Editing Normal Maps
From Mod Wiki
Using the AddNormals tool, two different normal maps can be combined.
Usage: addnormals <normalmap 1> <normalmap 2> [mask] <out filename> Example: addnormals bumpy_local.tga rusty_local.tga bumpyrusty_local.tga Example: addnormals bumpy_local.tga rusty_local.tga my_mask.tga bumpyrusty_local.tga
This tool ensures that no normal data is lost or incorrect normals get created. The end result is a new normalized normal map.
When using a mask image to combine two normal maps, this image has to be either grayscale or have its mask data stored in the red color channel.