I’m testing outputting fill and key to a Black Magic Decklink 8k Pro. I’ve created a simple scene with a sprite created from a png containing soft-edged shapes. The source RGBA, RGB and A are shown below.
I also added a cube, as it’s handy to see how the sprite looks when it intersects another object.
I’m using the Composure Compositing plugin to output to the Decklink. Initially I setup the Decklink output 1 as a Fill and Key pair and created a Comp with CG layer that outputted RGBA directly to the Decklink. The matte looked fine, but the fill looked like it was pre-multiplied with black. I’d expect to see hard edges on the fill output, similar to the source RGB channels of the PNG. No fading to black at the edges. If the fill and key were composited in a vision mixer, you would see black halos around the soft edges.
I then dug a bit deeper and created 2 Comps for separate fill and key and used a Compositiing Element Material Pass to send just the RGB channels to the fill comp and A channel to the key comp. I then outputted the fill to output 1 of the Decklink and the key to 2. The result was identical. The soft edges of the sprite graphic faded black.
I then looked at the RGBA channels in the preview window. It does look a little like there are black halos around the soft edges. Do I have to tell Unreal to interpret the png source as a straight fill?
I then went on and tried creating a emissive material for the Cube and added a PostProcessVolume object with Bloom enabled. I turned on “Use Emissive for Static Lighting” and increased Emissive Boost to 5 and baked the lighting. This worked great in the editor:
But wasn’t visible at all in the comp (either using split fill and key or RGBA). I realised that I had to add the bloom as a post process in the comp settings. I could then see the bloom on the RGB channels, but it was cut off by the alpha channel:
I could do with a bit of advise of how best to setup Unreal to handle straight matte and fill on the Decklink with proper handling of emissive materials, compositing semi-transparent materials etc. Thanks!