Added singleMaterials option in NormalPass

[giulioz]

Adds an alternative way for rendering the NormalPass using different material instances instead of a single override.
This fixes broken AO with Instancing.

[vanruesc]

Hi,

thanks for contributing!

The limitations of three's override material system not only affect instanced meshes but also skinned meshes. I understand that a hack is better than nothing, but the problem should be solved at the root and not in postprocessing. If you need the workaround right now, you'll have to use a customized version of the NormalPass in your projects.

Apart from the fact that traversing the scene each frame is bad, this workaround doesn't fully solve the problem: the individual override materials don't use the values of the original materials such as normal maps and displacement maps. A complete workaround would be rather complex on top of being slow. See https://github.com/gkjohnson/threejs-sandbox/tree/master/shader-replacement for a more complete implementation attempt.

The best solution would be Multiple Render Targets in combination with extensible built-in materials, but three doesn't support that yet.

[giulioz]

Thanks for your response.

I saw that in typical scenes (~10-30 elements) it's not too slow, since it's O(2N), but I understand that in some cases it may slow down the render loop too much.

I know that this is really an hack, but I think that, at least, this problem and a possible solution should be noted in the docs. For example, I didn't found any reference about how to do AO with instancing online.

Thanks again for your work.

[vanruesc]

I tried to implement a more efficient solution with layers but ran into weird material caching issues and massive FPS drops due to unexpected shader recompilations. So I think we have no other choice but to replace materials every frame. The scrapped version can be found here.

A workaround for override materials with instanced and skinned meshes has been added in postprocessing@6.14.0. Check this to find out how it can be used.