Transparency on Main Output / Program Output
Interesting observation...
I am using OBS-NDI and I would like the "Main Output" to display transparency (alpha channel?) when the selected scene does not fill the screen.
Currently, when I view the "Main Output," the background appears black. However, I have noticed that when I switch to the "Preview Output," I'm extremely pleased to see that it displays a transparent background instead of black.
To test this, I use NDI Studio (from NDI tools) and enable "Settings | Video | Show Checkerboard." The appearance of checkerboards indicates that transparency is being shown.
Does anyone have any insights to share regarding why it functions this way?
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As a VTuber, I can tell you that the alpha channel is my bestie. That's how my model gets overlaid into the scene without the hassle of dealing with chroma-keying.
Since I run VTube Studio and other accessory programs on its own machine, I have to send an alpha channel-bearing stream to my "studio PC" which builds up the stream.
By default, OBS Studio's color format is set to "NV12". In order to send an alpha channel-bearing stream out the back of OBS Studio, I set up the color format to be "BGRA" instead so then alpha is actually part of the signal that gets processed, then sent out the back of OBS Studio via the NDI Main Output. None of the other color formats support alpha channels.
To do this, go to Settings, then select "Advanced" in the left-hand pane. In the right pane, in the "VIdeo" sub-pane, there's a dropdown labeled "Color Format". Select "BGRA" as the format and you will have an alpha-ready stream out the back of that instance of OBS Studio.
NB: Do pay attention to the warning in red at the bottom of the dialog box. Processing in this domain can take a bit of resources, depending on the material you're using. Since this computer does not actually stream to the Internet, there is no issue here. However, it all comes down to what kind of system you have and what kind of hardware you have.
Please take a look at a screenshot of my RDP session to my VTuber machine for what you can expect to see on that machine. Pay particular attention to the settings window and the "Video" sub-pane on the right-hand pane, under "Color Format". Things are greyed out due to the fact that an NDI output out the back is active.
And here is the output on my studio machine, integrated with alpha channel and all in another RDP session to my studio machine, which also does not stream (that's for another computer):
I hope this helps.
--Katt. =^.^=