I'd like to create my base normal map in a 3d program (3ds) by baking high poly to low, then add fine detail in either Photoshop or my own program. I did research and there doesn't seem to be a good method, they are either blended in Photoshop (bad) or in a shader; I think that's because the tutorials aren't written by programmers.
I was thinking I can convert the two normal maps to vectors then combine them with a weighted sum, renormalize and convert them back to a normal texture. Will that be correct ? Do I just add the vector components or do I only offset them with the binormal and tangent parts of the fine detail map. I'm not really sure how to do it.