UE5 REQUEST: Colored specular for true physically based PBR materials (examples and use-cases included)

[Nick]

This has been an issue for ages now, but the Metalness workflow based on Disney’s BRDF isn’t physically based when combined into one shading pass for anything other than flawless binary metals.

Silicon, magnetite (one type of dark gunmetal), and other semiconductors have ~20% reflectivity and are physically impossible to create with Unreal 4’s “physically based” materials. The industry has had an entire generation to learn PBR and it would be nice to unlock the to 0-8% specular reflectivity value that’s currently hardcoded in the material editor.

The absolute biggest issue is the blending between metals and dielectrics, which the single-pass metalness shading (which is NOT the way Disney’s BRDF works) workflow cannot do in a physically correct manner. Colored specular allows transitions between metals and rust/dust/dirt/etc. while single-pass metalness does not.

Additional reference and use-cases:
Excellent summary talks about how Unreal is currently physically INaccurate by Billy Lundevall (who works at Quixel no less): https://www.youtube.com/watch?v=7J7bvtSoZkU and https://www.youtube.com/watch?v=bb1G18H5wAg

Brian Leleux, a fantastic AAA lighting artist and responsible for the Substance ACES LUT, has to remove all non-binary metalness values in order to get proper materials that respond correctly to lighting. He has to do this because of the issues outlined by Billy Lundevall—namely that Unreal’s PBR metalness is literally physically INaccurate and causes bad lighting values for anything other than laboratory-clean pristine metals: ArtStation - Presentation

IOR of Magnetite is 2.3439 or a specular reflectance of 16%, which is impossible to accurately create with Unreal’s “physically based” single-pass metalness https://refractiveindex.info/?shelf=main&book=Fe3O4

Silicon (like most semiconductors) is also impossible to accurately create with real physical values. It has measured IORs ranging from 3.4 to 4.3, or specular reflectivity of 30-38% Refractive index of Si (Silicon) - Pierce

[Frenetic_Pony]

I mean, sure. It’d be nice to see “UE5” transition get a lot of BRDF work. Metalness is limited, the anistropy added recently is hacky and doesn’t support area lights (what?) no energy preservation, the only pre-filtering offered is fairly basic and doesn’t cover displacement maps or anistropy, etc. etc.

Are there going to be changes, is it going to catch up or surpass Unity’s HDRP? I dunno, but OP I see and support your point.

[Nick]

Yeah, I’m not as familiar with Unity’s HDRP but anything that brings it to being more accurately “physically based” and less hacky, broken, and physically inaccurate would be an improvement. In general I would prefer they don’t force such heavy-handed hardcoded values (8% max dielectric specular? what, why? that’s not physically based at all).

If they’re worried about amateurs creating physically improbable materials, then bury the ‘unlock specular limits’ function in a checkbox under the shader details list for people who know what they’re doing, but never lock it to something that is so grossly inaccurate because they think their users are morons.

[Lunchbox]

Removing access to important functionality limits its use to power users who custom build to achieve these results.

I know this is about photorealism, but for the same reasons (not because I don’t know better) I want this feature and other similar ones (saturated emissive colors for one) so I can create better NPR projects.

These should be options, not to limit but empower those who know how to use them. Making us maintain custom builds for very common requests is tiresome when it can be a checkbox.