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These provide some colourisation information to the model where things are reflected. The textures alpha channel is used to modify the texture based on the environment map.
The default games version without PBR's is a bit lackluster.
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- TXI (urg) can define it with EnvMapTexture
- MTR can supposedly use Texture15 for it, this needs further testing
- You can specify a texture filename in the appearance.2da entry (not a terrible idea, but only works for creatures not placeables or tiles)
Using Alpha as Specular
As noted above alpha layers are used for the environment mapping "shiny/reflective" surfaces. It'd be nice to get these converted over to PBR's though (which gives us some nicer control).
In GIMP you can go Alpha → Greyscale like this: https://graphicdesign.stackexchange.com/questions/155206/alpha-to-colour-value-in-gimp
- Delete all the colour channels except alpha so RGB layers are black (do this by unselecting the alpha under Channels)
- (optional) Insert an all back layer below the now alpha only layer
- Use "Behind" fill with white paint bucket on the alpha layer
- Should fill it in with black being "no alpha" and white being "full alpha".
Now it's ok but might not be to what specular maps should be (where white == shiny).
Found that Colors → Exposure works well. Put black level to 0.1 (maximum) to keep the blacks and the Exposure upwards, say to 2.0 or so.