Recently I decided to create personalized folder icons in GIMP. I liked the look of Mac OS X Leopard folder icons, which are basically a simple colorless shape carved in the folder itself. This is how a similar effect can be recreated with GIMP.
Note on image formats: it appears that icon conversion works best if you use TIFF files. I do not know the reason, but icon2icns did not work for me properly when using PNG files.
Getting the base icon
Let's start with a simple folder icon. Normally you want to perform these steps for at least the 512x512 and 128x128 images to have the best effect. To do this go to any folder in Finder, and select Get Info.
In the info window, click on the icon, and press Command+C.
Go to preview, and select New From Clipboard in the File menu. You have the icon, save the 512x512 and 128x128 in TIFF format.
Editing the icon
Open the TIFF in GIMP, save the file in .xcf format. Then add a layer with the shape you want.
Duplicate both layers, so you have two folder layers and two shape layers. Add a new white layer, and merge the shape copy layer into it. You should have your shape on white background in the new merged layer. Blur this using Filters/Blur/Gaussian Blur.
Hide all layers expect the backgound copy. Right click on the original unblurred shape layer (text layer in this example), and set its alpha channel to selection. Invert the selection with Seleciton/Invert, and finally right click background copy and chose Add Layer Mask to add this inverted selection.
Then make sure that the picture, not the mask is selected in the layers dialog. Apply Filters/Map/Bump map to it. Set azimuth to around 90 (so light comes from top), and increase ambient as well to around 100. Once you are satisfied with the settings, click OK. We are almost there. If you see a black/white image here, you have not selected the correct layer or selected the channel instead of the actual layer.
Make a second copy of the background layer, make it visible behind our "cut through" layer. You may have noticed that the folder shadow became darker. To fix this select the middle part of the folder icon that covers the shape. Then select the Select/Invert menu item, and press Delete. The extra shadow should disappear, with the "hole" still covered completely. Remove the selection using Select/None.
Having the new copy layer selected go to Colors/Hue-Saturation. Change both Lightness and Saturation to around -40. We should have the carved in shape mostly finished.
To add a final touch we should make a darker thin border in the inside area. Select the text alpha to selection, and create a new transparent layer between the cut-through and the darker background images. Select/Grow with value 1 then Select/Border with value 3 and feather border. Then fill this new selection with black. Finally change the new layer mode to Overlay.
Finally, we are ready.
