Howto and what is cross processing?
Cross-processing has been around for as long as film has, and is a way of making your images more interesting and abstract by literally mixing colours. I followed the simple instructions here which I had blogged a long time ago but forgot! It came up in a search for “cross processing gimp”!
There’s no magic secret way of doing this. Simply open your image in your favourite image manipulation program, GIMP of course, or Photoshop if you must. Bring up the Curves tool, and play the Red, Green and Blue channels separately until you have something you like! Create S curves to increase the contrast of each colour, and make it pronounced to increase the effect!
Use the saturation tool to make the colours even more pronounced and remember, have fun!