Right now, only 8-bit RGBA PNG images are supported. It should be straightforward to open other images in this format (Image PixelRGBA8). Something better would be to abstract over the underlying image format completely and open all images as Data.Matrixes or something like that. But the API of Image PixelRGBA8 of behaves pretty much like what we would want from a matrix so the former could be implemented as a quick fix for now.

An obvious to-do. Initial implementation of pixelate takes magnitude to be 1—it should be changed so that it takes magnitude as a parameter. Should be easy to implement.

All image operations are implemented to work on images of equal size; they merely assume that the given two images are of same size. This is bad—it might result in a runtime error. Arithmetic operations should account for their operands not being of same size, either by cropping them or having a return type of Maybe (Image PixelRGBA8) and returning Nothing in that case.

It would really make more sense for flipVertical to be executed through flip --vertical and similarly for flipHorizontal as flip --horizontal. If no flag is given, the image should be flipped about the origin.

The generated histogram has width 300 although each pixel has the greatest magnitude 255 right now, since they are simply Word8s. Someone should go through the documentation of Haskell-chart to figure out how to handle that through the API.