Chroma keyer flickering

When using the chroma keyer, I'm running into an issue where the matte it generates is causing certain parts of the image to flicker as the footage it's keying from changes over time. I understand why this is happening, and even that it is expected behavior, but I'm curious if there are any common techniques to mitigate this. The issues comes from the fact that certain parts of the image have different color values over the course of the video footage, due to changes in lighting, shadows passing over areas, real world AO, etc. However, in my current case, I'm not really interested in the chroma keyer reacting to changes in lighting as much as I am hoping to define certain areas to be keyed based on a single frame, and would like those areas to stay as part of the matte for the rest of the clip. One idea I had was to use a specific frame as a screenshot to define the key so it stays the same for the rest of the clip (and apply the tracked transform data so it stays aligned with the comped image), but I don't know how to do that off the top of my head, and I'm also not sure it's the best way to solve the problem. Anyone have any ideas?

What you're describing doesn't sound like the way Chroma Keyer should be used. You very likely shouldn't be animating the Color Range controls, which is the only thing I can think of that would cause what you describe.

Are you sure you're not referring to the Clean Plate part of Delta Keyer? If so, Clean Plate is meant to be used with locked off cameras and yes—a freeze-frame of the clean (or cleaned) plate. If the camera is moving, then you shouldn't use the clean plate workflow. Delta functions quite well without the clean plate input, although it might take a bit more work to manage edges.

edit: It can also be helpful to denoise a greenscreen plate more aggressively for keying than you do for the actual composite to reduce sizzle caused by noise.

Yes, I am sure I'm using chroma keyer and not clean plate, and I am not animating any of the color range controls. Here's a video of what I'm seeing:

Maybe I'm wrong about what I think is happening; let me know if that looks like something else to you.

I see. You're keying off the variations in color of the wall? It's hard to say what all is contributing to the flicker without seeing full quality plates and your node graph, but from the size of the artifacts I'd guess there's some macro-blocking from temporal compression in addition to normal noise. It might help to pretreat the plate to enhance its contrast and saturation prior to pulling the key. And the aforementioned stronger denoise might knock some of that sizzle out, too.

In this specific case, I suspect your initial instinct to freeze the wall, key that still, and run the result through the tracker to apply it is the most expedient approach.

Keying is one of those things that never quite works the same way twice, so it's hard to declare what best practices should be.

Cool. Thanks Brian. Is there a good way to freeze a frame in Fusion? The only way I can think of to do that is to duplicate the clip in the timeline, turn the speed to 0, create a compound clip, and then jump back into Fusion, but I'd assume there's a better way to do that.

There's lots of ways, but the one I find most convenient is to use the time stretcher node with one keyframe - the frame number of the frame you want frozen. Be careful you don't inadvertently add another key frame or two down the line, though - it can happen easily. I always check in the spline graph before I render.

Thanks!

If you remove the animation entirely you don't have to worry about accidental keyframes. I actually use it that way so often that I've saved the default with animation removed.