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Composited Multi-Speed Action


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#1 Matt Dennie

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Posted 10 May 2012 - 01:49 PM

Ok so I have a difficult shot I am in the progress of planning. It is most similar to the effect used in Heroes where time stops, however one character moves through the scene at a normal speed, however time will not be stopped, only moving very slowly (thinking around 300fps). I'm trying to simplify the shot as much as possible. Instead of moving the camera on a steadycam through the scene we will probably keep it on a tripod and use several angles. The moving actor needs to interact with items in the scene that he will brake. This will be shown at a slow speed/high frame rate. Most likely in 3D/Maya. Oh, and there might be time ramping too. Anyway...

My main question is should I shoot the moving actor on a greenscreen at 24p and the bg separately at 300fps, or do you think it would be easier to do some rotoscoping when necessary?

Or maybe I have no idea what I'm doing and you can think of something better. Perhaps bottom line...do you think this shot should be entirely 3D? Or is there some possibility of pulling off most of it in camera?

Right now I'm leaning towards shooting the normal speed moving actor on a greenscreen then shooting/creating the background to match.
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#2 Gregory Gesch

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Posted 10 May 2012 - 11:19 PM

Hi Matt. You don't tell us what sort of scene it is (street, outer space), what he is interacting with or what he breaks - all of which make a difference. However on the presumption that he he could be moving through a real location/set where there are other people or objects and that he will be handling objects which he drops/throws (well I have to start somewhere), I would suggest you could think of doing most of it in camera with a portable greenscreen. Yes, locked off shots. Think of it in layers: Background (everything he walks in front of)- shoot clean plates at 300fps. Mid range (your actor)- 24fps moves through the set with a portable greenscreen behind him. Foreground - (anything that goes in front of the actor)- 300fps maybe you'd need to do several individual shots ie: another actor walking across the set with portable greenscreen behind him, a pillar - put into position and place greenscreen behind it, a bird in a cage - hang in position with greenscreen held behind it. The reason for doing it all on location is that your lighting and camera angles/lenses etc will all composit together without any problems [however be aware of shadows and reflections]. In the above scenario there would be 5 layers in your NLE: Background, Actor, Walking Character, Pillar, Birdcage. Using the portable greenscreen means you should only have to do minimal rotoscoping - probably feet. Then you probably need to make the breaking objects in a 3D program - depending on what they are, you might be able to use real objects in front of the greenscreen shot at 300 and matched up with his actions????????? In either case you need to be able to match up his interaction with the objects and what happens to them - green prop versions that he handles which are then keyed out and replaced with the 3D models, or perhaps you might be able to have 2 cameras running (24 for the actor and 300 for the prop) for the sections where he breaks things with the area where they are broken covered in green as well? Without any detail it's impossible to say, but that might give you something to think about. Good luck with it.
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#3 Paul Bartok

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Posted 11 May 2012 - 02:26 AM

I'm kind of finding hard to tell what your asking
But i think Gregory covered most of it
but from what I can tell you would want to shoot your actor on a green screen and shoot bg plates, with each character and or object shot as separate plates so you can have more control when composting, you want the character be at normal speed then just shoot him at 24fps and the rest at 300fps, could try even shooting your character at 30 or 40 and then slowing back down to 24 just so he doesn't seem so out of place, but that's just a thought
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#4 Matt Dennie

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Posted 16 May 2012 - 09:57 AM

Thanks for the advice already. It was helpful. Here's some better detail this time:

We have two people in this scene ( a man and a woman), and 3 zombies. We are in a church sanctuary. An explosion has just happened. The walls are burnt and there is ruble on the ground. The woman is lying on the ground in ruble. Zombies are approaching. She has a gun, but can not aim very well. She takes a shot, but it's not going to hit anything at first. Now, time slows down to a crawl. The man has super speed powers and looks over to her, then the bullet. He appears to move at normal speed as the bullet slowly travels across the scene. He walks over to each of the 3 zombies near the bullet, and moves them into the path of the bullet. Time now speeds up as he moves away from the zombies and we see the resulting triple headshot.

Oh, and to throw a wrench into the whole thing, all the explosion/debris/damage to the church has to be VFX. We can't actually burn the church (unless you know a way to do it with set design, I guess that might be a possibility)

If you have any tips on how to make this shot happen as simply as possible, let me know.
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#5 Mei Lewis

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Posted 17 May 2012 - 09:03 AM

It's complicated enough to shoot that it may be good to do an animatic of it to work out everything you'll need to shoot.
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#6 Will Earl

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Posted 27 May 2012 - 02:13 PM

I believe the most common way to approach these types of shots (specifically the use in Heroes and X-Men 2) is to shoot everything in camera at 24fps with actors or mime artists in FG and mannequins or cutouts in BG with rigging used to hold characters up in mid-air. CG elements are used to have frozen/slowly moving elements (ie.. spilled drinks, bullets, etc) appear in the shot and rigging is either creatively hidden in the shot or painted out in compositing.

Using this technique means you can move the camera around and you don't need to do motion control passes at different frame rates or even put up a greenscreen (even at the high-end of vfx, it is still hard to make greenscreen shots look good). It does however mean that you need to remove any elements in the shot which could give away the 24fps frame rate - ie... fire, smoke, water, dangling hair, etc.

For the burning church you might be able to get away with a Pepper's Ghost http://en.wikipedia.org/wiki/Pepper's_ghost style effect, but it'll probably be easier compositing high-speed flame elements.

Hope that helps.
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