A better way to composite one video sub-clip over another with chroma key?

Question

What I want is to chroma key a short segment from one video strip (a title overlay in the middle of a longer video), over another video strip background. I can't see any compositing options in the Effect Strip and Strip Modifier menus except for alpha/mask operations. So looks like I need to generate an alpha on my own, and apply it that way.

However, the only chroma key I am aware of in Blender is in the scene compositor. So the only solution I've been able to come up with is create one whole scene dedicated just to the title overlay clip, use the Matte output from the Chroma Key node to render the grayscale matte to disk, then reimport the render into a strip in my original scene and use that as my alpha.

This works, but it seems like more steps than there ought to be. I'm used to editors like Final Cut Pro that let you generate mattes via modifiers on a per-strip level. Is there no simpler way to achieve the same thing?

At the very least, can I do something to avoid having to manually render the matte frames?

Answer 1

You are right, you would: 1. use the chroma key source as a single input node then generate a key with the keyer node. This scene will appear in the VSE as a strip with an alpha channel

2. add an Alpha Convert node set to "Straight to Premultiplied"

3. Add the scene strip to the VSE timeline on top of the background strip. Check its blend type and make it "alpha over" instead of cross or replace.

You should then be able to trim it to length as required. Remember that to get audio you would have to add the original source strip to the VSE timeline of the compositor scene. But this only works in the latest versions of Blender.

Otherwise you will have to sync the sound up to the scene strip.