ForwardDecal: Allow custom alpha mode and fix using with OIT #21909
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IceSentry
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Nov 22, 2025
| blend = None; | ||
| // TODO tail blending would need to return color in shader to do alpha blending | ||
| // Alpha blending is also needed by forward decals. | ||
| blend = Some(BlendState::ALPHA_BLENDING); |
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I'm a bit confused. This is only relevant when using forward decal with OIT right? But in pbr.wgsl you added a comment saying forward decal is incompatible with OIT.
Also, do we have a way to check here if this is for a forward decal or just a standard mesh?
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Forward decal is just a quad mesh and has no specific mesh key, so we can't check it here.
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Objective
Forward decal is not transparent unless I set the alpha mode of base material (StandardMaterial) to Blend. This is because the
MaterialExtension::alpha_modedoesn't override standard material flags, which is still follow theStandardMaterial::alpha_modeand is opaque and discards alpha by default.And ForwardDecalMaterialExt overrides alpha mode so it can only do alpha blending, which is an unnecessary restriction in my opinion. I think it makes sense to allow use alpha modes other than Blend for forward decals.
There is also an issue that if OIT enabled, forward decal will be wrongly culled by opaque objects.
Solution
Make forward decal doesn't override alpha mode, and doesn't enter the code path of
OIT_ENABLEDso it can do alpha blending.Disabled prepass for forward decal so it still works if alpha mode is Opaque or Mask (thought the depth fade factor won't have effect).
Disabled shadow for forward decal as it generally shouldn't cast shadows.
Testing
Run the decal example:
Change alpah mode to Add:
