ShaderGraph with premultiply blending is not really premultiplied

Hello,

We have been struggling to get premultiplied effects done in ShaderGraph to work, and it turns out the problem is in Universal Render Pipeline package.

I looked into the com.unity.render-pipelines.universal@7.4.1\Editor\ShaderGraph\Includes\UnlitPass.hlsl, and found this:

Code (CSharp):

1. half4 frag(PackedVaryings packedInput) : SV_TARGET
2. {  
3.     Varyings unpacked = UnpackVaryings(packedInput);
4.     UNITY_SETUP_INSTANCE_ID(unpacked);
5.     UNITY_SETUP_STEREO_EYE_INDEX_POST_VERTEX(unpacked);
6.  
7.     SurfaceDescriptionInputs surfaceDescriptionInputs = BuildSurfaceDescriptionInputs(unpacked);
8.     SurfaceDescription surfaceDescription = SurfaceDescriptionFunction(surfaceDescriptionInputs);
9.  
10. #if _AlphaClip
11.     clip(surfaceDescription.Alpha - surfaceDescription.AlphaClipThreshold);
12. #endif
13.  
14. #ifdef _ALPHAPREMULTIPLY_ON
15.     surfaceDescription.Color *= surfaceDescription.Alpha;
16. #endif
17.  
18.     return half4(surfaceDescription.Color, surfaceDescription.Alpha);
19. }

Apparently when in premultiply mode, the generated shader does the multiplying for us. This is practically same as alpha-blended mode. Only difference is that blending mode in the shader is different, but from the user's perspective the ShaderGraph shader acts like alpha-blended shader.

If I comment out this:

Code (CSharp):

1. #ifdef _ALPHAPREMULTIPLY_ON
2.     surfaceDescription.Color *= surfaceDescription.Alpha;
3. #endif

it works as expected.

This is clearly a bug, and should be fixed.

However, as this has been broken for a long time now, makes me wonder if there is some fundamental issue to be solved...

 

We solved this as clean as we could by creating a forked copy of URP package with the fix, and switching that in place in manifest.json

But it would be great to have an official confirmation that the thing is broken.