to agracier:
I'll just make experiments.
To translate into a winter version of the texture I'm used to Photoshop:
- Invert, threshold, filtering, bonding layers...
I've tried other ways to turn a texture into a winter texture. And this works fairly well for turning a summer map into winter without need for serious remapping.
I find a good winter texture that is almost completely white, not completely but almost. A good one is GW_snowy_airfieldGr1.tga or something similar.
I then take a summer texture, create a new layer and import GW_snowy_airfieldGr1.tga into the new layer on top of the old. Then turn off new layer and go to old layer. There, with a magic wand tool set to different tolerances, I start selecting parts of the texture that are likely to be snow free in a winter landscape. For instance, roads or outlines of fields or tree tops. I select all of those I think appropriate, keep the selection active and then go to the new winter layer, where I delete the active selection, thus graphically 'removing snow'.
When both layers are turned on, you can see the effect, as if snow has been removed from roads or blown off tree tops ... when you are satisfied with the effect the merge both layers and you have a new winter texture, which should be given a new name of course, to avoid confusion.
You need to experiment a bit of course, finding the right balance and tolerance for the magic wand tool, or which RGB values to click on in the original layer. But it does work fairly well and moreover more importantly, allows you to keep the texture mapping as it is from a summer map.