Different alpha information than combined channels.

Report · Feb 13, 2020

I'm trying to do something that would be easy in a node based system. I would like the alpha channel on a exported file to only have the alpha information from one layer of the composite. To be overly simple, I would like the RGB channel to be a still, and the alpha channel to be animated. This is to create a segment that could be keyed later in a broadcast environment.

Report · Feb 14, 2020

I am not sure what you are asking for. You can pull an alpha channel from any layer and apply it to any other layer with Set Matte or a half dozen other effects.

Once a video is rendered the alpha channel affects all pixels. I don't know of any way to render a movie with an alpha channel the pokes a hole in an image when it is on but does not when it is off.

Report · Feb 14, 2020

What I'm doing is preparing an elements to be used by someone else in an edit. Here's a simplified idea. Let's say you have an all white logo or graphic, and you are fading on different parts at different times. At this point the information in the RGB channels is identical to the alpha channel. If you are halfway thru a fade, the alpha channel has the white at 50%, which is revealing that part of the RGB which is also at halfway to white. So the net result of the key is 25% white. Ideally, you'd want a fully solid still in the RGB channels, and the animation in the alpha.

Report · Feb 16, 2020

As far as I know or can tell by experimenting you can't add an alpha channel to any kind of RGB image and have that alpha channel not affect the RGB channels. If you make the alpha transparent the RGB goes away.

You can render a separate alpha only file and use that to control the alpha of any other layer.

In some apps, you can use multiple channels from EXR files to do other things, but that's a lot more complicated than most editors are comfortable with.

Report · Feb 16, 2020

I am intending it to affect the RGB channels. I just want a still in the RGB and animated alpha.

Report · Feb 16, 2020

Maybe this illustrates it better. If you have seperate alpha and RGB passes, you would generate this. This is what I'm trying to create in one export.

Report · Feb 16, 2020

I do not know of any video format or standard image format that works like that. You'll have to render a separate alpha only pass.

I believe your assumptions are wrong. Checking the color values of a white layer with alpha set to 50 shows no difference using opacity, a track matte or setting the alpha level to .5. Check the screenshots:

In the bottom screenshot, I rendered the comp to ProRez 444, Trillions of colors, RGB + Alpha, Straight Unmatted, which is an industry-standard. The RGB values of all 3 tests re identical. There is no change to the RGB values when you animate an alpha channel.

In any NLE you can adjust the opacity, and in Premiere Pro you can do a little manipulation of the channels but if you have a second layer you can gain even more control.

Report · Feb 17, 2020

I agree with Rick. I don't think your diagram accurately represents what happens in After Effects. The upper diagram suggests that when you change layer opacity the RGB layers have their opacity reduced, and then the alpha channel has its opacity reduced as well. That's not the case. When you make an opacity change, only the alpha channel changes. Your lower "ideal" diagram is exacvtly what happens in AE.

This image shows a pure white layer with a built-in alpha channel, with opacity set to 50%. I've added a 50% gray solid (100% opacity) underneath to show the opacity result is exactly 50% grey.

If you seperate the alpha channel and RGB channels by using the alpha channel as a Matte, with only the alpha layer set to 50% opacity, the result is exactly the same.

So when you adjust opacity in AE you are only changing the alpha channel, not the RGB channels.