$Header: /usr/people/sam/tiff/libtiff/RCS/README,v 1.17 93/08/26 14:58:20 sam Exp $

Configuration Comments:
----------------------
Aside from the compression algorithm support, there are
configuration-related defines that you can override in the Makefile
or in the default configuration file tiffconf.h:

COLORIMETRY_SUPPORT
		if this is defined, support for the colorimetry
		tags will be compiled in.
JPEG_SUPPORT	if this is defined, support for the JPEG-related
		tags will be compiled in.  Note that at the present
		time the JPEG compression support is not included.
YCBCR_SUPPORT	if this is defined, support for the YCbCr-related
		tags will be compiled in.  Note that you'll want
		YCBCR support for JPEG compression+decompression.
CMYK_SUPPORT	if this is defined, support for the CMYK-related
		tags will be compiled in.
MMAP_SUPPORT	if this is set, and OS support exists for memory
		mapping files, then the library will try to map
		a file if it is opened for reading.  If mmap does
		not exist on your system, or the mmap call fails
		on the file, then the normal read system calls are
		used.  It is not clear how useful this facility is.

By default tiffconf.h defines COLORIMETRY_SUPPORT, JPEG_SUPPORT,
YCBCR_SUPPORT, CMYK_SUPPORT.  MMAP_SUPPORT is not defined.

General Portability Comments:
----------------------------
I run this code on SGI machines (big-endian, MIPS CPU, 32-bit ints,
IEEE floating point).  Makefiles exist for other platforms that the
code runs on -- this work has mostly been done by other people.
The code runs on Macintosh and PC-based systems, although I don't
know all the particulars.

In general, I promise only that the code runs on SGI machines.
I will, however, gladly take back fixes to make it work on other
systems -- when the changes are reasonable unobtrusive.

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 w/o const
and systems w/o <stdarg.h>, but I am NO LONGER INTERESTED in these
antiquated environments.  With the general availability of gcc, I
see no reason to incorporate modifications to the software for these
purposes.

I've tried to isolate as many of the OS-dependencies as possible in
two files: tiffcomp.h and tif_<os>.c.  The latter file contains
OS-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).

Machine dependencies such as byte order are determined on the
fly and do not need to be specified.

Two general portability-related defines are:

BSDTYPES	Define this if your system does NOT define the
		usual BSD typedefs: u_char, u_short, u_int, u_longs.
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.

Note that tiffcomp.h defines HAVE_IEEEFP to be 1 (BSDTYPES is not defined).

Types and Portability:
---------------------
The software makes extensive use of 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:

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 (w/o penalty).  ttag_t must be either int,
unsigned int, pointer, or double because the library uses a varargs
interface and ANSI 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.

General Comments:
----------------
The library is designed to hide as much of the details of TIFF 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
doesn't care about (e.g. transfer function tables).

To add support for a new directory tag the following mods are needed:

1. Define the tag in tiff.h.
2. Add a field to the directory structure in tiffiop.h and define a
   FIELD_* bit.
3. Add an entry in the FieldInfo array defined at the top of tiff_dirinfo.c.
4. Add entries in TIFFSetField1() and TIFFGetField1() for the new tag.
5. (optional) If the value associated with the tag is not a scalar value
   (e.g. the array for TransferFunction), then add the appropriate
   code to TIFFReadDirectory() and TIFFWriteDirectory().  You're best
   off finding a similar tag and cribbing code.
6. Add support to TIFFPrintDirectory() in tiff_print.c to print the
   tag's value.

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.

To add support for a compression algorithm:

1. Define the tag value in tiff.h.
2. Edit the file tiff_compress.c to add an entry to the
   CompressionSchemes[] array.
3. 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.
4. Edit the Makefiles to include the new source file.

A compression scheme, say foo, can have up to 10 entry points:

TIFFfoo(tif)		/* initialize scheme and setup entry points in tif */
fooPreDecode(tif)	/* called once per strip, after data is read,
			   but before the first row in a strip 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 */
fooPreEncode(tif)	/* called once per strip/tile, before the first row in
			   a strip 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.

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's 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 TIFFFlushData1() 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 tif_postdecode
routine (set it to TIFFNoPostDecode) and doing the required work
internally.  For an example of doing this look at the horizontal
differencing code in the LZW decoding routines.

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, tiff_dumpmode.c has
the trivial code for the "nil" compression scheme, tiff_packbits.c is a
simple byte-oriented scheme that has to watch out for buffer
boundaries, and tiff_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.
