imagemagick

imagemagick is a command-line image manipulation tool.

options are done in order, so an option needs to be before other options to effect them. for example to turn off dithering with +dither, you have to put that before other things that would use dithering.

use the -negate option

use the -monochrome option

use -colorspace gray.

you can limit the number of colors with -colors and turn off dithering with +dither, though this seems to really highlight artifacts if using jpeg.

occationally, especially if you only have a few colors, this can actually produce larger file sizes than just normal indexed colors. if using PNG, you can prefix the filename with PNG8: to force normal colors which can then be indexed using something like pngo. do check the size though, this can also make the size much larger.

use the -flatten option for this. you can also set the -background color to a color beforehand if you want to replace the alpha with a specific color.

the -lat option takes window dimentions and an offset as an argument, and then does a local threshold to get 1-bit color that still preserves detail even with darker/lighter areas.

may benefit from -flatten and -colorspace gray beforehand. oddly this sometimes makes it output fully transparent images, not sure why but an offset of -1 (ie 30x30-1) seems to fix this...

-flip and -flop will vertically and horizontally flip an image respectively