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I have a large number of monochrome PNGs that are black on transparent. I would like to invert them to be white on transparent. I've tried using XnConvert but it doesn't maintain the transparency.

  • since "transparency" is another word for "alpha channel" and alpha is a greyscale image, and your source is "black and white", you can try compositing a white image with the source image specified as the alpha mask. Imagemagik (command line) may be able to handle this via the -compose switch (?). GIMP is open source and allows for batch commands as well. – Yorik Mar 3 '16 at 21:51
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ImageMagick will do it:

for a single image:

convert black.png -negate white.png

for a directory full of images:

mogrify -negate *.png

The "-negate" option only affects the pixel color components and doesn't change the alpha (transparency).

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    Are you sure about that? I just ran the command and it has flipped the alpha and non-alpha channel. So rather than black-on-transparent, it's now transparent-on-white – Matt Fletcher Sep 5 '17 at 9:41
  • I ran some tests and ImageMagick 6 works as I described, but ImageMagick 7 seems to be buggy. – Glenn Randers-Pehrson Sep 7 '17 at 0:08
  • Ahhh interesting! I wonder if it's a bug or intentional? Thanks for taking the time – Matt Fletcher Sep 7 '17 at 6:28
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For ImageMagic7 you can disable the negation of the alpha channel like this:

mogrify -alpha deactivate -negate *.png

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