Rock Overlays (GIMP)

This tutorial will teach you how to make semi-transparent overlays, for use in mapping programs that supports transparent PNGs as overlays or stamps. Thanks to Rob A who wrote the original tutorial that taught me how to get started. It can be found here.

Gimp is a free graphics program that can be obtained from their website. I have attempted to repeat this in but couldn’t. I would assume it could be replicated in Photoshop.

This was written using GIMP version 2.6.10

  1. Load up a source image of some rocks, cracks, ground, etc:

This can either be a graphic that you will cut a section out later or one with the background already transparent.

Make sure it is a copy of the original so you don’t accidentally save over it. You can always just save it as a new name now.

  1. Turn the picture to grayscale by removing all color.


It will ask what you want to Desaturate based upon. Choose Lightness (default)

You should now see a grayscale version.

  1. Auto Adjust the image levels, This stretches the image to use the entire grayscale range from black to white


Click Auto

The image should be similar to this.

  1. Add a layer mask


Select grayscale copy of layer and Check invert Mask

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

In the Layers, Channels… dock you will see the layer mask. The image now looks like this:

5. Select the image layer in the Layers, Channels.. dock. It should be the one that looks like your picture and on the right of the images. You will see the layer name in the graphic dock at the bottom. If it says background mask you have the wrong one selected.

6. Adjust the color level of the image to be in the lower range of the scale.


This will squeeze it into the lower half of the scale and make the source image from black to med grey

Change the light Level. 127 is a good starting place.

  1. Select the layer mask layer in the Layers, Channels.. dock. You will see the layer name in the graphic dock at the bottom. It should say background mask.

  1. Adjust the color level of the mask.


The settings here depend on the source image and the type of effect you are going for.

To start use these settings:

  • Set the input level left hand slider to 10

  • Set the gamma (middle slider) to 0.8 (lower is darker)

If you are doing something with sharp angles and deep cuts darker seems to work better. If you are doing rolling hills or shallow cuts like sand go a bit lighter.

You can put another layer behind the image to see what it will look like with different shades.

  1. At this point save a copy. This is a good base and many originals you can make several items from.


You will be asked to select a file location. Don’t for get to make it a .png file.

You might receive the following windows. You should choose Confirm, Export, and Save respectively.

  1. Now that you have an original copy tucked away (in my example case a usable rock overlay) you can take make one that the edges will blend with the ground around it.

10.If you have a image that fills the entire canvas then:

A. Select the entire image


B. Shrink the selection by a reasonable size


Shrink by about 50 pixels. If you image is small then it will probably be less.

C. Feather the selection.


Feather by about 100 pixels or about double what you shrank by.

D. Invert the selection


  • If you have an object smaller that the canvas like we have in the example then:

A.Select the entire image


b. Select the Magic wand (fuzzy) select tool. You can also use the rectangle, oval, or lasso select.


  • feather edges and set the radius to 100

  • Select Transparent Areas

Click in the outlying transparent area. This will select your image.

c. Invert the selection


  1. Select the background layer mask.

  1. Select the Flood Fill (Paint bucket) tool.

13. Flood fill the selected area with black. Fill a second time if you need more blending.

  1. Deselect all.


  1. Pick the Airbrush tool with a large fuzzy circle brush

  1. Changing the scale changes the size of the brush.

  1. Foreground color should be black.

  1. Set opacity to about 50%

  1. Start painting the edges on the mask layer to feather in the border. Try to follow the natural shape of the image. You can paint anywhere on the image to make some parts of the image more transparent. You can also paint the mask in white to make areas more opaque. You can also use the eraser later after it is flattened into a final .png if you want to blur up some edges.

  1. Save your image as a .png file.

Here is my final image. Well one of them. I will make several from the same base picture.

And on colored backgrounds. Normally there will be textures not solid colors.