Unity Compute Shader Realtime Ray Tracing

This is a Unity/HLSL port of the excellent Annotated Realtime Raytracing blog post by Kevin Fung.

The original GLSL shader runs live in a web browser on ShaderToy. Unity looks the same but at 138fps (Intel HD GPU).

Porting GLSL to HLSL

This was much easier than I thought it would be. Here are the changes I made:

• Added HLSL kernel (CSMain) that calls the GLSL fragment shader (mainImage)
• Added iGlobalTime and iResolution shader variables
• Replaced vec2 with float2
• Replaced vec3 with float3
• Replaced vec4 with float4
• Expand constructors with 1 argument to n, e.g. vec3(1.0) -> float3(1.0,1.0,1.0)
• Change const to static const
• Change struct initializer syntax, e.g. Material(vec3(0.0,0.2,1.0),1.0,0.0) -> { float3(0.0,0.2,1.0),1.0,0.0 }

Unity Compute Shaders

I learnt about Unity Compute Shaders from Coxlin's Blog. This article just multiplies 4 integers by 2.0 in a Computer Shader and prints the results to the console. Everything you need to know to get started.

Understanding the relationship between the [numthreads(x,y,z)] attribute in the shader, and the shader.Dispatch(kernelIndex, gx,gy,gz) call in C#, is fundamental. The kernel CSMain is executed (x*y*z) * (gx*gy*gz) times and the id passed into the kernel ranges from (0,0,0) to (x*gx, y*gy, z*gz).

Here is how the kernel is evaluated for each thread in pseudocode:

        const int numThreadsX, numThreadsY, numThreadsZ;

        static void kernel(int x, int y, int z) { }

        static void Dispatch(int threadGroupsX, int threadGroupsY, int threadGroupsZ) {
            for (var gx = 0; gx < threadGroupsX; gx++) {
                for (var gy = 0; gy < threadGroupsY; gy++) {
                    for (var gz = 0; gz < threadGroupsZ; gz++) {

                        // Dispatch group
                        for (var x = 0; x < numThreadsX; x++) {
                            for (var y = 0; y < numThreadsY; y++) {
                                for (var z = 0; z < numThreadsZ; z++) {
                                    kernel(
                                        gx * numThreadsX + x,
                                        gy * numThreadsY + y,
                                        gz * numThreadsZ + z
                                    );
                                }
                            }
                        }
                    }
                }
            }
        }