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Filter selection, palette index checks

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Palette index checking: checking on read was erroneously skipped.  pngcp can now
turn off the palette index checking or fix the errors (mainly to allow
comparison with libpng 1.6, which defaulted to ignoring it).  The API now
documents how the 'enabled' parameter works.  On read the check is on by default
(it was apparently off in libpng 1.6) however now if explicitly turned on the
warning message is stopped, this provides better interaction with
get_palette_max at the cost of a small API change (may need to be reviewed.)

Palette size errors: invalid entries in the palette are now set to #beaded to
make the errors more obvious and allow easy detection in memory.

Read transform pipeline caching: changes to fix the palette index check (which
are erroneously cached), the pipeline can now contain multiple caches.  E.g.
caching of packing of palette indices can be combined with caching of palette
RGB transforms.

Read code now checks for callbacks to read 0 data (and faults them).  Fixed the
reading of PNGs with multiple 0 length IDATs that failed as a result plus the
handling of zero length unknown.  (Which occurs, validly; the spurious warning
has been removed).

filter selection: the 1.6 and earlier sum of absolute differences algorithm has
been reintroduced with an option to disfavor some filters over others where the
sums are close (not yet exposed).  The selection code also logs the last known
occurence of each possible byte code across multiple lines.  This allows
detection of PNG images with lower bit depth than the format implies and,
therefore, allows the filtering to be turned off in those cases as well.

The default write zlib settings are still lower than libpng16.  The selection
algorithm is being refined (the overall results are worse than not doing any
filtering).

Signed-off-by: John Bowler <jbowler@acm.org>
This commit is contained in:
John Bowler
2016-05-15 16:32:01 -07:00
parent cf46e3748f
commit 5652acdd48
13 changed files with 298 additions and 98 deletions
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16
pngstruct.h
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@@ -393,13 +393,15 @@ struct png_struct_def
unsigned int num_trans :9; /* number of transparency values */
unsigned int transparent_palette :1; /* if they are all 0 or 255 */
#endif /* READ_tRNS */
#ifdef PNG_GET_PALETTE_MAX_SUPPORTED
unsigned int palette_index_max :9; /* maximum palette index found in IDAT */
#endif /* GET_PALETTE_MAX */
#ifdef PNG_CHECK_FOR_INVALID_INDEX_SUPPORTED
unsigned int palette_index_check_disabled :1; /* defaults to 0, 'enabled' */
unsigned int palette_index_check_issued :1; /* error message output */
#endif /* CHECK_FOR_INVALID_INDEX */
#ifdef PNG_PALETTE_MAX_SUPPORTED
unsigned int palette_index_max :8; /* maximum palette index in IDAT */
unsigned int palette_index_check :2; /* one of the following: */
# define PNG_PALETTE_CHECK_DEFAULT 0U
# define PNG_PALETTE_CHECK_OFF 1U
# define PNG_PALETTE_CHECK_ON 2U
unsigned int palette_index_have_max :1; /* max is being set */
unsigned int palette_index_check_issued :1; /* error message output */
#endif /* PALETTE_MAX */
#ifdef PNG_READ_tRNS_SUPPORTED
png_color_16 trans_color; /* transparent color for non-paletted files */
#endif /* READ_tRNS */