View Full Version : Understanding Alpha Channels

11-18-2016, 06:26 AM
I need help understanding how to use alpha images in my video editing program. I understand how chroma keying works but the images of the animated objects that I render always have that green edge or a green shading in the final images and the final video looks sloppy. I read where you can render your sequence as individual images and also render out your alpha channel separately and use that as your mask rather than going the chroma key route. I don't understand the process of using your image sequence and the individual alpha images together in a video editing program to achieve a cleaner version of chroma key effect. Any help would be greatly appreciated!

11-18-2016, 06:50 AM
Ummm...I should have known this would have already been answered. Found it here....


11-18-2016, 07:05 AM
You're also going to want to learn about premultiplied and 'straight' (non-premultiplied) alpha ...

11-18-2016, 08:59 AM
Thanks @SBowie
I read that the difference between straight and premultiplied is not actually in the alpha channel; in both cases, the alpha channel is identical. The difference is in the color channels. With straight channels, there is no change whatsoever to the color channels because of the alpha channel. With premultiplied channels, the color channels are modified by combining the background color with the foreground color. So, straight channels are pure color channels; premultiplied color channels need to have an operation done to them to remove the background color that they have been sullied with.
Which would be better to use for blending a moving foreground object with a moving background? I've seen YouTube videos that explain compositing an animated model over a static image but
I have animated a human model and I also have an image sequence as a backdrop none that I've watched so far go into straight VS premultiplied. Which alpha type would be better for this?

11-18-2016, 09:34 AM
Which would be better to use for blending a moving foreground object with a moving background?I don't have a particular view on which is 'better', but the thing you will want to know is which does your software support, or does it support both? It's not unusual to find that a given app either defaults to one type, or even insists on it.

11-18-2016, 09:44 PM
What I "typically" do, and I think this is fairly common, is to render the background black and premult the alpha unless your image/graphic makes a case for doing something different. That different would be make the background color similar to the image/graphic being premultiplied.

Also you mentioned "video editing" program vs video compositing. I would offer to composite in something like After Effects. It's going to have better tools and control for compositing than a video editor, plus you can export a clip out to the exact specs of your edit sequence, i.e. doesn't need to render preview files.