This tutorial discusses a fun way to make organic scratch patterns
for use as texture maps on 3d models.
Say you have a metal robot and you'd like a nice scratch pattern to
use as a bump map. Well, if you visit any local art store (or order
you can find
these things called "scratchboards". A scratchboard is a white piece of
paper or board covered in a dry black ink.
It's primary use is to produce white on black artwork, you use scratching tools to scratch the black ink off the white paper creating the art, like the tiger image on the packaging above.But beyond using it directly for artwork, you can also use it indirectly to make texture maps. Take a piece of the scratchboard and then find a surface that has lots of irregular bumps. For example, a corroded sheet of metal, concrete, gravel, mulch, wood, anything that has lots of tiny sharp surfaces. Then drag the scratchboard on these surfaces. The little bumps will remove parts of the black ink revealing the white underneath. Depending on how you drag it, you'll get different effects. Dragging it long and evenly on the surface will create more directional scratches like the kind you find on brushed metals. Dragging it in lots of different directions for short durations create small little knicks and scratches. You can also add a few manual scratches by using a knife, exacto blade, screw driver, or some sharpish object to add specific scratches in specific places...
Here's the scratchboard before...
And here's after
Then scan your scratchboard with a scanner and bring the result into
photoshop. Here's my photoshop processing tips:
If you're going to use it as a mask, keep it black on white. If
you're using it as a bump, adjust the colors using the Levels tool to
make it black on mid grey.