When trying to enhance the visibility of details in an image where some regions are low-contrast, I found none of the single-step solutions in ImageMagick or The GIMP satisfactory. (XV used to have a great button, but is non-free and hard to obtain now.)
But the following process produced the kind of results I wanted. It simulates a high/medium pass filter in The GIMP.
- Do Layer -> Duplicate Layer
- In the Layers Window (Ctrl-L) be sure the upper copy is selected
- Do Filter -> Blur -> Gaussian Blur with a medium size radius (e.g. 25)
- Do Colors -> Invert
- Do Colors -> Desaturate -> (Lightness)
- In the Layers Window (Ctrl-L) set the Mode to "Overlay"
- If satisfied, do Image -> Flatten Image. Otherwise undo to step 2 and try again with a different blur radius.
You may see a visible "halo" around dark elements, or "shadow" around light elements - that is it working!
Better results might be obtained by combining a number of layers with different blur radii.