RayTracing Material AO not populated in Substrate blendable or disabled modes.

Development Asset Creation
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1

In RayTracingHitShaders.usf, on the GetMaterialPayload() function, MaterialAO is only correctly populated when taking the adaptive Substrate path, but missing on the Substrate disabled or blendable GBuffer path.

I fixed this issue locally like this:

/**
 * Set material attributes for full materials
 **/
Payload.TranslatedWorldPos = DFFastToTranslatedWorld(MaterialParameters.AbsoluteWorldPosition, ResolvedView.PreViewTranslation);
Payload.WorldNormal = WorldNormal;
Payload.Radiance = EmissiveColor;
Payload.BaseColor = BaseColor;
Payload.Specular = Specular;
Payload.Roughness = Roughness;
Payload.Metallic = Metallic;
Payload.GBufferAO = GetMaterialAmbientOcclusion(PixelMaterialInputs);

Notice the last line, which is what I added.

Ernesto.

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2

Steps to Reproduce
With Substrate disabled or set to blendable GBuffer format (SUBSTRATE_GBUFFER_FORMAT == 0), add an AO output to a Material and notice that Payload.GBufferAO is not populated (zero).

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3

Hi,

Let’s assume, we set it as you suggested, the issue is that GBufferAO is never actually stored. FMaterialClosestHitPayload is packed into a FPackedMaterialClosestHitPayload struct, which does not have space for storing GBufferAO. So despite being set, it the value will be invalid if you plan to use it as hit payload after a ray query.

/Charles.

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4

Indeed, good solution. I forgot we had some spare space there. Great that solved your issue!

/Charles.

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5

Thank you for the explanation Charles. I ended up packing it in the top 8 bits of FlagsAndMipBias, which is currently unused.

void SetMaterialAO(float InMaterialAO)
{
	FlagsAndMipBias = (FlagsAndMipBias & 0x00FFFFFF) | ((uint(round(saturate(InMaterialAO) * 255.0f)) & 0xFF) << 24);
}
float GetMaterialAO()
{
	return float((FlagsAndMipBias >> 24) & 0xFF) / 255.0f;
}

We’ll see how the quantization looks like. Might not be something you want to adopt, but we’re currently using GetGBufferDataFromPayload(), which assumes the value is populated.

Thanks again,

Ernesto.

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