A decal is an image which adds a multiple display on the texture. In short: You can lay another texture on top of another texture.
Some wall or roof textures are quite nice. But sometimes, the texture isn't complete. You need a crack or stain on the wall or roof. You can achieve this effect by placing a so called 'decal'. A decal can also be a sign or roof, just to add something to that first texture (in this example, the wall).
Creating a decal is quite simple really.
1) Create a brush with a walltexture. I chose a brick wall
2) Copy this brush and place it somewhat further away from the first brush.
3) Select the second brush (the one you just copied). Browse for the nodraw decal texture. It's in tools. See image where to find.
4) Apply the nodraw decal texture
5) Select the front side by clicking CTRL-SHIFT-Left mouse button, on the surface in 3d view.
6) Browse for the decal shader
7) Pick a texture to your likings.
8) Place this brush inside your first texture. Don't worry, the decal texture will be displayed just fine. And you're done! The decal is created!
For comparison, I added a picture where you can compare the both brushes, one with, one without decal texture. The right is the one with decal
TUTORIAL CREATED BY: MAJOR PAYNE