Error in glTF PBR shader

[UX3D-haertl]

Description

Currently, mixed materials (e.g. metallic value of 0.5) are not energy conserving and therefore displayed incorrectly.
This issue highlights the error in the glTF spec:
KhronosGroup/glTF#2386
Here you can see the shader refactoring we had to do in the glTF-Sample-Viewer to fix the issue:
KhronosGroup/glTF-Sample-Viewer#556

The issue is clearly visible with the furnace test and mixed materials with iridescence extension (using a premixed F0 has a big impact on the visual output)

Reproduction steps

1. Load attached GLBs/HDR
2. Compare to glTF-Sample-Viewer output

Furnace test: No spheres should be visible
Iridescence: Changes from left to right should be gradual

iridescence_transmission_metallic_white.zip

furnace.zip

Code

This is the old incorrect pseudocode

const black = 0

c_diff = lerp(baseColor.rgb, black, metallic)
f0 = lerp(0.04, baseColor.rgb, metallic)
α = roughness^2

F = f0 + (1 - f0) * (1 - abs(VdotH))^5

f_diffuse = (1 - F) * (1 / π) * c_diff
f_specular = F * D(α) * G(α) / (4 * abs(VdotN) * abs(LdotN))

material = f_diffuse + f_specular

Here is the corrected version:
https://github.com/KhronosGroup/glTF/blob/main/specification/2.0/Specification.adoc#metal-brdf-and-dielectric-brdf

Babylon.js premixes colorReflectanceF0 and uses it e.g. in iridescence:

Babylon.js/packages/dev/core/src/Shaders/ShadersInclude/pbrBlockReflectivity.fx

Line 161 in c869b5b

outParams.colorReflectanceF0 = mix(outParams.dielectricColorF0, metallicColorF0, outParams.metallic);

Examples

Screenshots from Babylon.js Sandbox:

Screenshots from glTF-Sample-Viewer:

I opened a similar issue for three.js: mrdoob/three.js#31350

[sebavan]

We have a fix for it under a flag which was introduced by @MiiBond ? we can not make it the default for back compat purpose

[UX3D-haertl]

I assume this PR #16476?
Seems like this addresses most of the issues. But as far as I have seen it does not change iridescence.

[sebavan]

About iridescence, @Popov72 has worked on a fix in the past which should work with what @MiiBond did.

[UX3D-haertl]

I tested the assets with legacy energy conservation disabled:

All the spheres now have the same color, so the formula for mixing is correct. They are still brighter than the IBL. That might be a different bug.

The iridescence issue does not seem to be fixed by that option, or do I need to toggle another option as well?