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Using compute shadersΒΆ

This tutorial will walk you through the process of creating a minimal compute shader. But first, a bit of background on compute shaders and how they work with Godot.

Note

This tutorial assumes you are familiar with shaders generally. If you are new to shaders please read Introduction to shaders and your first shader before proceeding with this tutorial.

A compute shader is a special type of shader program that is orientated towards general purpose programming. In other words, they are more flexible than vertex shaders and fragment shaders as they don't have a fixed purpose (i.e. transforming vertices or writing colors to an image). Unlike fragment shaders and vertex shaders, compute shaders have very little going on behind the scenes. The code you write is what the GPU runs and very little else. This can make them a very useful tool to offload heavy calculations to the GPU.

Now let's get started by creating a short compute shader.

First, in the external text editor of your choice, create a new file called compute_example.glsl in your project folder. When you write compute shaders in Godot, you write them in GLSL directly. The Godot shader language is based on GLSL. If you are familiar with normal shaders in Godot, the syntax below will look somewhat familiar.

Note

Compute shaders can only be used from RenderingDevice-based renderers (the Forward+ or Mobile renderer). To follow along with this tutorial, ensure that you are using the Forward+ or Mobile renderer. The setting for which is located in the top right-hand corner of the editor.

Note that compute shader support is generally poor on mobile devices (due to driver bugs), even if they are technically supported.

Let's take a look at this compute shader code:

#[compute]
#version 450

// Invocations in the (x, y, z) dimension
layout(local_size_x = 2, local_size_y = 1, local_size_z = 1) in;

// A binding to the buffer we create in our script
layout(set = 0, binding = 0, std430) restrict buffer MyDataBuffer {
    float data[];
}
my_data_buffer;

// The code we want to execute in each invocation
void main() {
    // gl_GlobalInvocationID.x uniquely identifies this invocation across all work groups
    my_data_buffer.data[gl_GlobalInvocationID.x] *= 2.0;
}

This code takes an array of floats, multiplies each element by 2 and store the results back in the buffer array. Now let's look at it line-by-line.

#[compute]
#version 450

These two lines communicate two things:

  1. The following code is a compute shader. This is a Godot-specific hint that is needed for the editor to properly import the shader file.

  2. The code is using GLSL version 450.

You should never have to change these two lines for your custom compute shaders.

// Invocations in the (x, y, z) dimension
layout(local_size_x = 2, local_size_y = 1, local_size_z = 1) in;

Next, we communicate the number of invocations to be used in each workgroup. Invocations are instances of the shader that are running within the same workgroup. When we launch a compute shader from the CPU, we tell it how many workgroups to run. Workgroups run in parallel to each other. While running one workgroup, you cannot access information in another workgroup. However, invocations in the same workgroup can have some limited access to other invocations.

Think about workgroups and invocations as a giant nested for loop.

for (int x = 0; x < workgroup_size_x; x++) {
  for (int y = 0; y < workgroup_size_y;