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gamma handling fixes

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One serious bug; 8 was forced as a bit depth when caching a palette, even though
the palette have been expanded.  One less serious bug, the optimization for
avoiding gamma correction introduced significant errors if the app subsequently
did further corrections, only shown by using gAMA 1/1.52 files in pngstest.

Signed-off-by: John Bowler <jbowler@acm.org>
This commit is contained in:
John Bowler 2015-09-19 16:36:22 -07:00
parent 8a9a3fb4dd
commit 493bfd14a2
1 changed files with 24 additions and 2 deletions
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26
pngrtran.c
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@ -1479,6 +1479,16 @@ static int
png_gamma_significant(png_const_structrp png_ptr, png_fixed_point gamma_val,
unsigned int sbits)
{
#if 0
/* This seems to be wrong. The issue is that when the app asks for a higher
* bit depth output than the input has significant bits it causes gamma
* correction to be skipped (this was the intent) however there's no
* particular guarantee that the app won't go on to do further gamma
* processing - pngstest does this - and this messes up the results
* completely.
*
* TODO: work out how to optimize this correctly.
*/
/* The following table lists the threshold as a difference from PNG_FP_1 at
* which the gamma correction will make a change to at least an 'sbits'
* value. There is no entry for 1 bit values; gamma correction is never
@ -1517,6 +1527,15 @@ png_gamma_significant(png_const_structrp png_ptr, png_fixed_point gamma_val,
else if (gamma_val > PNG_FP_1 + gamma_threshold_by_sbit[sbits-2U][1U])
return gamma_val > PNG_FP_1 + png_ptr->gamma_threshold;
#else /* FIXUP */
if (gamma_val < PNG_FP_1)
return gamma_val < PNG_FP_1 - png_ptr->gamma_threshold;
else if (gamma_val > PNG_FP_1)
return gamma_val > PNG_FP_1 + png_ptr->gamma_threshold;
PNG_UNUSED(sbits)
#endif /* FIXUP */
return 0; /* not significant */
}
@ -3298,11 +3317,14 @@ png_init_gamma(png_transformp *transform, png_transform_controlp tc)
* necessary.
* 3) Neither is required; the transform can be eliminated.
*
* First default the bit depth if it is not already set.
* First default the bit depth if it is not already set. Note that if the
* output is a palette then 'row_bit_depth' refers to the palette size and
* 8U must be used here. tc->palette is irrelevant; it only tells us that
* the data came from a palette.
*/
if (tr->to_bit_depth == 0)
{
if (tc->palette) /* caching a palette */
if ((png_ptr->row_format & PNG_FORMAT_FLAG_COLORMAP) != 0U)
tr->to_bit_depth = 8U;
else