i thought this would be simple but its driving me nuts.
I have a video of a scene with a window. The sky is visible through the window. There is also an alpha channel in the video to make the window transparent if need be. I bring the video into AE with the expectation that I'll be able to control the alpha behaviour i.e the transparency of the window. But I can not find any way to do it. Set Channles is the closest I've found to an independent alpha control but it is simply all on or all off, I need to control the level of transparency (let alone animate it over time). I realise I could do this using another layer as a track matte but I really thought I could do it with just the one layer and some kind of channel control effect.
If you have a alpha channel you can not really control the opacity of it without using the method you mentioned. If you have a matte layer - separate as a black and white image- you can apply Levels on it and control the opacity by controlling the white level You can assign that as the alpha for your clip and control thus the transparency for the windows.
You can also duplicate the layer, invert the alpha channel, and control transparency this way.