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Diffusion Map

The Diffusion/Ref Map

Diffusion maps, or as Blender calls them "Ref Maps," work in a similar fasion to bump maps. Grayscale images are created most often from a bump map. The whiter areas will cause the material color to be affected by user lamps more easily, whereas darker areas will cause the material color to resemble the ambient light. In other words, the white areas will be brighter and show more of the material's natural color.

Try to imagine what a diffusion map for this wood plank should look like. The grooves need to be distiguished, deteroirated from the salt water -- deep, dark. But at the same time, the worn-down bumps need to be visible. Therefore, the map should be well contrasted, but keep an overall light-gray/white color.

Creating a Diffusion map with the Gimp

Open pineNor.jpg (your bump map) with the Gimp. You don't need to change this image to grayscale, because it's already been set that way. Open the Levels editor (remember the following steps can be also be achieved to a degree with brightness/contrast settings). To get a high contrast, you need to increase both the white and dark elements at the same time. Therefore, move the far right and far left triangles of the input levels towards each other, and use the middle triangle to favor the black. Why favor the black? You could favor the white, but then you'd be loosing the thickness of the darker areas. The settings I used can be seen in Figure 7.
Figure 7: Example Levels Settings
Example Levels
Because my image was too white, I needed to decrease the overall whitness (or increase overall blackness), and so I adjusted the output levels accordingly.
That's pretty much it. Save this image as pineRef.jpg, and check it out in Blender.

From the Gimp to Blender

Bring this texture in just like you did with the bump map, but this time use the third texture channel (the Blender Manual describes that the order of texture maps is important; in this example, the first channel should be color, the second bump, and the third diffusion). In the material buttons, change the texture from "col" to "ref," and remember to adjust the material's natural diffusion to 0.
Figure 8: Ref Map Settings
Ref Slider Set to 0
Ref Map Settings
   

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