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Better filter checking

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Improve pngvalid coverage of filter combinations, remove the (new in 1.7) code
which disabled previous-row filters on the first row of an image in some cases.

Signed-off-by: John Bowler <jbowler@acm.org>
This commit is contained in:
John Bowler
2015-11-24 22:24:18 -08:00
parent 8bffb48bc1
commit 1775bdeb24
2 changed files with 46 additions and 107 deletions
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93
pngwutil.c
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@@ -2264,44 +2264,6 @@ png_write_filter_row(png_structrp png_ptr, png_bytep prev_pixels,
}
}
}
/* The filters are pre-calculated in png_set_filter, however if the
* image is interlaced some passes may still be too narrow or short to
* allow certain filters. In any case the first row of the pass
* doesn't need to consider PAETH or UP (AVG is still different).
*/
if (first_row_in_pass)
{
if ((filters_to_try & PNG_FILTER_UP) != 0U)
{
filters_to_try &= PNG_BIC_MASK(PNG_FILTER_UP);
filters_to_try |= PNG_FILTER_NONE;
}
if ((filters_to_try & PNG_FILTER_PAETH) != 0U)
{
filters_to_try &= PNG_BIC_MASK(PNG_FILTER_PAETH);
filters_to_try |= PNG_FILTER_SUB/*equialent to PAETH here*/;
}
/* If this leaves the AVG filter it will be used on the first row
* this is handled in the filter implementation by setting prev_row
* to NULL below.
*/
}
/* Check for a narrow image; the blocking will never return just one
* pixel at the start unless the pass is only one pixel wide, this test
* needs to happen after the one above on PAETH:
*/
if (width == 1U)
{
if ((filters_to_try & PNG_FILTER_SUB) != 0U)
{
filters_to_try &= PNG_BIC_MASK(PNG_FILTER_SUB);
filters_to_try |= PNG_FILTER_NONE;
}
}
} /* start of row */
else if (prev_row != NULL)
@@ -2392,61 +2354,6 @@ png_set_filter(png_structrp png_ptr, int method, int filtersIn)
return;
}
/* New in 1.7.0: adjust the mask according to the image characteristics.
* This used to happen on every row, doing it here means that these checks
* happen only once every png_set_filter call, or once per image.
*/
if (filters != PNG_FILTER_NONE)
{
/* Test to see if there are enough rows to allow previous-row filters to
* work. Note that the AVG filter is still significant because it uses
* half the value of the previous pixel as the predictor, but it is
* ignored in this case.
*/
if (png_ptr->height <= (png_ptr->interlaced == PNG_INTERLACE_NONE ? 1U :
(png_ptr->width == 1U ? 3U : 2U)))
{
/* Replace 'up' by the equivalent 'none': */
if ((filters & (PNG_FILTER_UP)) != 0)
{
filters &= PNG_BIC_MASK(PNG_FILTER_UP);
filters |= PNG_FILTER_NONE;
}
/* Replace 'paeth' by the equivalent 'sub': */
if ((filters & PNG_FILTER_PAETH) != 0)
{
filters &= PNG_BIC_MASK(PNG_FILTER_PAETH);
filters |= PNG_FILTER_SUB;
}
/* Remove 'avg' unless it is the only filter in which case 'none' is
* used. (This chooses compression speed of very short images over a
* probably pointless compression option for a one line image; short
* images are common, the sub-case which benefits from AVG is not.
*/
if ((filters & PNG_FILTER_AVG) != 0)
{
filters &= PNG_BIC_MASK(PNG_FILTER_AVG);
if (filters == 0U)
filters |= PNG_FILTER_NONE;
}
}
/* Also check for SUB on narrow images; it's equivalent to NONE on the
* first pixel.
*/
if (png_ptr->width <= (png_ptr->interlaced == PNG_INTERLACE_NONE ? 1U :
(png_ptr->height == 1U ? 3U : 1U)))
{
if ((filters & PNG_FILTER_SUB) != 0)
{
filters &= PNG_BIC_MASK(PNG_FILTER_SUB);
filters |= PNG_FILTER_NONE;
}
}
}
debug(filters != 0U && (filters & PNG_BIC_MASK(PNG_ALL_FILTERS)) == 0U);
png_ptr->filter_mask = png_check_bits(png_ptr, filters, 8);