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Chroma key scale lock

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Neil Weaver
Chroma key scale lock
on Aug 17, 2009 at 12:22:32 pm

Hi all,

We are doing a fashion shoot next week on green screen where we are digitally mocking up a catwalk for the models to walk down.

I think it would be nice to have a slight pull out on camera as the models approach the camera, but this will obviously affect the scale of the digital background.

We'll be putting some 'X's on the green screen to act as a guide, but how do I tell After Effects I want to lock the scale of the digital background to the markers?

Thanks in advance!



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Barend Onneweer
Re: Chroma key scale lock
on Aug 17, 2009 at 6:27:59 pm

Hi,

For tracking scale you need two markers. In AE's tracker controls you can select a combination of scale, position and rotation tracking. This should be fairly easy as long as your two markers are never obscured by the models.

This is assuming that you are not digitally adding in the floor, because then you'll need a more complicated approach since you'll have parallax. You 'd need 3D tracking.

Barend

Raamw3rk - digital storytelling and visual effects


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Neil Weaver
Re: Chroma key scale lock
on Aug 17, 2009 at 7:41:58 pm

Thanks for responding, Barend. Yes, we are digitally adding the floor - how does that change the process?



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Barend Onneweer
Re: Chroma key scale lock
on Aug 17, 2009 at 9:18:23 pm

In that case you'll need to reproduce the camera movement in 3D space and you'll need a 3D floor layer in AE.

It may seem like a small difference, but 3D tracking and matchmoving isn't all that straightforward to get right.

If you're planning a small push just to get a subtle tension in the shot, you might consider shooting locked off and doing a small push-in digitally. It's not the same but it does have some effect, with minimal effort.

For 3D tracking you'll need markers on the floor too, and not too few. You'll track the 3D camera move in an application like Boujou, Syntheyes (most affordable), PFTrack or Matchmover. Then export the camera solve to After Effects and place it in your virtual set.

If you're interested in these techniques and have time to investigate and experiment, go for it. Otherwise, get someone else to do it or fake it per my earlier suggestion.

If you're going to do the push-in digitally it helps if you shoot at higher resolution than the delivery format. Shooting Red @ 4k gives you some room when you're finishing in HD.

Raamw3rk - digital storytelling and visual effects


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Neil Weaver
Re: Chroma key scale lock
on Aug 18, 2009 at 12:23:24 pm

Hmm, I don't think the budget on this allows for the type of work you've outlined there. The digital pull out would work though - especially as the final clips are for the web anyway.

Thanks for your suggestions, Barend!



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