Convert jpg to png with predefined color codes?

Question

I've got some maps, based on 66 colors, that I unfortunately saved in jpg for a couple of months ago.

I've discovered that I can convert to png, and set it to be 66 colors, BUT, is there anyway to predefine those 66 colors, so for example the colors close to #888888 will be converted to #888888 and not to 878787 and 878887.

One issue is also that the maps don't contain all 66 colors,(usually around 30) so now I might get 4 different type of white. (that's why I need this solution)

is this even possible with imagemagick?

user_2836621 · Answer

You can make a 66x1 pixel "swatch" of the 66 colours you want by taking one of your original PNG images and extracting the unique colours like this:

magick original.png -unique-colors swatch.png

Then you can apply it like this:

magick unhappy.jpg -remap swatch.png result.png

More details here.

Here's a concrete example. Here's our map:

First, make a swatch of the 5 colours we want to appear in our output image:

magick xc:"rgb(10,100,140)" xc:"rgb(240,190,120)" xc:"rgb(70,130,30)" xc:"rgb(220,230,230)" xc:"rgb(40,80,50)" +append swatch.png

Now remap all the colours in the original to the 5 colours in the swatch:

magick map.jpg +dither -remap swatch.png result.png

Alternatively, we could let ImageMagick choose the best colours for the swatch like this:

magick map.jpg -colors 7 -unique-colors swatch.png

And remap just the same as before but using the colours ImageMagick chose:

magick map.jpg +dither -remap swatch.png result.png

Note that you can use hex codes (or HSL, or Lab colours) just the same:

convert xc:"#0a658c" xc:"#f0be78" xc:"#46821e" xc:"#dce6e6" xc:"#285032" +append swatch.png

Note that the above commands assume ImageMagick v7. If you are obliged to use old v6 syntax, replace magick with convert.

Convert jpg to png with predefined color codes?

1 Answers