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Motion Tracking a Chroma Key Suit

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Cody Sinclair
Motion Tracking a Chroma Key Suit
on Jan 27, 2012 at 8:47:01 am

Hi, im new to this forum (this being my first thread) so I hope you all can help me out here. Im trying to achieve an effect not so different from a digital suit for a movie im going to be filming soon. I dont really know how to describe the effect I want, but the closest thing as a reference that I know of is the recent Green Lantern movie, where the actor would be wearing a chroma key suit (track markers?) and I would then key it out and track the effect to it. Problem is, besides what I listed, I have no idea where to start. Could a regular chroma key suit work? Would I need to place tracking markers of some sort on it? I would be keying out the body of the character, so since im replacing a 3d object for essentially another, would the replacement have to be CG, or could a simple background insert be tracked?

...please help, as you can see im lost

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Jeff Brown
Re: Motion Tracking a Chroma Key Suit
on Jan 27, 2012 at 4:18:35 pm

Try starting with a Google search on "3D motion tracking" followed by a search on "3D motion capture". You should find a wealth of information to get you started.


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Michael Szalapski
Re: Motion Tracking a Chroma Key Suit
on Jan 27, 2012 at 7:02:02 pm

In addition to Jeff's very helpful suggestion, I would like to add some things.

A chroma key suit is only useful if what you're replacing the actor with is going to be significantly thinner than the actor or if it is to have holes in it. A flat green suit alone would be impossible to track and would actually be more of a hindrance. You absolutely need tracking markers on him.

I'm assuming you're shooting a blank plate too so that you have actually have an image to fill in the background behind your character (if it's a different size from your actor).

Now, a motion tracking rig can be pretty costly, do the research Jeff suggested and you should learn pretty quickly if it will be what you want.

- The Great Szalam
(The 'Great' stands for 'Not So Great, in fact, Extremely Humble')

No trees were harmed in the creation of this message, but several thousand electrons were mildly inconvenienced.

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Andrew Somers
Re: Motion Tracking a Chroma Key Suit
on Jan 27, 2012 at 8:55:43 pm

First, to echo what Michael was saying, there is a huge misconception that if you put an actor in a green or blue suit that you can then press a magic keying button and make him disappear - nothing could be farther from the truth.

It *is* useful to have the actor in a monotone blue or green (or red, depending) suit to make auto-tracking masks easier, IF auto-tracked masks will be needed.

What is more important is the CLEAN PLATE that is the scene behind the actor - how are you going to capture that? Either your camera is locked down, or you are using a motion control rig with repeatable moves. OR your are going to shoot only the PLATE, and then get the motion from a stage.

And what level of motion capture do you need? Do you need it to drive a fully articulated "robot" 3D character?

Which method is best is hard to say, since you have not really told us anything about the shot.

So I will make some assumptions - You don't want to see the actor at all, only the ""3D robot", against the background. With this assumption, here is *one* possible workflow:

Shoot the plate for the shot where you want the action to occur. You should have the actor there for some rehearsal takes to nail down the camera move you want, but shoot it with the actor NOT in the shot at all. This could he hand held, on a dolly, or however you like - motion control is not needed, but a stop watch and someone calling out "key points" to match the rehearsals would be useful. (You should ideally shoot some of the rehearsals as a reference).

Take accurate measurements of some of the prominent features of the environment (exact size of door frames, etc) as this will be useful is setting scale in the 3D track pass, next.

NOW: Take your CLEAN PLATE, and do a complete 3D motion track using PFTrack, BouJou, SynthEyes, etc.

From this clean plate you can determine the "environment".

Now, Go to a stage to shoot the motion - put the actor in a skin-tight suit, covered with very high contrast dots, and shoot him with multiple cameras at the same time (from different angles). It is important that these cameras are all "genlocked" - i.e. the shutters are all "in sync: with each other, all opening and closing at the same time.

If necessarily, add elements on the stage that match physical properties of the background clean plate (a chair, a railing to jump over, whatever).

Load this footage into a mocap program and use that motion mesh to develop your 3D objects and animation, then render than and composite it on the clean plate.

That is the *basic* run down - in practice, it usually takes a *team* of people that are specialists in each phase. If you are planning on doing this all yourself, expect the learning curve for each phase of development.

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Andrew Somers
Re: Motion Tracking a Chroma Key Suit
on Jan 27, 2012 at 9:05:23 pm

If, on the other hand, all you want to do is put the actor into an "ironman" suit that folly encompasses his body, then you can just shoot the actor in a monotone skin tight suit covered with high contrast dots, and shoot him on your actual set. When you shoot him, you need to use your main camera for the plate, but you must *also* cover him with at least two additional "witness" cameras (in genlock/frame sync).

Then use something like PFtrack and extract the motion.

Built the "suit" in a 3D app, using the motion/mesh from PFTrack to animate, render and then composite over the actor.

A "green screen" suit is not needed in this case - black with highly reflective dots is probably preferable.

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