The key to this tutorial is to help you improve your scenes and your texturing ability, by showing the basics of using different kinds of maps for different scenarios. I'm a car modeler so I mainly use this technique for applying decals to my car's paint. The problem is that many don't know that the same can be done with the Brazil Advanced material car shader...
To start off with I'm using a car door to apply the material to, First off if you don't know how to make the car paint material then keep reading, if you do, then proceed to the next step. Open up the material editor and make a Brazil Advanced material, from there open up the Base Shader rollout, which looks like Figure 1.
Click the dropdown and select Car Paint, now you will have something that looks similar to Figure 2. Change the Diffuse color to any color you would like, it doesn't really matter. Next drag that color to the Falloff slot, and pick copy, then to the Flakes, and Candy slots, now click on the Candy slot and lighten it up just a little bit.Your car paint material should now be ready. Render and it should look something like Figure 3.
Now to the fun part, we are going to make the actual decals now, I'm using just a random decal from my collection, all I have done is gone into Photoshop and made an alpha map of it, where the black spots are will be parts that cannot be seen and will just show the car paint, and the white part will be the decal. So once done you should have something like this for your maps...
Decal: Texture Map Decal: Alpha Map
Ok, first off we need to make the UVW maps of where we want the decals to be located, so go to the modifiers menu and select UV coordinates and then select UVW map. Once you have the map, scroll down to the size of it in the modifiers rollout, there should be an option called Alignment, select the box that says Bitmap Fit, and choose your Decal map. Now that this is done go ahead and make this the 2nd map channel. Now click on the UV Mapping tree and select Gizmo, now go to scale and scale it to the size you need it, make sure you scale everything the same or it will be stretched or skewed.
Now open back up the material editor and go down to Basic Surface Properties and put a Composite map in the Color (Cs) Slot, it should look like this when you have done that
Ok, now in the first map slot, put a Mask, (This is so you can see the decal, but not the area around it). Now in the first slot put your Decal Map, not the Alpha Map, and put these settings...
Now drag the Decal map and put it into the 2nd slot, make sure you hit copy, and not instance, now just switch the map with your alpha map and your done...
Now to add more decals, you just add more UVW Maps and do the same thing in the other slots in the Composite map, when done your image should like this...
Here is a sample of what else can be achieved with this technique.
I hope this tutorial has been helpful and if you have any questions or comments
feel free to e-mail me here