High pass in GIMP
High pass filter sharpens images, first of all the edges of color spans. Though standard Gimp distributive doesn’t provide this filter, its operation sequence can be easily reproduced by these easy steps. There is plugin for high pass filter too but I’m not sure about the link.
Open the image.
Duplicate the background layer twice (Layer — Duplicate layer) and desaturate the layer copies (Color — Desaturate).
Apply the Gaussian blur filter for the upper layer (Filters — Gaussian blur) with the parameters you need. The less the blur you apply, the sharper image you get, but too little blur makes highlights appear. After applying the filter change the blending mode to Grain Extract in the listbox named Mode. (Default listbox value is «Normal»).
You get an odd grayscale image. Now, merge the upper layers (Layer — Merge down).
Now that’s it, the sharpened image. Apply the Overlay blending mode for the upper layer.
Can you hardly notice the effect? Duplicate the black-and-white layer, but be careful about the edges — that’s where high pass filter does his job. So here is an example with four layer duplicates. As you see, some cleavage appears.