UE5.1 PostProcess Custom Shader 100x slowdown (Kuwahara Filter)

Development Rendering
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1

I am running a PostProcess material that invokes HLSL code which involves a nested for-loop doing scene texture lookups.

 for (int i = 0; i < 4; i++)
        {
            for (int j = 0; j <= Radius.x; j++)
            {
                for (int k = 0; k <= Radius.y; k++)
                {
                    [stuff]

                    col = SceneTextureLookup(uvPos, 14, false);
                    [stuff]

                }
            }
        }

In Unreal 5.0.3 this material cost me about 0.5ms. In 5.1 it is costing me over 80ms, with no apparent changes to the material. I verified SM6 DX12 and Vulkan. Clearly there is a change in the rendering pipeline that is affecting this, but I could not find an obvious culprit in the changelog.

Does anybody know what might have changed?

Only other thing of note is that I am using ViewportUVToSceneTextureUV(UV,14) in a separate custom node…other than that, the material itself is straightforward.

1 Like
2

They recently did some changes to the shader code for Unreal Engine Version 5.1. I am not sure if I know the full reason for why you are experiencing these issues, all I know is that the default shading model is set to 6.

Here are the release notes: Unreal Engine 5.1 Release Notes | Unreal Engine 5.1 Documentation. I would sugget that you perhaps go through all relevant changes, to see what may have changed, and then alter your shader in accordance.

3

Hi, thanks for your reply. Yes, I confirmed the behavior in SM6 and in Vulkan, so it seems that just the switch to SM6 would not explain it. I did go through the changelog but it was not granular enough to show me actual API differences in the RHI, so now I’m looking through Github and taking a look at differences in frames between 5.0.3 and 5.1 in RenderDoc.

1 Like
4

Oh god, I’m sorry you have to do that… please let me know if you figure it out.

Hopefully someone else are able to point out the reason why it is happening.

5

Hey!
I experience the same issue with a directional Kuwahara shader!
To be honest, I was quite happy to find this topic and to see that this seems to be in issue with SM6 - i though my shader implementation sucks big time! :slight_smile:

Anything came up about this issue yet?
I switched back to SM5 but get warnings that Nanite and Virtal Shadow Maps won’t work unless I switch to SM6.
With SM6, my PPM performance drops and is unusable.

6

5.1.1 still got the same problem!
Hope they fix this soon - it’s unusable