I accidentally discovered an interesting editing technique. It requires an editing software where one can use layers and have the following layer modes. Here are the layers and settings from top down:
- Vignetting layer: New alpha transparent layer painted black, circular hole cut into, given generous 'gaussian blur', and set for 'soft light' layer mode.
- Base duplicate: Turned into outline (cartoon effect), invert, and set for 'soft light'.
- Base duplicate: Layer mode set for 'difference'.
- Base duplicate: Layer mode set for 'addition'.
- Base photo: Sharpened with 'unsharp mask'.
The trick is to utilize 'difference' layer mode that turns only part of the picture invert (negative): "Difference mode subtracts the pixel value of the upper layer from that of the lower layer and then takes the absolute value of the result." What exactly becomes invert in picture much depends on how to use this layer mode.
If you just duplicate the base layer and turn the duplicate into 'difference', then you get black picture. Therefore I added another duplicate set for 'addition' where: "the pixel values of the upper and lower layers are added to each other", which then just makes the basic image lighter for 'difference'.
Disadvantages is that some straight lines may get nasty looking white and sharp outlines. Therefore I also turned one copy of base image into black outlines and kind hid the bad looking edges behind of that. And that's basically all there is done. However, some fine tuning may needed.
Very good idea in general is to try 'unsharp mask' on some of the layers. For example I used it for base photo layer on some of the images. Later on in discovered it can be a good idea to use 'unsharp mask' also for layer with outlines. One can also try to turn 'difference' layer invert and leave out the 'addition' layer. That also gives rather interesting results. And finalizing the whole thing with vignetting, contrast and sharpening usually always makes the outcome much better.
Read more about GIMP layer modes: http://docs.gimp.org/en/gimp-concepts-layer-modes.html