Now we can finally start to work on the alpha map. In the UV screen we create the texture image with all texels set to one of the primary colours. I chose green because I’m going to use the green channel for grass, which is the most common texture on my terrain. Make sure to use the RGB sliders to make sure you have one primary colour set to 1 and the others to 0. I let the dimensions at the default 1024x1024, but you may want to increase it for large terrains. Downscaling is easy with any image editor, upscaling is harder. Besides, alpha maps often contain large areas of the same colour so they should be compressed pretty well.@ben_g
A problem i am having is that after i paint the map, if i change a single vertex, i have to re-project the UVs, which makes the entire UV map not line up anymore.
Is there something that would allow me to "paint" on the 3D mesh, and then have that painting projected onto onto the UV image?