Using keyable objects

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  08 August 2013
Using keyable objects

Hey guys, I am an animator and looking at playing around with some green screen and compositing type stuff. I watched a cool spot the other day that I want to see if I can replicate it in a test. It was a hand with a CG hammer in it.

So my question would be, is there a standard time when you would use a green object (in this example it would be the hammer) and key it out to add a 3D version in? You will still have to rotoscope things I would assume? Is that method used for getting the position of the thumb right for holding onto the handle? Thanks for any help is it greatly appreciated

I'd be using C4D and CS6 for any post work if that matters.
  08 August 2013
In that particular instance, yes, a chroma-green object held in the hand could be useful if it's lit properly. The fact that it's green isn't enough—you have to have relatively even lighting across the parts of the object that are going to be used in a matte. If you've got dark, moody lighting, with the hammer half in shadow, then it doesn't matter what color it is; you're not going to get a good key from it.

Depending on the skin tone of the hand holding the hammer, you may run into some difficulty with spill—green light reflected onto the hand by the hammer's surface. Typically, spill is controlled by moving the subject away from the screen, but that's not going to be possible in this instance. It can take some work to get a good key in that instance, and sometimes roto is simply faster.

Since you're dealing with a rigid object, roto may very well be much more straightforward than a key. If the hand is not moving around too much, then Mocha (a version comes with After Effects) will probably get you a good matte very quickly.

It's sounds like a pretty simple shoot, so I suggest you do it both ways. You'll be able to see for yourself the advantages of each method, and if one isn't working, you have the option of using the other.

Now, for the more general question, I'm going to answer the negative version of it: When does it not make sense to use a green stand-in object?

The point of chroma key is to quickly get an edge for an object that is in front of something that's going to be CG. If the stand-in is in front of everything else, then there's no reason for it to be green—you're just going to cover it up, and you don't need any edge information. If you're erasing Lt. Dan's legs, you don't need to wrap Gary Sinise's legs in blue fabric (Forrest Gump). There's nothing in front of them, so there's really not any point to it.

I worked on a shot recently where the production wanted a creature (practical) to climb up a glass surface, then peek around the edge, such that we'd see the creatures head in the clear and its body through the glass. Somehow, they got the notion that it would be easier to track if they used a green board with markers on it instead of using glass. Well, obviously that didn't work out because we couldn't see the creature through the green board! And to make matters worse, it didn't even track well; I had to build the motion manually. We wound up recreating the entire thing in CG using the plate as reference.
Bryan Ray
  08 August 2013
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