the typedefs and follow the function + prototypes for the library API. +
++ libtiff uses a machine-specific set of routines for managing + dynamically allocated memory. + _TIFFmalloc, _TIFFrealloc, and _TIFFfree + mimic the normal ANSI C routines. + Any dynamically allocated memory that is to be passed into the library + should be allocated using these interfaces in order to insure pointer + compatibility on machines with a segmented architecture. + (On 32-bit UNIX systems these routines just call the normal malloc, + realloc, and free routines in the C library.) +
++ To deal with segmented pointer issues libtiff also provides + _TIFFmemcpy, _TIFFmemset, and _TIFFmemmove + routines that mimic the equivalent ANSI C routines, but that are + intended for use with memory allocated through _TIFFmalloc + and _TIFFrealloc. +
++ libtiff handles most errors by returning an invalid/erroneous + value when returning from a function call. + Various diagnostic messages may also be generated by the library. + All error messages are directed to a single global error handler + routine that can be specified with a call to TIFFSetErrorHandler. + Likewise warning messages are directed to a single handler routine + that can be specified with a call to TIFFSetWarningHandler +
++ The library is modeled after the normal UNIX stdio library. + For example, to read from an existing TIFF image the + file must first be opened: +
+
+ #include "tiffio.h"
+ main()
+ {
+ TIFF* tif = TIFFOpen("foo.tif", "r");
+ ... do stuff ...
+ TIFFClose(tif);
+ }
+
+ The handle returned by TIFFOpen is opaque, that is + the application is not permitted to know about its contents. + All subsequent library calls for this file must pass the handle + as an argument. +
++ To create or overwrite a TIFF image the file is also opened, but with + a "w" argument: +
+
+ #include "tiffio.h"
+ main()
+ {
+ TIFF* tif = TIFFOpen("foo.tif", "w");
+ ... do stuff ...
+ TIFFClose(tif);
+ }
+
+ If the file already exists it is first truncated to zero length. +
+ |
+ Note that unlike the stdio library TIFF image files may not be + opened for both reading and writing; + there is no support for altering the contents of a TIFF file. | +
+ libtiff buffers much information associated with writing a + valid TIFF image. Consequently, when writing a TIFF image it is necessary + to always call TIFFClose or TIFFFlush to flush any + buffered information to a file. Note that if you call TIFFClose + you do not need to call TIFFFlush. +
++ TIFF supports the storage of multiple images in a single file. + Each image has an associated data structure termed a directory + that houses all the information about the format and content of the + image data. + Images in a file are usually related but they do not need to be; it + is perfectly alright to store a color image together with a black and + white image. + Note however that while images may be related their directories are + not. + That is, each directory stands on its own; their is no need to read + an unrelated directory in order to properly interpret the contents + of an image. +
++ libtiff provides several routines for reading and writing + directories. In normal use there is no need to explicitly + read or write a directory: the library automatically reads the first + directory in a file when opened for reading, and directory information + to be written is automatically accumulated and written when writing + (assuming TIFFClose or TIFFFlush are called). +
++ For a file open for reading the TIFFSetDirectory routine can + be used to select an arbitrary directory; directories are referenced by + number with the numbering starting at 0. Otherwise the + TIFFReadDirectory and TIFFWriteDirectory routines can + be used for sequential access to directories. + For example, to count the number of directories in a file the following + code might be used: +
+
+ #include "tiffio.h"
+ main(int argc, char* argv[])
+ {
+ TIFF* tif = TIFFOpen(argv[1], "r");
+ if (tif) {
+ int dircount = 0;
+ do {
+ dircount++;
+ } while (TIFFReadDirectory(tif));
+ printf("%d directories in %s\n", dircount, argv[1]);
+ TIFFClose(tif);
+ }
+ exit(0);
+ }
+
+ Finally, note that there are several routines for querying the + directory status of an open file: + TIFFCurrentDirectory returns the index of the current + directory and + TIFFLastDirectory returns an indication of whether the + current directory is the last directory in a file. + There is also a routine, TIFFPrintDirectory, that can + be called to print a formatted description of the contents of + the current directory; consult the manual page for complete details. +
++ Image-related information such as the image width and height, number + of samples, orientation, colorimetric information, etc. + are stored in each image + directory in fields or tags. + Tags are identified by a number that is usually a value registered + with the Aldus (now Adobe) Corporation. + Beware however that some vendors write + TIFF images with tags that are unregistered; in this case interpreting + their contents is usually a waste of time. +
