[UPROPERTY](Programming/UnrealArchitecture/Reference/Properties)(EditAnywhere, BlueprintReadWrite, Category="PNG")
bool bOutputAlpha
Should we accumulate the alpha channel and write it into the resulting image? This requires r.PostProcessing.PropagateAlpha to be set to 1 or 2 (see "Enable Alpha Channel Support in Post Processing" under Project Settings > Rendering). This adds ~30% cost to the accumulation so you should not enable it unless necessary. You must delete both the sky and fog to ensure that they do not make all pixels opaque.