Uncompressed GetViewportScreenShot

Is there a method of getting using GetViewportScreenShot to get an uncompressed image? Right now I get blur around the actors.

UGameViewportClient* gameViewport = GEngine->GameViewport;
FViewport* inViewport = gameViewport->Viewport;
TArray<FColor> data;
FIntRect Rect(0, 0, inViewport->GetSizeXY().X, inViewport->GetSizeXY().Y);
GetViewportScreenShot(inViewport, data, Rect);

What I’m trying to do is add backgrounds in post processing. To do this I’ve set removed all background stuff so that the screenshot should show a value of 0,0,0,0. What I get around an object or between is something like this 6, 4, 8, 0. Using a rendertarget to build a 2D texture works without this blur but I can’t use it due to the large amount of caching that it does.