++ libtiff reads the contents of a directory all at once + and converts the on-disk information to an appropriate in-memory + form. While the TIFF specification permits an arbitrary set of + tags to be defined and used in a file, the library only understands + a limited set of tags. + Any unknown tags that are encountered in a file are ignored. + There is a mechanism to extend the set of tags the library handles + without modifying the library itself; + this is described elsewhere. +
++ libtiff provides two interfaces for getting and setting tag + values: TIFFGetField and TIFFSetField. + These routines use a variable argument list-style interface to pass + parameters of different type through a single function interface. + The get interface takes one or more pointers to memory locations + where the tag values are to be returned and also returns one or + zero according to whether the requested tag is defined in the directory. + The set interface takes the tag values either by-reference or + by-value. + The TIFF specification defines + default values for some tags. + To get the value of a tag, or its default value if it is undefined, + the TIFFGetFieldDefaulted interface may be used. +
++ The manual pages for the tag get and set routines specifiy the exact data types + and calling conventions required for each tag supported by the library. +
++ libtiff includes support for a wide variety of + data compression schemes. + In normal operation a compression scheme is automatically used when + the TIFF Compression tag is set, either by opening a file + for reading, or by setting the tag when writing. +
++ Compression schemes are implemented by software modules termed codecs + that implement decoder and encoder routines that hook into the + core library i/o support. + Codecs other than those bundled with the library can be registered + for use with the TIFFRegisterCODEC routine. + This interface can also be used to override the core-library + implementation for a compression scheme. +
++ The TIFF specification says, and has always said, that + a correct TIFF + reader must handle images in big-endian and little-endian byte order. + libtiff conforms in this respect. + Consequently there is no means to force a specific + byte order for the data written to a TIFF image file (data is + written in the native order of the host CPU unless appending to + an existing file, in which case it is written in the byte order + specified in the file). +
++ The TIFF specification requires that all information except an + 8-byte header can be placed anywhere in a file. + In particular, it is perfectly legitimate for directory information + to be written after the image data itself. + Consequently TIFF is inherently not suitable for passing through a + stream-oriented mechanism such as UNIX pipes. + Software that require that data be organized in a file in a particular + order (e.g. directory information before image data) does not + correctly support TIFF. + libtiff provides no mechanism for controlling the placement + of data in a file; image data is typically written before directory + information. +
++ libtiff provides a high-level interface for reading image + data from a TIFF file. This interface handles the details of + data organization and format for a wide variety of TIFF files; + at least the large majority of those files that one would normally + encounter. Image data is, by default, returned as ABGR + pixels packed into 32-bit words (8 bits per sample). Rectangular + rasters can be read or data can be intercepted at an intermediate + level and packed into memory in a format more suitable to the + application. + The library handles all the details of the format of data stored on + disk and, in most cases, if any colorspace conversions are required: + bilevel to RGB, greyscale to RGB, CMYK to RGB, YCbCr to RGB, 16-bit + samples to 8-bit samples, associated/unassociated alpha, etc. +
++ There are two ways to read image data using this interface. If + all the data is to be stored in memory and manipulated at once, + then the routine TIFFReadRGBAImage can be used: +
++
+ #include "tiffio.h"
+ main(int argc, char* argv[])
+ {
+ TIFF* tif = TIFFOpen(argv[1], "r");
+ if (tif) {
+ uint32 w, h;
+ size_t npixels;
+ uint32* raster;
+
+ TIFFGetField(tif, TIFFTAG_IMAGEWIDTH, &w);
+ TIFFGetField(tif, TIFFTAG_IMAGELENGTH, &h);
+ npixels = w * h;
+ raster = (uint32*) _TIFFmalloc(npixels * sizeof (uint32));
+ if (raster != NULL) {
+ if (TIFFReadRGBAImage(tif, w, h, raster, 0)) {
+ ...process raster data...
+ }
+ _TIFFfree(raster);
+ }
+ TIFFClose(tif);
+ }
+ exit(0);
+ }
+
+ Note above that _TIFFmalloc is used to allocate memory for + the raster passed to TIFFReadRGBAImage; this is important + to insure the ``appropriate type of memory'' is passed on machines + with segmented architectures. +
++ Alternatively, TIFFReadRGBAImage can be replaced with a + more low-level interface that permits an application to have more + control over this reading procedure. The equivalent to the above + is: +
+
+ #include "tiffio.h"
+ main(int argc, char* argv[])
+ {
+ TIFF* tif = TIFFOpen(argv[1], "r");
+ if (tif) {
+ TIFFRGBAImage img;
+ char emsg[1024];
+
+ if (TIFFRGBAImageBegin(&img, tif, 0, emsg)) {
+ size_t npixels;
+ uint32* raster;
+
+ npixels = img.width * img.height;
+ raster = (uint32*) _TIFFmalloc(npixels * sizeof (uint32));
+ if (raster != NULL) {
+ if (TIFFRGBAImageGet(&img, raster, img.width, img.height)) {
+ ...process raster data...
