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Question: how far does a 4K camera input pass-through in a Full-HD production? (for good chroma-key)
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I've a question: we usually drive our vMix production in Full-HD (1080p), but the camera inputs are 4K. The interesting aspect now is: how far does the 4K camera input content really pass-through in such a Full-HD production?
Why? Well, in my OBS Studio based productions I fetch a 4K camera input primarily in order to get a better quality for the chroma-key, because the chroma-key is still applied to the 4K resolution and afterwards(!) the content is reduced for use in all Full-HD scenes.
In vMix I guess when I apply the chroma-key directly to a Camera input, the chroma-key is applied in 4K, even if the input is part of a Full-HD production, right? But in my vMix productions I've first create Virtual Inputs from the Camera inputs and on those Virtual Inputs the chroma-key is applied. So, in this case, is the chroma-key still applied to the 4K resolution or does the Virtual Input perhaps already implicitly reduce the 4K content to Full-HD? Does the Virtual Input's chroma-key really still apply to the 4K resolution?
PS: I especially ask, because optically I would say the chroma-key on the Virtual Inputs is already applied in Full-HD. But my eyes could be wrong...
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In vmix everything is put on to a canvas the size of your production. If you have a 720P sdi input it will be scaled up to your 1080 canvas. If you have a 4K input it will be scaled down to your 1080 canvas before any processing.
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Oh, this would explain why my eyes think the chroma-key'ed 4K cameras look worse in vMix than in OBS Studio. But if this is how it actually happens in vMix, this means that ingesting the cameras via SDI in 4K for a Full-HD production is entirely useless (and just cause extra downscale processing) in vMix (even if a chroma-key would be applied directly onto the camera input), right?
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I'm not sure that is the case. You will probably need Admin to explain exactly when inputs are resampled but I'm pretty sure it is later in the process. Quick test that shows it isn't immediate... Set up a 1080p project. load a 4K image like https://i.redd.it/dm24xf6b05r11.png that you can tell if it is being resampled (it is a 1px black and white checkerboard so will look grey when resized to 1080) Create a virtual input of that image (still grey) add a colour input. On the colour input select the virtual input as as layer (input shows grey) Then zoom that layer to 2x and you will see the 1px checkerboard is retained so the input wasn't scaled to 1080p immediately I would expect all input types to be handled in the same way, so this would apply for cameras as it does for images.
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Assuming your have the CPU power to accommodate, you can set the project for 4K. Then stream at something lesser.
That would also allow you to make a local recording at camera native resolution.
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Rank: Advanced Member
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Joined: 3/7/2022(UTC) Posts: 71 Location: Munich Was thanked: 33 time(s) in 17 post(s)
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Yes, using a 4K project would avoid any early downsamplings, but our PCs would be not powerful enough for a full 4K production, I think. And except for the chroma-key of the cameras (which would be a lot more precise in 4K) I not even would need 4K anywhere else in the production. But RichDanby's posting is also interesting: he doesn't think it is true that vMix actually does an early downsampling. I've to check this myself...
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Question: how far does a 4K camera input pass-through in a Full-HD production? (for good chroma-key)
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