How to make a custom decal?

[Mudbill]

I guess I could answer some of your questions.

A specular map is sort of like a filter for your diffuse texture. It tells the game where these shine bumps are supposed to be. A specular map can have different looks, but often they are just black and white, with noticeable lines to define the shiny areas of the texture.

I'll just make an example of Behemoth's forest leaves:
https://www.frictionalgames.com/forum/thread-29948.html

You see, each of these have 3 texture files. Diffuse (left), normal map (top right), specular map (bottom right). The spec looks like some black and white blobs, but will be applied to the diffuse.

A black spec map will not shine. A white one will shine like crazy. The whiter the file is, the more it will shine.