Making Rock Overlays using the GIMP

This tutorial will tech you how to make semi-transparent rock overlays, for use in Dundjinni, Dungeonforge, or any art program that supports transparent PNGs as overlays or stamps.

1. You must have GIMP. Get it Head over to the gimp website if you already don't use it and download this excellent, free image editing program. (Though I expect all these steps have an equivalent in Photoshop, or other programs).

2. Load up a source image of some rocks, cracks, or whatever:

3. Remove the colour (Layer->Colors->Desaturate):

4. Auto Adjust the image levels (Layer->Colors->Level, click auto). This stretches the image to use the whole greyscale range, from black to white:

5. Add a layer mask, (Layer->Mask->Add Layer Mask) and select greyscale copy, and invert:

You will now have a layer mask where the darker parts of the image are opaque, and the lighter parts are transparent:

6. Create a New White layer, and drag it under the rock layer:

7. Select the whole image (Select->All) then shrink the selection by a reasonable size (Select->Shrink). I picked 40 here, but it depends on the image size. Feather the selection by double the size you shrunk it (Select->Feather). I used 80. Now invert the selection (Select->Invert):

8. Select the rock layer, and ensure the layer mask is selected (it is selected by default when created). Flood fill the with black:

9. Flood fill a second time if the fringes of the rocks can still be seen. (No image of this step, sorry.)

10. Pick a big fuzzy brush. I created one 40 pixels large like this:

11. Remove the selection (Select->None). Using the Paintbrush tool, colour black, and an opacity of around 50:

start painting in from the edges on the mask to feather in the border. Try to follow the shape of the rocks. You can also paint in the middle of the image to make some parts of the image more transparent. Feel free to change brushes. You can also paint the mask in white to make areas more opaque Here is my final effort:

12. Now select the rock layer image (not the mask - see the white outline in the thumbnail is on the image, not the mask):

13. Adjust the colour level of the image (Layer->Colors->Level) to squeeze it into the lower half of the scale (I changed the light Output Level to 127) This make the source image from black to med grey without this step it will look funny on darker backgrounds:

14. Select the layer mack, and adjust the colour level of the mask (Layer->Colors->Level). I set the input level left hand slider to 10, and moved the gamma (middle slider) to 0.8 to darken the image. The values used really depend on the source image. It is best to try this over a variety of backgrounds, not just white.

15. Here is the final image in the GIMP, with the background turned off. Don't forget to save!

16. Now Save As a PNG and pick the export option to get an image you can use:

Hope this was informative!

-Rob A>

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