Baking normal map from uv layout 1 to uv layout 2

Obscura

Hi! I have a normalmapped lowpoly model, with 2 uv channels. 1 has no overlappings, and the second has some overlappings. Originally I wanted to use the first uv channel for my normalmap, but now I need to use the second, because my client wants this. So, I would like to bake out my normalmap from uv layout 1 to uv layout 2. There is no problem with this, theoretically it should work, except a few minor errors because of the overlappings. But.... I get errors on the not overlapped areas too. When I compare the newly baked normalmap with the original, I see that the green channel is stronger/weaker at some areas. So basically the new normalmap shows different angles from what it should. Unfortunately I can't show pictures. Anyone know a good method for this?

I didn't made highpoly model, so I can't simply rebake the normals from it. The normalmap is made with nDO.

EarthQuake

How are you baking the normal map?

You can't bake a normal map as if it is a diffuse map. You will likely need to apply the normal map to the lowpoly as your "high poly" and then bake that to your low poly with the 2nd uv set. Exactly how to do this will vary by app, so knowing which app you use will help.

You can also try offsetting your overlapping bits 1 unit in UV space.

Obscura

I know that I can't bake it as a diffuse map. I use 3dsmax, and the normalmap is applied to the lowpoly (bump->normal bump). Then I use the render to texture, and I'm not using the projection. I simply set the output channel to the second uv channel, and I'm baking a normalmap. It is baking down, and it is looking good at some areas but it doesn't look as the original normalmap at the most of places. Thanks for the tip about the offsetting by the way.

About the normals: I was thinking on using 2 lowpoly meshes. One with the normalmap applied, and one without, and then I would bake to the "bare" one. Would this work better, than baking from itself?

Edit: Using two lowpolies worked fine. Thanks!

FelixL

You have to convert the normal map from tangent space to world space, then bake over this world space map to the new model/UV layout. Then apply the world space normal map to the new model, and convert it to tangent space again.

MARI lets you do this with world to tangent and tangent to world modifiers.
The LODmaker in cryengine lets you do this as well, although I am not sure if it's in the SDK.

As for xnormal et al, I'm not sure. 3dsMax should have some ways to do this, though.