Renderers: Add automatic support for roughness adjustment to alleviate specular aliasing

[gkjohnson]

Description

There have been a couple issues in the forum recently regarding issues with specular aliasing with high detail normal maps and geometry (post 1, post 2) so I wanted to make an issue to track this. It can be particularly noticeable in VR environments.

My understanding is this is caused by different subpixel details being sampled as the camera moves causing a "shimmering" effect in addition to a lack of continuity with sibling pixels.

Solution

I haven't read them in depth but there are some papers on the topic here (Square Enix 2019) and here (Valve 2015, accompanying video). At a high level it seems that adjusting the roughness factor based on the subpixel detail (as determined by derivatives) can help alleviate the twinkling artifacts.

Alternatives

Generating custom normal map mipmaps and an associated roughness texture (a la the old RoughnessMipmapper) may be a more ideal solution (and is referenced in the above Valve talk) but doing this automatically may pose some difficulties, though maybe the node material system will help with that?

Additional context

No response