Created
August 17, 2024 00:24
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Cursed compute shader issue in unity. Plop Ashl onto a game object, assign the compute shader in the inspector, and watch the console. You might need to save a scene and restart unity to see the red log messages!
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| //////////////////////////////////////////////////////////// | |
| // Ashl.cs | |
| using UnityEditor; | |
| using UnityEngine; | |
| [ExecuteAlways] | |
| public class Ashl : MonoBehaviour { // add this to a game object | |
| public ComputeShader cs; // assign the compute shader in the inspector | |
| private static GraphicsBuffer gpuBuffer; | |
| static uint frameid; | |
| void OnEnable() => SceneView.duringSceneGui += SceneViewOnduringSceneGui; | |
| void OnDisable() { SceneView.duringSceneGui -= SceneViewOnduringSceneGui; | |
| gpuBuffer?.Dispose(); gpuBuffer = null; | |
| } | |
| void SceneViewOnduringSceneGui( SceneView obj ) { | |
| // on every 20th GUI frame, run apply instead of preview | |
| if( Event.current.type == EventType.Repaint ) | |
| Dispatch( frameid++ % 20 == 0 ? BoxAction.Apply : BoxAction.Preview ); | |
| SceneView.RepaintAll(); // <-- just to make it re-trigger repaint like an update loop | |
| } | |
| void Dispatch( BoxAction action ) { | |
| gpuBuffer ??= new GraphicsBuffer( GraphicsBuffer.Target.Structured, 1024, 4 ); | |
| cs.SetBuffer( 0, "_gBufSelection", gpuBuffer ); | |
| cs.SetInt( "_BoxAction", (int)action ); | |
| cs.Dispatch( 0, 1024 / 64, 1, 1 ); // run the compute shader | |
| if( action == BoxAction.Apply ) { | |
| uint[] dataAll = new uint[gpuBuffer.count]; | |
| gpuBuffer.GetData( dataAll ); // downloads the data from the GPU | |
| uint data = dataAll[0]; | |
| // the data SHOULD be 0xFFFFFFFF at this point | |
| bool works = data == 0xFFFFFFFF; | |
| string color = works ? "ffffffaa" : "ff2222ff"; | |
| string msg = works ? "works fine so far" : "BUG HAPPENED"; | |
| Debug.Log( $"<color=#{color}>{msg}: {data:X}</color>" ); | |
| } | |
| } | |
| enum BoxAction { Preview = 1, Apply = 2 } | |
| } | |
| //////////////////////////////////////////////////////////// | |
| // shader.compute | |
| #pragma kernel CSMain | |
| #pragma exclude_renderers d3d11_9x | |
| #pragma exclude_renderers d3d9 | |
| uint _BoxAction; | |
| RWStructuredBuffer<uint> _gBufSelection; | |
| #define BOX_APPLY 2 | |
| [numthreads(64,1,1)] | |
| void CSMain(const uint chunkId : SV_DispatchThreadID) { | |
| uint dims, stride; | |
| _gBufSelection.GetDimensions(dims, stride); | |
| if (chunkId >= dims) | |
| return; | |
| if (chunkId < 10) { | |
| if (_BoxAction == BOX_APPLY) | |
| _gBufSelection[chunkId] = 0xFFFFFFFF; | |
| else | |
| _gBufSelection[chunkId] = 0xAAAAAAAA; | |
| } else | |
| _gBufSelection[chunkId] = 0; | |
| } |
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My testing:
So my guess is that yes, this is an actual Unity bug that started happening in 2018, probably because of some sort of DX11 specific optimization that tries to reuse constant buffers or whatever, and occasionally fails.
All the replies on the internet suggesting that "GetData probably needs some sort of flush call from you, and/or you should use async readback" are mostly cargo culting. This seems very much just like a bug within Unity.