Folks may need additional clarity on how compression quality works (-quality) for the PNG format.
Section "PNG Compression" under formats should be expanded to included the behavior outlined in png.c.
/* Untangle the "-quality" setting:
Undefined is 0; the default is used.
Default is 75
10's digit:
0 or omitted: Use Z_HUFFMAN_ONLY strategy with the
zlib default compression level
1-9: the zlib compression level
1's digit:
0-4: the PNG filter method
5: libpng adaptive filtering if compression level > 5
libpng filter type "none" if compression level <= 5
or if image is grayscale or palette
6: libpng adaptive filtering
7: "LOCO" filtering (intrapixel differing) if writing
a MNG, otherwise "none". Did not work in IM-6.7.0-9
and earlier because of a missing "else".
8: Z_RLE strategy (or Z_HUFFMAN_ONLY if quality < 10), adaptive
filtering. Unused prior to IM-6.7.0-10, was same as 6
9: Z_RLE strategy (or Z_HUFFMAN_ONLY if quality < 10), no PNG filters
Unused prior to IM-6.7.0-10, was same as 6
Note that using the -quality option, not all combinations of
PNG filter type, zlib compression level, and zlib compression
strategy are possible. This will be addressed soon in a
release that accomodates "-define png:compression-strategy", etc.
*/
Folks may need additional clarity on how compression quality works (
-quality) for the PNG format.Section "PNG Compression" under formats should be expanded to included the behavior outlined in
png.c.