GM TIFF updates

I have spent some time re-implementing how GraphicsMagick's TIFF coder
reads and writes colormapped and grayscale TIFF files.  The previous
implementation used hand-coded/tuned cases for every depth and
condition.  While this approach is as fast as possible, it requires
that every case be specifically supported.

The modified TIFF coder uses some new implementation functions that
support reading and writing generic "bit streams".  The functions
provide for writing or reading a specified number of bits (1-32)
to/from a byte array.  A result of using this simple interface is that
the supportive code is now very concise and compact since the same
code handles every possible bit-depth as well as a transparency
channel.  The generic approach is a bit slower, but it can be expected
to succeed for cases that have never been tested (e.g. 10-bit
grayscale TIFF) and since there is only one code path to test, it
doesn't require as much validation testing.

The new implementation eliminates a number of bugs (mostly having to
do with transparency) and enables writing more compact TIFF files
since low-color or shallow-gray images can be encoded using fewer
bits.

The currently supported bits-per-sample values when writing are 1,4,8,
and 16.  A mechanism may be provided in the future to override the
default logic so that other values may be used.  We believe that
bits-per-sample values ranging from 1-16 (even odd values!) should be
supported for read.

Bob
======================================
Bob Friesenhahn
[email protected]
http://www.simplesystems.org/users/bfriesen



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