Can’t figure out how to read back data from an AppendStructuredBuffer in a compute shader in UE5.2.
I have this simple shader which fills an AppendStructuredBuffer with 1s:
AppendStructuredBuffer<int> OutputAppendBuffer;
[numthreads(THREADGROUPSIZE_X, THREADGROUPSIZE_Y, THREADGROUPSIZE_Z)]
void Main(uint3 ID : SV_DispatchThreadID)
{
OutputAppendBuffer.Append(1);
}
Here is the declaration in C++:
class COMPUTESHADERDECLARATIONS_API FAppendBufferTestCSDeclaration : public FGlobalShader
{
public:
DECLARE_GLOBAL_SHADER(FAppendBufferTestCSDeclaration);
SHADER_USE_PARAMETER_STRUCT(FAppendBufferTestCSDeclaration, FGlobalShader);
BEGIN_SHADER_PARAMETER_STRUCT(FParameters, )
SHADER_PARAMETER_RDG_BUFFER_UAV(AppendStructuredBuffer<int32>, OutputAppendBuffer)
END_SHADER_PARAMETER_STRUCT()
public:
static bool ShouldCompilePermutation(const FGlobalShaderPermutationParameters& Parameters)
{
return IsFeatureLevelSupported(Parameters.Platform, ERHIFeatureLevel::SM5);
}
static inline void ModifyCompilationEnvironment(const FGlobalShaderPermutationParameters& Parameters, FShaderCompilerEnvironment& OutEnvironment)
{
FGlobalShader::ModifyCompilationEnvironment(Parameters, OutEnvironment);
OutEnvironment.SetDefine(TEXT("THREADGROUPSIZE_X"), THREADGROUPSIZE_X);
OutEnvironment.SetDefine(TEXT("THREADGROUPSIZE_Y"), THREADGROUPSIZE_Y);
OutEnvironment.SetDefine(TEXT("THREADGROUPSIZE_Z"), THREADGROUPSIZE_Z);
}
};
I then dispatch the shader and try to read back the data like so:
check(IsInRenderingThread())
FRDGBuilder GraphBuilder(RHICmdList);
// Get shader reference
TShaderMapRef<FAppendBufferTestCSDeclaration> ComputeShader(GetGlobalShaderMap(GMaxRHIFeatureLevel));
if (ComputeShader.IsValid())
{
FAppendBufferTestCSDeclaration::FParameters* PassParameters = GraphBuilder.AllocParameters<FAppendBufferTestCSDeclaration::FParameters>();
// Hard coded group count for testing purposes.
FIntVector GroupCount = {1, 1, 1};
uint32 NumDispatchedThreads = GroupCount.X * GroupCount.Y * GroupCount.Z * THREADGROUPSIZE_X * THREADGROUPSIZE_Y * THREADGROUPSIZE_Z;
FRDGBufferRef OutputAppendBufferRef = GraphBuilder.CreateBuffer(
FRDGBufferDesc::CreateBufferDesc(sizeof(int32), NumDispatchedThreads),
TEXT("OutputAppendBuffer"),
ERDGBufferFlags::None
);
PassParameters->OutputAppendBuffer = GraphBuilder.CreateUAV(FRDGBufferUAVDesc(OutputAppendBufferRef));
GraphBuilder.AddPass(
RDG_EVENT_NAME("ExecuteAppendBufferTestCS"),
PassParameters,
ERDGPassFlags::AsyncCompute,
[&PassParameters, ComputeShader, GroupCount](FRHIComputeCommandList& RHICmdList)
{
FComputeShaderUtils::Dispatch(RHICmdList, ComputeShader, *PassParameters, GroupCount);
}
);
TRefCountPtr<FRDGPooledBuffer> PooledAppendBuffer;
GraphBuilder.QueueBufferExtraction(OutputAppendBufferRef, &PooledAppendBuffer, ERHIAccess::CPURead);
GraphBuilder.Execute();
// Read data
FRHIBuffer* AppendBuffer = PooledAppendBuffer->GetRHI();
int32* AppendBufferData = (int32*)RHILockBuffer(AppendBuffer, 0, sizeof(int32) * NumDispatchedThreads, RLM_ReadOnly);
// Probably unnecessary copy of data
int32* AppendBufferDataCopy = new int32[sizeof(int32) * NumDispatchedThreads];
FMemory::Memcpy(AppendBufferDataCopy, AppendBufferData, sizeof(int32) * NumDispatchedThreads);
for (uint32 i = 0; i < NumDispatchedThreads; i++)
{
UE_LOG(LogTemp, Display, TEXT("Output data : %d"), AppendBufferDataCopy[i])
}
delete[] AppendBufferDataCopy;
RHIUnlockBuffer(AppendBuffer);
}
else
{
UE_LOG(LogTemp, Warning, TEXT("Shader reference of 'FAppendDataTestCSDeclaration' is invalid."))
}
I dispatch one thread group of 8 threads, and this is the output:
LogTemp: Display: Output data : 1
LogTemp: Display: Output data : 15
LogTemp: Display: Output data : 27
LogTemp: Display: Output data : 40
LogTemp: Display: Output data : 43
LogTemp: Display: Output data : 44
LogTemp: Display: Output data : 45
LogTemp: Display: Output data : 46
It doesn’t matter what I fill the buffer with, only the first element will be correct. The other elements are always filled with the same values like above. If I use a RWStructuredBuffer instead everything works like expected.
Any help very much appreciated!