How to import a splatting texture

I need to import a terrain splatting drawn with an external tool (Gimp). This is because I need to quick draw lines and other shapes to be asphalt. Any suggestion?

Fractscape comes with a script that applies an external splatmap to the terrain. (The readme is a bit outdated, since the terrain APIs are now public.) Note that if you’re drawing a splatmap manually, you should make sure everything is normalized yourself. i.e., r+g+b+a = 1.0 for every pixel.