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write palette check corrections

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The write palette check is off-by-one when checking the maximum palette
index against the number of entries however, because of the
implementation, the simple correction would fail if no palette check had
been performed (for example for a non-palette image).  This corrects
both errors so that the code outputs a warning (but not an error) if a
user of libpng writes an image with a PLTE which is one entry short.

The write palette check can be turned off on colour type 3 images
(colour mapping images) but this is done by setting the 'maximum'
palette index in the image to (-1).  The ammended code works because it
only executes for paletted images, it is dependent on palette checks
being compiled in and they will always be checked unless the stored
'max' value is less than 0.

Signed-off-by: John Bowler <jbowler@acm.org>
This commit is contained in:
John Bowler 2024-01-16 16:09:29 -08:00
parent 2ea11e2235
commit f9348b77da
1 changed files with 2 additions and 1 deletions
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3
pngwrite.c
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@ -369,7 +369,8 @@ png_write_end(png_structrp png_ptr, png_inforp info_ptr)
png_error(png_ptr, "No IDATs written into file");
#ifdef PNG_WRITE_CHECK_FOR_INVALID_INDEX_SUPPORTED
if (png_ptr->num_palette_max > png_ptr->num_palette)
if (png_ptr->color_type == PNG_COLOR_TYPE_PALETTE &&
png_ptr->num_palette_max >= png_ptr->num_palette)
png_benign_error(png_ptr, "Wrote palette index exceeding num_palette");
#endif