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Video mixing fill and matte layers live on a video mixer: problems with black tint

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Jim Hoyle
Video mixing fill and matte layers live on a video mixer: problems with black tint
on Nov 5, 2018 at 8:18:10 am

https://helpx.adobe.com/after-effects/using/alpha-channels-masks-mattes.htm... describes the functionality of a fill and matte layer. I have to create two layers of my RGBA footage so that a video mixer can create an overlay on top of the original live video feed. When testing, if there is supposed to be white half-transparent smoke, that results in black smoke because there is black in the fill layer and the mixer doesn't interpret the 50% black in the fill layer as 50% transparent but 50% transparent AND black instead. What I mean is, the fill and matte layers are correct but I can't make low opacity objects completely transparent but they become more or less black instead.

This happens already when doing the Luma Matte (Track Matte) in AE. Even more difficult to fix it in the live video mixer.

How do I fix this problem in AE and what is generally the proper way to fix it in a video mixer? I don't have the model of the video mixer but a general tip would probably do. (We can assume for now that a one RGBA file is not possible, because I have been told that the video mixer requires the two separate files.)


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Bouke Vahl
Re: Video mixing fill and matte layers live on a video mixer: problems with black tint
on Nov 5, 2018 at 11:01:19 am

In general:
There are two ways of alpha, straight and premultiplied.
Your alpha channel should be a nice bw gradient, but your video should be ugly as hell. (Straight alpha, image does look weird by itself, as the places where it is NOT transparant are fully filled.)
Most NLE software wants straight alpha as well...

If that does not work, the problem lies in you mixer. YOu have choosen the wrong key type, or some parameter about sensitivity is wrong.

Bouke
http://www.videotoolshed.com


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Jim Hoyle
Re: Video mixing fill and matte layers live on a video mixer: problems with black tint
on Nov 5, 2018 at 11:29:47 am

Thank you. How do I make this work perfectly in AE, if I have to? I mean, I can simply make a nice RGBA file and there's no problem whatsoever (perfect transparency without any black tint). But if I get two files from somewhere, fill with black background and matte, how do I put them in my AE project so that the black doesn't get any black tint on opaque regions? If I just put the layers on top of each other (matte above), then Luma (for Track Matte), I get that black tint.


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Bouke Vahl
Re: Video mixing fill and matte layers live on a video mixer: problems with black tint
on Nov 5, 2018 at 11:42:10 am

you need to learn about straight alpha vs premultiplied alpha.
You can set it on output, and you can interpret footage as being one or the other.
Toy around, look at the differences. Especially look at the fill signal without the key applied.

Bouke
http://www.videotoolshed.com


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Jim Hoyle
Re: Video mixing fill and matte layers live on a video mixer: problems with black tint
on Nov 5, 2018 at 12:34:23 pm

What I understand from https://www.provideocoalition.com/alpha-channels-premultiplied-vs-straight/ is that straight is used on a basic RGBA video where there is no black (or other color) for transparent parts, just pure transparency (at whatever opacity). And premultiplied has black (or other color) visible for the parts that are meant to be transparent.

It's easy to interpret straight RGBA without problems (of black).

But I couldn't figure out how to get rid of the black on a premultiplied fill (+ matte) layer in AE. It would be nice to know how to do that in AE. Interpretation of the footage doesn't seem to make any difference in that case.


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Bouke Vahl
Re: Video mixing fill and matte layers live on a video mixer: problems with black tint
on Nov 5, 2018 at 1:02:57 pm

I don't get your problem.
You want to feed a mixer. Thus you do not want premultiplied alpha.
So, render to straight alpha and be done with it.

Next, toy around, and take at least half a day to toy by yourself. This is the best way to learn.

Bouke
http://www.videotoolshed.com


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Jim Hoyle
Re: Video mixing fill and matte layers live on a video mixer: problems with black tint
on Nov 5, 2018 at 1:58:23 pm

Ok I will try to find the solution. I have different cases I'm trying to solve, but one is that the video mixer doesn't support a video with straight alpha (or at least I'm told it doesn't).

On how to blend fill+matte layers in AE perfectly: I will then try to find how to do the same thing as in Photoshop there is Layer Style / Blend If Gray. That kind of functionality should help a lot.


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Bouke Vahl
Re: Video mixing fill and matte layers live on a video mixer: problems with black tint
on Nov 5, 2018 at 2:03:00 pm

You really need to learn.
A digital file can have fill and alpha in one. And a hardware mixer does not accept video and alpha over SDI, so you need to feed it alpha on one input, fill on the other.
The fill probably MUST be straight rendered, thus be ugly.

To use video with alpha in AE there is absolutely no need to feed two files. You can render to a file with alpha embedded.

Bouke
http://www.videotoolshed.com


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