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Calculate PI via compute in UE5

This example approximates PI in parallel using the Monte Carlo method. When running you’re only going to be able to get about 2-3 decimal places of accuracy.

Get the code

$ shadeup-unreal
 > [COMPUTE] Compute Shader
 > PI

Usage

1. Generate or download the template using the instructions above
2. Rebuild your project
3. Create or navigate to an existing Blueprint
4. Right-click and add the node called “Execute PICompute Shader”

Notes

• The mechanism used for detecting when the results are ready on the CPU is for example purposes (see F[NAME]Interface::DispatchRenderThread - RunnerFunc). A better strategy would be to queue and check a list of buffers per tick in an external singleton.

Helpful reading

• Using Compute Shaders in Unreal Engine 4
• Overview of Shaders in Plugins
• UE4ShaderPluginDemo
• Render Dependency Graph

Sample snippet

// pi.usf

#include "/Engine/Public/Platform.ush"

// Calculate π using the monte carlo method
// https://en.wikipedia.org/wiki/Monte_Carlo_method

// Output has 1 element: [numInCircle]
RWBuffer<uint> Output;

// Seed the random number generator
float Seed = 0.0f;

float random( float2 p )
{
    float2 r = float2(
        23.14069263277926,
         2.665144142690225
    );
    return frac( cos( dot(p,r) ) * 12345.6789 );
}

[numthreads(THREADS_X, THREADS_Y, THREADS_Z)]
void PIExample(
	uint3 DispatchThreadId : SV_DispatchThreadID,
	uint GroupIndex : SV_GroupIndex )
{
	float2 RandomPosition = float2(
		random(float2(GroupIndex * THREADS_X + DispatchThreadId.x, 10 + Seed)),
		random(float2(GroupIndex * THREADS_X + DispatchThreadId.x, 20 * Seed))
	);
	float Dist = length(RandomPosition);

	if (Dist < 1.0f) {
		uint Dummy;
		InterlockedAdd(Output[0], 1, Dummy);
	}
}