+ }
+ _TIFFfree(raster);
+ }
+ TIFFRGBAImageEnd(&img);
+ } else
+ TIFFError(argv[1], emsg);
+ TIFFClose(tif);
+ }
+ exit(0);
+ }
+
+ However this usage does not take advantage of the more fine-grained + control that's possible. That is, by using this interface it is + possible to: +
++ The first item means that, for example, image viewers that want to + handle multiple files can cache decoding information in order to + speedup the work required to display a TIFF image. +
++ The second item is the main reason for this interface. By interposing + a "put method" (the routine that is called to pack pixel data in + the raster) it is possible share the core logic that understands how + to deal with TIFF while packing the resultant pixels in a format that + is optimized for the application. This alternate format might be very + different than the 8-bit per sample ABGR format the library writes by + default. For example, if the application is going to display the image + on an 8-bit colormap display the put routine might take the data and + convert it on-the-fly to the best colormap indices for display. +
++ The last item permits an application to extend the library + without modifying the core code. + By overriding the code provided an application might add support + for some esoteric flavor of TIFF that it needs, or it might + substitute a packing routine that is able to do optimizations + using application/environment-specific information. +
++ The TIFF image viewer found in tools/sgigt.c is an example + of an application that makes use of the TIFFRGBAImage + support. +
++ The simplest interface provided by libtiff is a + scanline-oriented interface that can be used to read TIFF + images that have their image data organized in strips + (trying to use this interface to read data written in tiles + will produce errors.) + A scanline is a one pixel high row of image data whose width + is the width of the image. + Data is returned packed if the image data is stored with samples + packed together, or as arrays of separate samples if the data + is stored with samples separated. + The major limitation of the scanline-oriented interface, other + than the need to first identify an existing file as having a + suitable organization, is that random access to individual + scanlines can only be provided when data is not stored in a + compressed format, or when the number of rows in a strip + of image data is set to one (RowsPerStrip is one). +
++ Two routines are provided for scanline-based i/o: + TIFFReadScanline + and + TIFFWriteScanline. + For example, to read the contents of a file that + is assumed to be organized in strips, the following might be used: +
+
+ #include "tiffio.h"
+ main()
+ {
+ TIFF* tif = TIFFOpen("myfile.tif", "r");
+ if (tif) {
+ uint32 imagelength;
+ tdata_t buf;
+ uint32 row;
+
+ TIFFGetField(tif, TIFFTAG_IMAGELENGTH, &imagelength);
+ buf = _TIFFmalloc(TIFFScanlineSize(tif));
+ for (row = 0; row < imagelength; row++)
+ tiffreadscanline(tif, buf, row);
+ _tifffree(buf);
+ tiffclose(tif);
+ }
+ }
+
+ TIFFScanlineSize returns the number of bytes in + a decoded scanline, as returned by TIFFReadScanline. + Note however that if the file had been create with samples + written in separate planes, then the above code would only + read data that contained the first sample of each pixel; + to handle either case one might use the following instead: +
+
+ #include "tiffio.h"
+ main()
+ {
+ TIFF* tif = TIFFOpen("myfile.tif", "r");
+ if (tif) {
+ uint32 imagelength;
+ tdata_t buf;
+ uint32 row;
+
+ TIFFGetField(tif, TIFFTAG_IMAGELENGTH, &imagelength);
+ TIFFGetField(tif, TIFFTAG_PLANARCONFIG, &config);
+ buf = _TIFFmalloc(TIFFScanlineSize(tif));
+ if (config == PLANARCONFIG_CONTIG) {
+ for (row = 0; row < imagelength; row++)
+ tiffreadscanline(tif, buf, row);
+ } else if (config == planarconfig_separate) {
+ uint16 s, nsamples;
+
+ tiffgetfield(tif, tifftag_samplesperpixel, &nsamples);
+ for (s = 0; s < nsamples; s++)
+ for (row = 0; row < imagelength; row++)
+ tiffreadscanline(tif, buf, row, s);
+ }
+ _tifffree(buf);
+ tiffclose(tif);
+ }
+ }
+
+ Beware however that if the following code were used instead to + read data in the case PLANARCONFIG_SEPARATE,... +
+
+ for (row = 0; row < imagelength; row++)
+ for (s = 0; s < nsamples; s++)
+ tiffreadscanline(tif, buf, row, s);
+
+ ...then problems would arise if RowsPerStrip was not one + because the order in which scanlines are requested would require + random access to data within strips (something that is not supported + by the library when strips are compressed). +
