Dynamic masking/cropping

[fingerbreaker]

Need some help figuring out how to accomplish this: I am overlaying a chroma-keyed clip onto a video background. There is an object in the background clip (let's say it's a table) that I want it to look like the overlayed clip is behind. However the camera view in the background clip is panning such that the edge of the table is moving within the image. How do I dynamically crop/mask the overlay clip to be coincident with the edge of the table?

My first thought was to add a mask frame to the same chroma-keyed clip, but it looks like you can only do one or the other (chroma or mask). And, as far as I can tell, there's no way to apply a dynamic (moving) mask frame.

My second thought was to add a cropping effect to the overlay clip and use key frames to have the cropping edge move to match the table edge. However, when I tried this, the moving cropped edge is not smooth at all, even in the rendered video. It advances in steps and doesn't look convincing at all. If there's a way to fix the choppiness, that would probably work. Any ideas there?

I also thought of adding a second copy of the background clip as an overlay clip and using a mask frame to isolate the table and overlay it on top of the chroma-key clip. But again, I don't know how to make a mask frame move to stay with the edge of the table as it slides up.

What other tools could I use for this effect? By the way, I don't have these actual clips shot yet - this is just planning stages, otherwise I'd post stills for reference but hopefully from my description it's clear what I'm trying to do.

Thanks in advance, VideoStudio experts!
FB

[BrianCee]

To keep a tracker on a moving object Videostudio provides "Track Motion"


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[fingerbreaker]

To use "Track motion" in this scenario, what would I be tracking to the moving object? For the sake of discussion, let's say it's someone seated behind the table. As the camera pans down, the top edge of the table appears to move up, cutting away more and more of the person "sitting" behind it. The person would themselves be stationary more or less. I don't want to track the person to the table edge because there is relative motion between the two.

[BrianCee]

Not sure if I'm exactly following your problem - I assumed you had a still as the overlay and you wanted to move the overlay clip in sympathy to the movement of the table in the background video - perhaps some one else can be more helpful


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[fingerbreaker]

I'll take another shot at describing the scenario: The background and the overlay are both video, not stills. Imagine the background video to be a side view looking at a table while the chroma-keyed overlay is of a person positioned as if sitting behind the table. The overlay clip is truncated in some way so you just see from the torso up to create the illusion that they are behind the table. As the background video perspective pans down below the plane of the table top, you should see less and less of the person as the edge of the table moves up in the view. Let's say at the end of the clip, you can only see the person's head above the table. Does that make sense?

Cropping really should be the best solution because I can set as many key frames throughout the clip to match the cropped bottom border of the overlay clip to the table top, but as I said, when I tried that the cropped edge doesn't slide smoothly at all it jumps in little steps. I've used dynamic cropping before and recall that it was very smooth, so I'm wondering if it's the combination of the chroma key mask with simultaneous cropping that's making the software work too hard to be able to do it smoothly?

Hope that helps explain the problem a little better.

Thanks,
FB

[fingerbreaker]

Update after some more research: I opened an older project that I knew used "dynamic" cropping and it moves very smoothly, so I'm not sure what is going on with my new project where the crop edge moves very choppily.

Also, I should have specified that I'm using pro x7. It looks like the "video mask" option introduced in x8 would be another option for this (and superior to cropping in that I could theoretically do a complex shape, although in my case I just need a straight line anyway). But now that I think about it, I can't use video mask on the same overlay clip that I'm chroma keying on, so I guess I have to use cropping and figure out why it's not transitioning smoothly.

Well, sometimes just having someone to listen to helps one work their problems out! Thanks.

FB