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Using version 1.4 downloaded from the GM snapshots on 6/17/2014 using the GM commandline in Windows. Attempts with 1.3.18 actually crashed the tool. I am running this command:

gm convert "%~dpnx1" -dither -colors 254 -treedepth 2 -compose Copy -matte
-type PaletteMatte "%~dpn1_8bit.png"

The input PNG is a 32bit PNG with an 8-bit alpha channel and the output needs to be an 8-bit paletized PNG file with the source's transparency. When the output of this command is generated, the alpha is dropped and space covered is converted to all white (255,255,255). No alpha channel exists at that point. I have attempted several iterations of this, including doing a composite of the matte which only produced a 32bit output:

gm composite "%~dpn1_8bit.tif" -compose Copy "%~dpn1_matte.tif" -matte 
-type PaletteMatte "%~dpn1_8bMatted.png"

So far, even XNView fails to do this. Any help would be appreciated. Thank you!!

user3750278
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  • BTW, please don't say "use PNGquant" or "Imagemagick" as their conversions appear worse than GM's 8bit color reduction. – user3750278 Jun 18 '14 at 01:46
  • So using GM's dither to 255 colors, then compositing in the extracted matte to make a truecolor file with a matte and only 256 colors, then using pngquant or IM to ouput the PNG got me very close to what I wanted. Still, seeing this fixed in GraphicsMagick would make my life easier. – user3750278 Jun 20 '14 at 21:18

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