Vulkan Clustered: ReflectionProbe roughness is affected by global Texture Mipmap Bias

[BraqueDown]

Godot version

v4.0.alpha.custom_build [baded9f]

System information

Windows 10, Vulkan Clustered, Ryzen R5 3600, RTX 3070 (512.15)

Issue description

Enabling FXAA or TAA, or lowering the viewport's resolution scale causes reflections created by ReflectionProbes to reduce in apparent roughness. The size of the viewport itself doesn't affect anything. Also, raising resolution scaling above 1.0 doesn't affect it either.

ReflectionProbe.Scaling.mp4

This does not affect the size of the specular lobe, nor does it affect reflections from VoxelGI, SDFGI, or the sky cubemap.

This issue does not occur on the Vulkan Mobile backend.

Steps to reproduce

1. Add a mesh and apply a metallic material, and raise it's roughness.
2. Open the project settings and toggle either FXAA or TAA, and reduce resolution scaling below 1.0.

or download attached MRP and hit Play (F5).

Minimal reproduction project

ReflectionProbe Scaling Roughness Bug.zip

[Chaosus]

сс @Calinou

[Calinou]

It looks like FXAA and TAA affect rendering within the reflection probes, but this may be intended to improve reflection smoothness. It does look strange on higher roughness values, but that's the price to pay to avoid aliasing in low-roughness reflections. See also #49789.

On the other hand, not using FXAA/TAA within reflection probe rendering should speed up their rendering noticeably, with only a small decrease in quality in low-roughness reflections. This also applies to some other tonemapping effects, such as debanding. Disabling them within reflections may be a better tradeoff if possible.

The resolution scale issue may not be caused by resolution scale itself, but by very low 3D buffer resolutions instead. See also #64683.

This issue does not occur on the Vulkan Mobile backend.

Vulkan Mobile doesn't support TAA, and FXAA may not be working either.

[BraqueDown]

It looks like FXAA and TAA affect rendering within the reflection probes, but this may be intended to improve reflection smoothness. It does look strange on higher roughness values, but that's the price to pay to avoid aliasing in low-roughness reflections.

I think you may be misunderstanding my issue. Toggling FXAA and TAA does not (to my knowledge) cause ReflectionProbes to rebuild, and that should only affect in-cubemap aliasing (and of course texture aliasing as a result of how cubemapped surfaces are rendered).

The problem here is that these effects are changing the way that the roughness mipmaps are being blended. When the effects are on (or the res-scale is reduced), ReflectionProbe reflections act as though the surfaces they are rendering on have decreased in roughness.

Metallic surface with 0.5 roughness, Post-AA disabled

Same surface, Post-AA enabled. The previous mipmap level is clearly fading out to the next one up.

---

The resolution scale issue may not be caused by resolution scale itself, but by very low 3D buffer resolutions instead.

I'm not sure what you mean by this?

Vulkan Mobile doesn't support TAA, and FXAA may not be working either.

I'm aware TAA isn't supported on mobile, and indeed FXAA doesn't seem to be working either. But the res-scale slider still works just fine on the mobile backend (on my desktop at least), and this issue still doesn't occur there.

[Calinou]

I'm not sure what you mean by this?

When using a low resolution scale on a low-resolution viewport (e.g. in a small window), the final 3D buffer size will be very small (around 240p). It's difficult to properly generate reflection probe mipmaps at such a low size.

[BraqueDown]

Why would ReflectionProbe resolution depend on the viewport resolution? ReflectionProbes have their own project setting that determines their resolution.

EDIT: I did a quick test, and building the cubemaps at 0.25 viewport scaling in project settings indeed has no effect on the cubemap resolution, but does have a minor effect on the appearance of the ground texture (mipmapping? AF?)

Scene reloaded with scaling at 0.25

Scene reloaded with scaling at 1.00

[BraqueDown]

By happenstance, I messed with the Texture Mipmap Bias setting in another project and noticed that it also affects the mips of ReflectionProbes.

Lo and behold the tooltip flat out states that this is the cause of the problem:

These auto-offsets for FXAA/TAA and viewport scaling were implemented in #64223 and #64225, both by @Calinou, so it's very funny that they didn't realise.

[Calinou]

@BraqueDown Good catch 🙂

I think this can be fixed by using a different sampler for radiance cubemaps, one that does not take the mipmap LOD bias override into account:

godot/servers/rendering/renderer_rd/shaders/forward_clustered/scene_forward_clustered.glsl

Lines 1089 to 1093 in e82d66f

	specular_light = texture(samplerCubeArray(radiance_cubemap, material_samplers[SAMPLER_LINEAR_WITH_MIPMAPS_CLAMP]), vec4(ref_vec, lod)).rgb;
	specular_light = mix(specular_light, texture(samplerCubeArray(radiance_cubemap, material_samplers[SAMPLER_LINEAR_WITH_MIPMAPS_CLAMP]), vec4(ref_vec, lod + 1)).rgb, blend);
	#else
	specular_light = textureLod(samplerCube(radiance_cubemap, material_samplers[SAMPLER_LINEAR_WITH_MIPMAPS_CLAMP]), ref_vec, sqrt(roughness) * MAX_ROUGHNESS_LOD).rgb;

The same change should be carried over elsewhere in the shader (e.g. for fog rendering), and in scene_forward_mobile.glsl.