++ The strip-oriented interfaces provided by the library provide + access to entire strips of data. Unlike the scanline-oriented + calls, data can be read or written compressed or uncompressed. + Accessing data at a strip (or tile) level is often desirable + because there are no complications with regard to random access + to data within strips. +
++ A simple example of reading an image by strips is: +
+
+ #include "tiffio.h"
+ main()
+ {
+ TIFF* tif = TIFFOpen("myfile.tif", "r");
+ if (tif) {
+ tdata_t buf;
+ tstrip_t strip;
+
+ buf = _TIFFmalloc(TIFFStripSize(tif));
+ for (strip = 0; strip < tiffnumberofstrips(tif); strip++)
+ tiffreadencodedstrip(tif, strip, buf, (tsize_t) -1);
+ _tifffree(buf);
+ tiffclose(tif);
+ }
+ }
+
+ Notice how a strip size of -1 is used; TIFFReadEncodedStrip + will calculate the appropriate size in this case. +
++ The above code reads strips in the order in which the + data is physically stored in the file. If multiple samples + are present and data is stored with PLANARCONFIG_SEPARATE + then all the strips of data holding the first sample will be + read, followed by strips for the second sample, etc. +
++ Finally, note that the last strip of data in an image may have fewer + rows in it than specified by the RowsPerStrip tag. A + reader should not assume that each decoded strip contains a full + set of rows in it. +
++ The following is an example of how to read raw strips of data from + a file: +
+
+ #include "tiffio.h"
+ main()
+ {
+ TIFF* tif = TIFFOpen("myfile.tif", "r");
+ if (tif) {
+ tdata_t buf;
+ tstrip_t strip;
+ uint32* bc;
+ uint32 stripsize;
+
+ TIFFGetField(tif, TIFFTAG_STRIPBYTECOUNTS, &bc);
+ stripsize = bc[0];
+ buf = _TIFFmalloc(stripsize);
+ for (strip = 0; strip < tiffnumberofstrips(tif); strip++) {
+ if (bc[strip] > stripsize) {
+ buf = _TIFFrealloc(buf, bc[strip]);
+ stripsize = bc[strip];
+ }
+ TIFFReadRawStrip(tif, strip, buf, bc[strip]);
+ }
+ _TIFFfree(buf);
+ TIFFClose(tif);
+ }
+ }
+
+ As above the strips are read in the order in which they are + physically stored in the file; this may be different from the + logical ordering expected by an application. +
++ Tiles of data may be read and written in a manner similar to strips. + With this interface, an image is + broken up into a set of rectangular areas that may have dimensions + less than the image width and height. All the tiles + in an image have the same size, and the tile width and length must each + be a multiple of 16 pixels. Tiles are ordered left-to-right and + top-to-bottom in an image. As for scanlines, samples can be packed + contiguously or separately. When separated, all the tiles for a sample + are colocated in the file. That is, all the tiles for sample 0 appear + before the tiles for sample 1, etc. +
++ Tiles and strips may also be extended in a z dimension to form + volumes. Data volumes are organized as "slices". That is, all the + data for a slice is colocated. Volumes whose data is organized in + tiles can also have a tile depth so that data can be organized in + cubes. +
++ There are actually two interfaces for tiles. + One interface is similar to scanlines, to read a tiled image, + code of the following sort might be used: +
+
+ main()
+ {
+ TIFF* tif = TIFFOpen("myfile.tif", "r");
+ if (tif) {
+ uint32 imageWidth, imageLength;
+ uint32 tileWidth, tileLength;
+ uint32 x, y;
+ tdata_t buf;
+
+ TIFFGetField(tif, TIFFTAG_IMAGEWIDTH, &imageWidth);
+ TIFFGetField(tif, TIFFTAG_IMAGELENGTH, &imageLength);
+ TIFFGetField(tif, TIFFTAG_TILEWIDTH, &tileWidth);
+ TIFFGetField(tif, TIFFTAG_TILELENGTH, &tileLength);
+ buf = _TIFFmalloc(TIFFTileSize(tif));
+ for (y = 0; y < imagelength; y += tilelength)
+ for (x = 0; x < imagewidth; x += tilewidth)
+ tiffreadtile(tif, buf, x, y, 0);
+ _tifffree(buf);
+ tiffclose(tif);
+ }
+ }
+
+ (once again, we assume samples are packed contiguously.) +
++ Alternatively a direct interface to the low-level data is provided + a la strips. Tiles can be read with + TIFFReadEncodedTile or TIFFReadRawTile, + and written with TIFFWriteEncodedTile or + TIFFWriteRawTile. For example, to read all the tiles in an image: +
+
+ #include "tiffio.h"
+ main()
+ {
+ TIFF* tif = TIFFOpen("myfile.tif", "r");
+ if (tif) {
+ tdata_t buf;
+ ttile_t tile;
+
+ buf = _TIFFmalloc(TIFFTileSize(tif));
+ for (tile = 0; tile < tiffnumberoftiles(tif); tile++)
+ tiffreadencodedtile(tif, tile, buf, (tsize_t) -1);
+ _tifffree(buf);
+ tiffclose(tif);
+ }
+ }
+
+ Some other stuff will almost certainly go here... +
++ Last updated: $Date: 2005/12/28 06:53:18 $ +
+ +