A snow mask occupies alpha channel of vehicles _n texture. It's function is (obviously) to mark areas that will be covered in snow during winter. It is also currently used to generate rust on vehicles. As with other masks the texture is grayscale with black areas for no snow and white areas that are to be covered with snow/rust.
Again, there are multiple ways how to achieve a decent snow mask. One such may be:
- desaturate color/diffuse mask or otherwise get rid of its colors
- duplicate the desaturated layer
- apply some (gaussian) blur on the duplicate, about 10 to 15px
- subtract the duplicate from the unblurred desaturated diffuse
- paint black areas without snow
- alter brightness and contrast to get the final result
NOTE 1: If you cover your cargo ona vehicle with snow, keep in mind that it will also be covered with rust.
NOTE 2: If you cannot get a decent result, you can alsways leave the mask black.