Using The TIFF Library
======================

.. image:: images/cat.gif
    :width: 113
    :alt: cat

``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 :doc:`functions` that accompany this software.
An alphabetic list of all the public functions with a brief description can be found at :ref:`list_of_routines`

.. warning::
    The following hyperlink does no more work, at least no libtiff introduction:

    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>`_.


How to tell which version you have
----------------------------------

The software version can be found by looking at the file named
:file:`VERSION`
that is located at the top of the source tree; the precise alpha number
is given in the file :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 :c:func:`TIFFGetVersion`
routine will return a pointer to a string that contains software version
information.
The library include file :file:`<tiffio.h>` contains a C pre-processor
define :c:macro:`TIFFLIB_VERSION` that can be used to check library
version compatibility 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 :file:`tiff.h` and
:file:`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.
:c:func:`_TIFFmalloc`, :c:func:`_TIFFrealloc`, and :c:func:`_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 :c:func:`malloc`,
:c:func:`realloc`, and :c:func:`free` routines in the C library.)

To deal with segmented pointer issues ``libtiff`` also provides
:c:func:`_TIFFmemcpy`, :c:func:`_TIFFmemset`, and :c:func:`_TIFFmemmove`
routines that mimic the equivalent ANSI C routines, but that are
intended for use with memory allocated through :c:func:`_TIFFmalloc`
and :c:func:`_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 :c:func:`TIFFSetErrorHandler`.
Likewise warning messages are directed to a single handler routine
that can be specified with a call to :c:func:`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:

.. highlight:: c

::

    #include "tiffio.h"
    main()
    {
        TIFF* tif = TIFFOpen("foo.tif", "r");
        /* ... do stuff ... */
        TIFFClose(tif);
    }

The handle returned by :c:func:`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.

.. warning::

    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 :c:func:`TIFFClose` or :c:func:`TIFFFlush` to flush any
buffered information to a file.  Note that if you call :c:func:`TIFFClose`
you do not need to call :c:func:`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; there 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 :c:func:`TIFFClose` or :c:func:`TIFFFlush` are called).

For a file open for reading the :c:func:`TIFFSetDirectory` routine can
be used to select an arbitrary directory; directories are referenced by
number with the numbering starting at 0.  Otherwise the
:c:func:`TIFFReadDirectory` and :c:func:`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:
:c:func:`TIFFCurrentDirectory` returns the index of the current
directory and
:c:func:`TIFFLastDirectory` returns an indication of whether the
current directory is the last directory in a file.
There is also a routine, :c:func:`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 in :doc:`addingtags`.

``libtiff`` provides two interfaces for getting and setting tag
values: :c:func:`TIFFGetField` and :c:func:`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 :c:func:`TIFFGetFieldDefaulted` interface may be used.

The manual pages for the tag get and set routines specify 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 :c:func:`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 :c:func:`TIFFReadRGBAImage` can be used:

::

    #include "tiffio.h"
    main(int argc, char* argv[])
    {
        TIFF* tif = TIFFOpen(argv[1], "r");
        if (tif) {
            uint32_t w, h;
            size_t npixels;
            uint32_t* raster;
            
            TIFFGetField(tif, TIFFTAG_IMAGEWIDTH, &w);
            TIFFGetField(tif, TIFFTAG_IMAGELENGTH, &h);
            npixels = w * h;
            raster = (uint32_t*) _TIFFmalloc(npixels * sizeof (uint32_t));
            if (raster != NULL) {
                if (TIFFReadRGBAImage(tif, w, h, raster, 0)) {
                    ...process raster data...
                }
                _TIFFfree(raster);
            }
            TIFFClose(tif);
        }
        exit(0);
    }

Note above that :c:func:`_TIFFmalloc` is used to allocate memory for
the raster passed to :c:func:`TIFFReadRGBAImage`; this is important
to insure the "appropriate type of memory" is passed on machines
with segmented architectures.

Alternatively, :c:func:`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_t* raster;
                
                npixels = img.width * img.height;
                raster = (uint32_t*) _TIFFmalloc(npixels * sizeof (uint32_t));
                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:

* repeatedly fetch (and manipulate) an image without opening
  and closing the file
* interpose a method for packing raster pixel data according to
  application-specific needs (or write the data at all)
* interpose methods that handle TIFF formats that are not already
  handled by the core library

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 :file:`tools/sgigt.c` is an example
of an application that makes use of the :c:func:`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:
:c:func:`TIFFReadScanline`
and
:c:func:`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_t imagelength;
            tdata_t buf;
            uint32_t row;
            
            TIFFGetField(tif, TIFFTAG_IMAGELENGTH, &imagelength);
            buf = _TIFFmalloc(TIFFScanlineSize(tif));
            for (row = 0; row < imagelength; row++)
                TIFFReadScanline(tif, buf, row, 0);
            _TIFFfree(buf);
            TIFFClose(tif);
        }
    }

:c:func:`TIFFScanlineSize` returns the number of bytes in
a decoded scanline, as returned by :c:func:`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_t imagelength;
            tdata_t buf;
            uint32_t 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, 0);
            } else if (config == planarconfig_separate) {
                uint16_t 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).

Strip-oriented Image I/O
------------------------

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; :c:func:`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_t* bc;
            uint32_t 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.

Tile-oriented Image I/O
-----------------------

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_t imageWidth, imageLength;
            uint32_t tileWidth, tileLength;
            uint32_t 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
à la strips.  Tiles can be read with
:c:func:`TIFFReadEncodedTile` or :c:func:`TIFFReadRawTile`,
and written with :c:func:`TIFFWriteEncodedTile` or
:c:func:`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);
        }
    }

Other Stuff
-----------

Some other stuff will almost certainly go here...
