Hey Stimpanzee I think you will enjoy .
For an epic friday project a couple months ago I added a new secondary bottom normal map input to the clear coat shading model so you can get accurate shading for carbon fiber substrate as well as attempt to create metallic flakes using a noisy normal. It is not checked in yet (it needs to go through code review as I am not a full fledged coder) but there is a will make it in by 4.11 or so. Currently is implemented as a project setting that has to be enabled since there is some rendering overhead any time you introduce more dynamic branching into the deferred rendering path.
quick attempt at flakes with a normal map (tempAA is off since tempAA kind of removes the sparkles unfortunately):
The basic gist of how its done:
-New normal map input via “Custom Output” node (same system used for landscape grass)
-Normalmap is packed into gbuffer using vector to octahedron since we only have 2 spare gbuffer channels to store in (custom data had a single channel left and opacity). Note that packed normal is 8-bit which is why I chose to make the bottom normal the secondary rather than the top (top requires full precision for smooth vertex normals).
-Still in the bass pass, the clearcoat mask value is packed as 7-bit and if there has been a custom-output clearcoat bottom normal node placed, write a 1 into the free bit in CC mask, else write a 0.
-Then its a branched IF inside of the clearcoat shading model that unpacks the bit from the clearcoat mask. If the bit is 0, do the standard clearcoat shader. If the bit is 1, do the extra work for the bottom normal.