Development notes, Reusable code

Compute shaders and Camera.Render in Unity: workaround for re-ordering

Recently working on the 1.3 release of my Panorama Capture script I encountered a mysterious and difficult-to-reproduce issue on certain older GPUs such as NVIDIA GTX 675M where I made a sequence of calls to Camera.Render() and ComputeShader.Dispatch(), and Unity would blithely re-order them without regard for the read/write dependencies between them, resulting in very strange panorama images like this:

distorted

The function of the compute shader was to take the result of the rendering and store it in a compute buffer, so that the RenderTexture could then be re-used. This is roughly what my code looked like:

renderTexture = new RenderTexture(width, height);
ComputeBuffer computeBuffer = new ComputeBuffer(10 * width * height, 4);
copyShader.SetTexture(kernelIdx, "source", renderTexture);
copyShader.SetBuffer(kernelIdx, "result", computeBuffer);
cam.targetTexture = renderTexture;
for (int i=0; i < 10; i++) {
  // Set cam.transform.position/rotation based on i
  camera.Render();
  copyShader.SetInt("startIdx", i * width * height);
  copyShader.Dispatch(kernelIdx, (width  + threadsX - 1) / threadsX,
                                 (height + threadsY - 1) / threadsY, 1);
}
pixels = new uint[10 * width * height];
computeBuffer.GetData(pixels);

The goal is to render 10 images and copy them all into computeBuffer in order. But on some GPUs, the Render() and Dispatch() calls are done out-of-order – sometimes all the Render() calls are done before the Dispatch() calls, resulting in all 10 images in the computeBuffer being identical. Other times, the Dispatch() calls are done just one iteration early or late, shifting the resulting images up or down in the buffer, or resulting in duplication of certain images. I don’t know whether this is a Unity bug or a GPU memory model limitation, but I needed to find a workaround.

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