RDG - Rendering Material to Render Target Doesn't Give Correct Alpha Channel

[Sweenus987 (Ryan_Sweeney987)]

I’m rendering a material to a render target on the render thread, the material renders fine except there are no alpha channel values other than 0. I’ve tried setting the material to be a Post Process material with alpha out enabled and I’ve tried Surface > Unlit > Alpha Composite Blend/Translucent/Masked and no luck.

The render target format is set as RGBA16f so the RT has alpha enabled.

Does anyone have any idea? I’ve seen others with a similar issue but using the BP function to do it.

To render on the render thread I use the following code. It’s run inside a Scene View Extension function and is based on the UKismetRenderingLibrary::DrawMaterialToRenderTarget() function.

// Get the material
UMaterialInterface* MaterialInterface = Resource->GetMaterial();
UMaterial* Material = MaterialInterface->GetMaterial();

// Ensure it's compiled
Material->EnsureIsComplete()

const FMaterialRenderProxy* MaterialRenderProxy = MaterialInterface->GetRenderProxy();
		
FRDGTextureRef TemporaryTexture = Resource->CreateTemporaryTexture(GraphBuilder);
FVector2D TempTextureSize = FVector2D(TemporaryTexture->Desc.Extent.X, TemporaryTexture->Desc.Extent.Y);
FScreenPassRenderTarget TemporaryTextureRenderTarget = FScreenPassRenderTarget(TemporaryTexture, ERenderTargetLoadAction::ENoAction);

check(TemporaryTextureRenderTarget.IsValid());

// Create the tile item used for rendering the material
FCanvasTileItem TileItem(FVector2D::ZeroVector, MaterialRenderProxy, TempTextureSize);

const FSceneViewFamily& ViewFamily = *View.Family;

FCanvas& RenderCanvas = *FCanvas::Create(GraphBuilder, TemporaryTextureRenderTarget.Texture, nullptr, ViewFamily.Time, View.GetFeatureLevel());
RenderCanvas.SetRenderTargetRect(TemporaryTextureRenderTarget.ViewRect);
RenderCanvas.DrawItem(TileItem);
RenderCanvas.Flush_RenderThread(GraphBuilder);

// Copy the texture to the render target
URenderTargetProxy* OutRenderTarget = Resource->GetRenderTarget();
FRDGTextureRef OutRenderTargetTexture = Resource->GetPooledTextureRef();
AddCopyTexturePass(GraphBuilder, TemporaryTexture, OutRenderTargetTexture);

[RecourseDesign]

Hi, Does the Material use “Pre-Multiply Alpha” mode, and you’re plugging in 1-alpha to the alpha?

[Sweenus987 (Ryan_Sweeney987)]

Yup, this is how the material output is currently set up:

In the render target, the result I get is just the green outline but the alpha channel is then all 0.

Also, I’m using 5.3 for this, in case that makes any difference

[RecourseDesign]

Before doing the Draw, Clear the render target to 0,0,0,1 - and add a “1-” between the Mask A and the Opacity - that should get it working.

[Sweenus987 (Ryan_Sweeney987)]

That did the job, thank you!!!