Access reflection probe in shader?

arkaein · February 26, 2025, 4:05pm

Godot Version

4.3

Question

I have an effect which I currently create by layering an alpha pass overlay onto a base material using a second material pass using a shader. For a variety of reasons I’d like to consolidate this into a single material, so I used the conversion utility to convert the base standard material into a shader, then added my separate overlay code.

This works, however my scene uses a reflection probe, and the new shader material doesn’t appear to sample the reflection probe in any way.

I’d like to try adding the reflections back in myself, but it isn’t clear how. The ReflectionProbe class doesn’t appear to provide access to the texture. Shader built-ins don’t appear to provide any access, just the write-only RADIANCE and IRRADIANCE values. The rendering server has a lot of functions dealing with reflection probes, but I still don’t see a way to access the texture.

Anyone know a way to do this, or incorporate reflection probes into shaders in any way?

Calinou · February 26, 2025, 11:56pm

Sampling the (ir)radiance maps directly is not supported yet:

github.com/godotengine/godot-proposals

Allow irradiance and radiance to be manually sampled in `spatial` shaders

opened 09:36PM - 22 Jun 21 UTC

Lauson1ex

topic:rendering topic:shaders topic:3d

### Describe the project you are working on Third-person, high-fidelity visuals… 3d game. ### Describe the problem or limitation you are having in your project Godot doesn't implement specular occlusions on its BDRF model, so I implemented a more complete model for my materials using bent normal maps (like Unreal, Substance, etc). I need to sample radiance and irradiance manually based on the bent normal vector instead of the normal vector. I can already _sort of_ work around this by tricking Godot to sample it for me, by sending the bent normal vector to the `NORMALMAP` shader built-in, but then I had to implement my own lighting algorithms in the `light` shader using the actual normal vectors, otherwise lighting looks obviously wrong. You can see how this is inconvenient, plus I'm not _really_ sampling the irradiance/radiance VoxelGIs/SHs/Reflection Probes; I'm merely tricking Godot into shading the indirectly-shaded fragments with the correct color using the bent normal vectors. Plus, I'm limited to only a single sample per fragment. Implementing the lighting functions myself is fine... mostly, since I had to only do it once, but since `ATTENUATION` doesn't separate distance attenuation from *shadow* attenuation, it introduces a lot of headaches implementing physically-correct attenuation in the `light` shader _Note: this probably should be its own proposal._ ### Describe the feature / enhancement and how it helps to overcome the problem or limitation I propose a way to manually sample irradiance and radiance directly in the `spatial` shader. This way, I can simply give a normalized vector to the sampling function, and it will return me the color for that voxel's/SH's/probe's coordinate. This way I don't have to reimplement light functions myself, and simply use the `diffuse_burley`, `specular_schlick_ggx` render modes. ### Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams I propose two interfaces for sampling irradiance and radiance: * `vec3 IRRADIANCE(vec3 normal)` for sampling the nearest voxel for that fragment when using VoxelGI, the nearest Spherical Harmonic when using lightmaps, or the nearest SDF probe when using SDFGI; * `vec3 RADIANCE(vec3 normal, float lod)` for sampling the environment panorama, or a ReflectionProbe, when those are defined; the `lod` parameter defines the blur level of the pre-filtered importance-sampled radiance, for roughness levels. ### If this enhancement will not be used often, can it be worked around with a few lines of script? I could work around this issue by using a simple microfacets shadowing algorithm for specular occlusions. This is fine for rough materials, but looks really bad for glossy reflections in my opinion and only works for Lambert diffuse lighting. Bent normals are absolutely necessary for convincing specular occlusions and the _only_ alternative for Burley. However, there are many more applications for sampling radiance and irradiance manually, namely having more flexibility in creating light shaders, or creating stencil shadows for 3rd person platformer characters with correct environment shadow colors. ### Is there a reason why this should be core and not an add-on in the asset library? Alas, this is core.

or incorporate reflection probes into shaders in any way?

The reflection’s visibility is determined by ROUGHNESS (sharper at lower roughness) and METALLIC (more visible at lower roughness). Higher SPECULAR values will make the reflection more visible, although only 0.5 (the default) is considered physically accurate.

arkaein · February 27, 2025, 3:55pm

Okay, so it sounds like as long as I want reflection probes to have an effect I need to stick with the multi-pass rendering.

Maybe you have a suggestion for a different workaround. One of the reasons I want to put this color overlay into the base material is that on top of all of this I do a localized heat haze effect which uses a screen reading texture. Because screen reading textures won’t sample render passes using the transparency pipeline my color overlays vanish behind the haze effect. However I haven’t figured out a way to apply my color overlay as a second pass without writing ALPHA.

I can skip the ALPHA write and write a new color completely, discarding fragments below an threshold for my overlay, but that gives a very different appearance than my desired effect, which includes both gradations in the color overlay as well as some blending with the base material underneath.

If I could somehow use ALPHA writes in my shader, but convince Godot to apply this pass before creating the screen texture it would solve my problem, if not quite as elegantly as incorporating the color overlay directly into the base material.

Calinou · February 28, 2025, 12:59am

You could migrate the heat haze effect to the compositor API, so that you can still use transparent materials:

arkaein · February 28, 2025, 4:42pm

Thanks, that’s sounds promising. I’ll try to give that a go today.

arkaein · February 28, 2025, 7:24pm

Well, I feel like an idiot. The compositor effect wasn’t going to be a great fit as i wanted to isolate my haze effect to specific meshes which may have other unhazed objects between them and the camera.

However I tried setting these objects render priority to a low negative number and ensuring that they hit the transparency pipeline, and the haze worked without conflict! I’m pretty sure I had tinkered with render priorities before, but may have got the ordering backwards and not realized that lower priorities come later.

Anyways, I believe my original problem is solved, if not exactly in the way I was thinking when I posted my original question. Appreciate thye suggestions in any case.