links and tests against zlib with a prefix; tests have been clarified; and
irrelevant or obsolete things (as defined by the autotools man page) have
been removed.
png_malloc_default png_free_default.
Updated some left over "1.6.0beta32" in code sources.
Fixed a "png_structp" prototype (should be png_structrp) in arm_init.c
Updated the version-number hack in pngvalid.c
handling png_struct members rearranged - partly to reorder to avoid packing,
partly to put frequently accessed members at the start and partly to make
the grouping more clear. png_set_filter code has been rewritten and the
code shared with png_write_start_row moved to a common function. Comments
in png.h have been made more clear. Minor fixes to
contrib/libtests/timepng.c and some of the png_*_tRNS logic, including
more error detection in png_set_tRNS.
These changes cause 16-bit arithmetic to be used for 8-bit data in the gamma
corrected compose and grayscale operations. The arithmetic errors have
three sources all of which are fixed in this commit:
1) 8-bit linear calculations produce massive errors for lower intensity
values.
2) The old 16-bit "16 to 8" gamma table code erroneously wrote the lowest
output value into a table entry which corresponded to multiple output
values (so where the value written should have been the closest to the
transformed input value.)
3) In a number of cases the code to access the 16-bit table did not round;
it did a simple shift, which was wrong and made the side effects of (2)
even worse.
The new gamma code does not have the 16-to-8 problem at the cost of slighly
more calculations and the algorithm used to minimize the number of
calculations has been extended to all the 16-bit tables; it has advantages
for any significant gamma correction.
READ_GAMMA. Prior to 1.6.0 switching off READ_GAMMA did unpredictable things
to the interfaces that use it (specifically, png_do_background in 1.4 would
simply display composite for grayscale images but do composition
with the incorrect arithmetic for color ones). In 1.6 the semantic
of -DPNG_NO_READ_GAMMA is changed to simply disable any interface that
depends on it; this obliges people who set it to consider whether they
really want it off if they happen to use any of the interfaces in
question (typically most users who disable it won't).
management system (CMS). In practice it is not possible to obtain cHRM
information reliably using just the end-point tags because they do not exist
in the vast majority of profiles. Instead it is necessary to run the endpoints
through the ICC colorimetric intent transform (as described in the v4
spec). Since this is likely to be too much code inside libpng for too
little gain (it implies a fairly complete CMS implementation) the code
has been changed to allow an external CMS to be used. This code is
temporarily disabled until a suitable set of test cases using one or more
external CMS implementations have been implemented.
