The new alpha channel support over HD-SDI, means that you can now output, for instance, a transparent lower third graphics from one vMix system and take it in as an input on another vMix system. This is possible only in vMix 21 and was initially added in the beta version 184.108.40.206.
The vMix documentation for this can be found here:
The instructions, however, is not very elaborate on how to take the output and using it as an input on another vMix system.
First, set up the output using vMix with one of these capture cards:
-Decklink 4K Extreme / Extreme 12G / Pro
-Decklink Duo 2 / Quad 2 (when Port 1 is configured to Port 1 + 3 mode in Desktop Video Utility)
-UltraStudio 4K / 4K Extreme / 4K Extreme 3 / HD Mini
In this example we will be using a Blackmagic Decklink Duo 2 to output and a Blackmagic Quad 2 to take the input.
Using Blackgmagic Desktop Video Setup utility, change the Connector Mapping on the first input to SDI 1 & SDI 2.
In vMix select the first Blackmagic Device, in my case Decklink Duo (1) 1 and select "Straight" in the Alpha Channel dropdown. You can only see this dropdown if you set the Connector Mapping as instructed above.
Now run two HD-SDI cables from Decklink Duo (1) and Decklink Duo (2) to to your other vMix system.
The physical order on Decklink Duo 2 is: reference, (1), (2), (3), (4)
Use (1) and (2)
In my case the second vMix system im using a Decklink Quad 2, so I will run HD-SDI cables from (1) and (2) to (1) and (3) on the Quad 2 card.
(1) and (3) in vMix corresponds to Decklink Quad (1) and Decklink Quad (2). Add these inputs to vMix.
Now you should see the inputs coming in to the second vMix system: Decklink Quad (1) shows a black and white rendition and Decklink Quad (2) shows a color representation.
Configure the Decklink Quad (2) input.
Go to the tab "Color Key" and in the "Key/Fill Input" dropdown select Decklink Quad (1).
Now in the preview you can instantly see that you have the output from the first vMix system coming in to the second vMix system with alpha key, meaning that in my case the transparent lower third that I created, is still transparant after having being output over HD-SDI.
This was not possible in vMix before version 21.0.34.
Note that you do not the new vMix 21 on the second system that takes the two inputs, only on the vMix machine that is outputting alpha channel HD-SDI output.
At this time we are only aware of alpha channel support with the already mentioned capture cards. It might work with others though. If it doesn't work with your capture card, you can instead rely on NDI:
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