Using The TIFF Library
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:
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.
Library Datatypes
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.
Memory Management
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.
Error Handling
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
Basic File Handling
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:
To create or overwrite a TIFF image the file is also opened, but with a "w" argument:
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 Directories
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:
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.
TIFF Tags
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.
TIFF Compression Schemes
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.
Byte Order
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).
Data Placement
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.
TIFFRGBAImage Support
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:
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:
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.
Scanline-based Image I/O
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:
A simple example of reading an image by strips is:
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:
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:
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:
Some other stuff will almost certainly go here...