09-25-2019, 07:49 AM
Quote:...I'd like to isolate some colors in a photo.
It is to hopefully reveal a hidden message. I'm thinking I can remove some colors (maybe convert them to a specific color like white) so what's left are all colors except what I picked.
Is the 'photo' an indexed image? Typically a gif or a png maybe a bmp, top of the Gimp window will tell you. If it is it is worth trying;
Colors -> Map -> Set Color map
Choose an limited palette, Used volcano here. screenshot https://i.imgur.com/XvcxwBL.jpg
With a bit of luck any 'hidden text' will show. https://i.imgur.com/G5g1jlR.jpg
Originally, all to do with editing an indexed image colormap. Look for a suspicious entry in the colormap. https://i.imgur.com/sZ3KFgd.jpg Assign a new color. https://i.imgur.com/YJ2virv.jpg
On the other hand, If a jpg, tif or other RGB image then the number of colors (apart from grayscale) will be in the thousands and probably some other method than changed colours used to hide the text. Sometimes fast-fourier-transform (FFT) is used.