UV  Maps

 

A 3D program uses X, Y, and Z in Units to make a mesh and paint program use X and Y as pixels, so to keep things simple 3D programs call the X and Y for pixels U and V.

 

A UV map is how the image texture is mapped to the 3D surface and there are several way to do this but they all have one problem, you can not get the surface to match up with the pixels at angles.

 

You get this:

 

 

So what happens is you get an over scan by the 3D program of the image texture. Trainz dose not use the textures you make, it re-makes them as mit-map witch also gives an over scan. What this all comes to is you have to leave a bit of room around the surface, about 1 to 3 pixels.

 

The surface part is called an island, a lot of them do look like islands on a map.

 

 

 

 

If you use a 3D paint program you can not have the islands overlap and it could give problems for PBR. Sometimes you can getaway with overlaps if it dose not show much in game, like the underside of a car body that every thing is the same color.

 

This is something I am having to do that I did not have to before. In testing with a 3D paint program overlapping is a big NO NO.

 

In Blender the best way to do the UV unwrapping is to use Smart UV Project with a 0.003 separation.

But some times you need to use the others.

 

All the islands should have about the same pixel density, that is the same scale.

 

Blender can only do a UV unwrap on one object at a time and you need to do more so there's a handy plug-in that will unwrap all selected at one time. It's here:https://github.com/ndee85/Multi-Object-UV-Editing