Reading output buffer - Compute Shaders

Development Rendering
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Like many people trying to get into compute shaders with Unreal, start with the excellent demo of Temeran: GitHub - Temaran/UE4ShaderPluginDemo: A tutorial project that shows how to implement HLSL Pixel and Compute shaders in UE4

However, for my project I’m more interested in having a data array as an output rather than a render target such as a texture. Furthermore, I also require a texture as input. So I’ve made some attempts at altering the demo project to fit these goals, but I’m getting stuck at finding the right manner of reading out the data again using structured buffers. So ultimately, somewhere in my game thread I would like to receive the output of the compute shader in some tarray structure. Here is my adjustments to the original demo code so far:

ComputeShaderExample.cpp:

...

	BEGIN_SHADER_PARAMETER_STRUCT(FParameters, )
		SHADER_PARAMETER_TEXTURE(Texture2D<FVector4>, InputTexture)
                ...
		SHADER_PARAMETER_UAV(RWStructuredBuffer<FVector4>, OutputBuffer)
	END_SHADER_PARAMETER_STRUCT()

...

void FComputeShaderExample::RunComputeShader_RenderThread(FRHICommandListImmediate& RHICmdList, const FShaderUsageExampleParameters& DrawParameters, FTextureRHIRef inputTexture, FUnorderedAccessViewRHIRef outputBuffer_UAV)
{
	QUICK_SCOPE_CYCLE_COUNTER(STAT_ShaderPlugin_ComputeShader); // Used to gather CPU profiling data for the UE4 session frontend
	SCOPED_DRAW_EVENT(RHICmdList, ShaderPlugin_Compute); // Used to profile GPU activity and add metadata to be consumed by for example RenderDoc

	UnbindRenderTargets(RHICmdList);
	
	FComputeShaderExampleCS::FParameters PassParameters;
	PassParameters.InputTexture = inputTexture;
        ...
	PassParameters.OutputBuffer = outputBuffer_UAV;

	TShaderMapRef<FComputeShaderExampleCS> ComputeShader(GetGlobalShaderMap(GMaxRHIFeatureLevel));
	FComputeShaderUtils::Dispatch(RHICmdList, *ComputeShader, PassParameters, 
								FIntVector(FMath::DivideAndRoundUp(DrawParameters.GetRenderTargetSize().X, NUM_THREADS_PER_GROUP_DIMENSION),
										   FMath::DivideAndRoundUp(DrawParameters.GetRenderTargetSize().Y, NUM_THREADS_PER_GROUP_DIMENSION), 1));
}

ShaderDeclarationDemoModule.cpp:

void FShaderDeclarationDemoModule::Draw_RenderThread(const FShaderUsageExampleParameters& DrawParameters)
{
	check(IsInRenderingThread());

	if (!DrawParameters.RenderTarget)
	{
		return;
	}

	FRHICommandListImmediate& RHICmdList = GRHICommandList.GetImmediateCommandList();

	QUICK_SCOPE_CYCLE_COUNTER(STAT_ShaderPlugin_Render); // Used to gather CPU profiling data for the UE4 session frontend
	SCOPED_DRAW_EVENT(RHICmdList, ShaderPlugin_Render); // Used to profile GPU activity and add metadata to be consumed by for example RenderDoc

	OutputBuffer_array_.SetNum(32);
	OutputBuffer_resource_.ResourceArray = &OutputBuffer_array_;
	OutputBuffer_buffer_ = RHICreateStructuredBuffer(sizeof(float), sizeof(FVector4) * 32, BUF_ShaderResource | BUF_UnorderedAccess, OutputBuffer_resource_);
	OutputBuffer_UAV_ = RHICreateUnorderedAccessView(OutputBuffer_buffer_, /* bool bUseUAVCounter */ false, /* bool bAppendBuffer */ false);


	FComputeShaderExample::RunComputeShader_RenderThread(RHICmdList, DrawParameters, DrawParameters.RenderTarget->GetRenderTargetResource()->TextureRHI, OutputBuffer_UAV_);
}

ShaderDeclarationDemoModule.h:

struct FShaderUsageExampleParameters
{
	UTextureRenderTarget2D* RenderTarget;
        ...
	TArray<FVector4> OutputBuffer;

	FIntPoint GetRenderTargetSize() const
	{
		return CachedRenderTargetSize;
	}

	FShaderUsageExampleParameters()	{ }
	FShaderUsageExampleParameters(UTextureRenderTarget2D* InRenderTarget, 
		TArray<FVector4> OutputBuffer)
		: RenderTarget(InRenderTarget),
		  OutputBuffer(OutputBuffer),
		  ...
	{
		CachedRenderTargetSize = InRenderTargetDepth ? FIntPoint(InRenderTarget->SizeX, InRenderTarget->SizeY) : FIntPoint::ZeroValue;
	}

...

private:
	TResourceArray<FVector4> OutputBuffer_array_;
	FRHIResourceCreateInfo OutputBuffer_resource_;
	FStructuredBufferRHIRef OutputBuffer_buffer_;
	FUnorderedAccessViewRHIRef OutputBuffer_UAV_;

ShaderUsageDemoCharacter.cpp

AShaderUsageDemoCharacter::AShaderUsageDemoCharacter()
{
	...
	capture_2D_ = CreateDefaultSubobject<USceneCaptureComponent2D>(TEXT("CaptureComp"));
}

void AShaderUsageDemoCharacter::PostInitializeComponents()
{
	Super::PostInitializeComponents();
	render_target_2D_ = NewObject<UTextureRenderTarget2D>();
	capture_2D_->SetRelativeRotation(FRotator(0, 0, 0));
	capture_2D_->SetRelativeLocation(FVector(0, 0, 0));
	capture_2D_->AttachTo(this->RootComponent);
	capture_2D_->CaptureSource = ESceneCaptureSource::SCS_FinalColorLDR;
	render_target_2D_->InitCustomFormat(1024, 1024, PF_B8G8R8A8, true);
	capture_2D_->TextureTarget = render_target_2D_;
	capture_2D_->bAlwaysPersistRenderingState = true;
	capture_2D_->bCaptureEveryFrame = false;
	capture_2D_->bCaptureOnMovement = false;
	capture_2D_->bUseCustomProjectionMatrix = false;
}
...
void AShaderUsageDemoCharacter::Tick(float DeltaSeconds)
	...
	capture_2D_->CaptureScene();
	FShaderUsageExampleParameters DrawParameters(render_target_2D_,  output_buffer_);
	{
		...
	}
	FShaderDeclarationDemoModule::Get().UpdateParameters(DrawParameters);

	// HERE I WOULD LIKE TO READ OUT THE DATA BUFFER OF THE SHADER

ShaderUsageDemoCharacter.h

…

.
public:
	UPROPERTY(BlueprintReadWrite, EditAnywhere)
	class USceneCaptureComponent2D* capture_2D_;

	UPROPERTY(BlueprintReadWrite, EditAnywhere)
	class UTextureRenderTarget2D* render_target_2D_;

	TArray<FVector4> output_buffer_;