# Output Key/Fill without Decklink Card



## tobisk (Jan 4, 2021)

I am using OBS to play out Graphics to a Video Mixer (ATEM Mini). At the moment, I am outputting via one HDMI and using either Luma key (where possible) or Chroma Key. However I just ran into the issue, that I wanted semi-transparent sections of an image that contains black and white, so neither chroma nor luma key is usable. What I already tried is outputting a key and a fill image both via HDMI, just as full-screen images and it worked on the Video Mixer, however I want OBS to do this (the images just were desktop backgrounds).

So what I want to do is OBS to output two full-screen-projectors of the program output over two HDMI ports, however, one of them has to be the Fill (regular output) and one has to be the key (the transparency information).

I know that there is a decklink duo, but as the budget is tight and the ATEM mini only has HDMI ins (so no SDI possible here and Converters are not an option), the best option would be to actually use multiple ports on the Graphics card (Having 4 of them, each can output Video, also already tried outputting 4 Full screen projectors of scenes, which the PC could handle, however I then have no Transitions and have to manually create the key/fill videos instead of just rendering them from my Editing software)


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## loganawilliams (Apr 3, 2021)

I literally wanted this exact same thing. I would love somebody to come up with the functionality, but I did come up with a temporary work around.

If you use the NDI Plugin (https://obsproject.com/forum/resources/obs-ndi-newtek-ndi™-integration-into-obs-studio.528/) you have alpha output support from OBS. However, you have to then loop this back into OBS to send the alpha channel as white for the Atem to read.

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Turn on your Main Output in the NDI Output Tool.

Now this is where we get creative. Our alpha is exporting to NDI, but now we need create two Scenes — one that will be the projector for the Clean Output and one that will be the Alpha Channel Output.

Under each of these scenes add in the NDI source of your OBS NDI Output. You'll want to make them different sources, so you can add filters on one. Remember hardware acceleration and low latency in the settings, since NDI can add latency.

On the NDI Source on the Alpha Channel Scene add the Color Correction filter and turn the brightness all the way up. Now you have your Key to output to the Atem.

As I mention above, I recommend using NDI on both projectors, so the alpha channel stays in sync with the projector or else you will have ghosting.

Let me know if you have any questions.


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## Meierhans (Apr 4, 2021)

If you just want output the alpha channel of a source as BW to an outputs (lets say one of the four virtual cams), you can add a source mirror to a group with a solid black layer below, then tweak Color Correction to wipe out all RGB data, thus leaving only alpha channel. Pretty much the same way we create blurred drop shadows from a source, just in white. It might not me 100% accurate, but serves the purpose.


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