A decal is usually a new, unique piece of geometry that's spawned just above the surface. Whatever's behind it has no effect on the decal itself - the decal just draws on top.
http://www.inlandstudios.com/index.php?show=software.decalcreator There is a brief explanation of decals on this page as well a trial-ware plug-in for Maya. There is also a video at the bottom of the page that shows how a decal works on another mesh.
Yeh, sure, I'm assuming I would draw a decal to the UV texture. But what I'm saying is, when I do that, there is the possibility that the decal would go onto another polys UV data. What I'm trying to figure out is, how do I stop that?! RipX
My best guess would be to give it a second set of UV coordinates where the entire mesh is uniquely unwrapped, similar to how AO or Lightmaps are often baked. With unique UV's, your seams will always be on polygon edges, not UV edges. Not sure if your system allows that, though. I've heard of bullets that cast rays, and the…