#21 PNGCrush alters images that don't use black

[Anonymous]

If source image is a 8-bit depth palette PNG that only
uses 4 unique colours but does not use black colour,
reducing bit depth to 2 and palette length to 4 colour
via switches "-reduce -brute -rem alla -bit_depth 2
-plte_len 4" causes PNGCrush to turn one colour to
black (see attachment acme). In addition, using
-reduce switch does not reduce output images bit depth
or palette length in such case.

[Nobody/Anonymous]

Sample picture 'acme'

[Glenn Randers-Pehrson]

It is an error to use -plte_len 4 when there is a palette index present in the image with value greater than 4. Currently pngcrush will issue a warning if you try to do that. Pngcrush does not currently have a capability of regenerating the palette. You can use ImageMagick to do that:

convert acme.png -define png:exclude-chunks=all acme_im.png

The "-define png:exclude-chunks=all" directive suppresses some extra chunks (bKGD, text, cHRM, gAMA) that ImageMagick would otherwise insert.