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Bug "Clip" function fails on raytracing passes with "Opcode Discard not valid in shader model lib_6_3"

Discussion in 'HDRP Ray Tracing' started by merpheus, Oct 13, 2021.

  1. merpheus


    Mar 5, 2013
    I am trying to use the clip function in raytracing passes (not via shadergraph but with regular code) but I get "Opcode Discard not valid in shader model lib_6_3(anyhint)" message(there are variants for closehit too, the discard code works with regular raster passes. It gives this error on raytracing.

    If clipping/discard is not supported on raytracing shaders, is there a way to do clip other than using this?
  2. INedelcu


    Unity Technologies

    Jul 14, 2015
    clip and discard are rasterization instruction available in pixel shaders only.

    You can get a similar functionality in ray tracing using anyhit shaders where alpha can come from a texture. Anyhit shaders are usually used for alpha tested / cutout Materials. If you use RayTracingAccelerationStructure.AddInstance functions you'll need to specify just RayTracingSubMeshFlags.Enable - this will enable anyhit shaders.

    Some example:

    _Cutoff and _MainTex are shader properties.

    void AnyHitMain(inout RayPayload payload : SV_RayPayload, AttributeData attribs : SV_IntersectionAttributes)
    uint3 triangleIndices = UnityRayTracingFetchTriangleIndices(PrimitiveIndex());
    Vertex v0, v1, v2;
    v0 = FetchVertex(triangleIndices.x);
    v1 = FetchVertex(triangleIndices.y);
    v2 = FetchVertex(triangleIndices.z);
    float3 barycentricCoords = float3(1.0 - attribs.barycentrics.x - attribs.barycentrics.y, attribs.barycentrics.x, attribs.barycentrics.y);
    Vertex v = InterpolateVertices(v0, v1, v2, barycentricCoords);
    float alpha = _MainTex.SampleLevel(sampler__MainTex, _MainTex_ST.xy * v.uv +, 0).w;
    if (alpha < _Cutoff)