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Creating objects/ greenscreen question...?

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Patrick Kaplin
Creating objects/ greenscreen question...?
on Jan 14, 2009 at 1:49:28 am

Ok, so I'm going to be embarking on my first experience with 3d animation in one of my films. The shot consists of a persons fingers morphing into ambiguous forms (think Terminator 2 and Matrix for reference). I have a 3d compositor helping me with this project.

It has been suggested to me that the best way to do this would be to cover the actor's hand in greenscreen paint, place reference balls on her hand, then film the actress against a greenscreen background. The only part of this advice that I don't understand is why we would need to film in front of a greenscreen...? I'd much rather have the actor still be in the real world with the exception of their arms.

What's the best way to achieve this effect usually?

Sorry for the lamens terms, but 3d compositing is a whole new world to me!

Thanks in advance.


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Adam Benson
Re: Creating objects/ greenscreen question...?
on Jan 15, 2009 at 6:19:29 pm

Hello Patrick,
You're right. You do not need to shoot this on the greenscreen. Here's my greenscreen rant.
Green/Blue screen is for creating MASKS or MATTES! Again, it is for creating MASKS or MATTES! That's it. Nothing more. The temptation for new-to-the-industry vfx guys to green screen everything is something I come across all the time. People watch the behind the scenes on their favorite DVD and see the effects work being shot on the green screen, and they don't ask why, they just replicate it by shooting everything they do on a green screen.
Green screen work is often problematic, and should be used only when truly necessary. Use the right tool for the right job. Using green screen for everything just creates unnecessary work. Here's the breakdown of the issues you will face when using green screens.
1. Spill. Green and blue screens both have a certain amount of spill associated with them. And while it is possible to minimize and nearly eliminate that spill during photography, that is not the norm, that is the exception to the rule. Most newbies improperly light, and subsequently get the actors too close to the screen to weed out this problem. SO, you have to apply spill suppression. No big deal, but the simple truth is that spill suppression removes color data. That's how it works. So, while the affect may be negligible, you are still losing data and data loss, while sometimes necessary, is something you do not want in your effects work.
2. Artifacting. Unless you are shooting on film (analog), digital photography is made up of square (or nearly square) pixels, not smooth lines, SO when you try and pull a key you will get a certain amount of artifacting (jagged lines around a mask), which will often create a kind of halo around the matte or mask that you are trying to create. Granted, the higher the resolution of your digital camera the less and less this is a problem, but if you are shooting on DV25 (standard def DV) your keys will suffer.
3. Color and Lighting. Unless you've got yourself a really good DP and Gaffer getting the lighting and color to look the same from the green screen to the main set is very tricky. This can keep you from selling the final shot to the audience. For instance, if the shadows or highlights on the arm don't go the same direction as the shadows and highlights in the background, then your shot is hosed.
Now, I said that they are for masks. Let me explain that a little more. The main reason for using a green or blue screen is for either cutting something out, or removing something you don't want. Actors are shot in front of green screens so that the compositor can pull a key (the term "key" came from an early film that created a literal, physical, cardboard keyhole for the camera to pass through so that it looked like the audience was peering through a keyhole, hence, any time you matte something out it's called "pulling a key") The green screen allows compositors to create a traveling matte that perfectly cuts a "keyhole" around the actor or object that you want to layer onto something else as it is moving. Another application for keying something out is if you have something, let's say a television, that an actor is passing in front of, but you haven't got anything on the TV just yet; you're adding it in post. If you cover the TV screen with a green screen, then when the actor passes in front of the television, you can, once again, cut a keyhole around the actor and apply that keyhole to your television layer that you add in post, so that the television screen appears to remain behind the actor. Again, it's simply for masking out the actor as the pass. Nothing more.
I have seen many productions where they'll cover ANYTHING that they're doing in post with green paper, even though no one is passing in front of it. This is wrong. If you don't need to create a mask or a matte, you don't need to shoot it on a green screen.
Now, onto your specific problem. If you are not using the actor's arm at all, and are just using it for an animation reference, then don't use the actors arm at all. Shoot a reference plate for the animator, and then shoot a background plate for the compositor to put the CG morphing arm into. There is no reason to green screen the arm, just so that you can cut it out and replace it with something else. That doubles your work load. If you need the arm for tracking purposes... same story. Shoot a reference pass (something that will never be used in anything other than tracking or animation reference) for the tracker or animator, and shoot the back ground plate with no arm in it at all. LASTLY, if you ARE using the arm, but only part of the arm, then all you have to do is shoot the foreground plate (with the arm) and the background plate (without the arm) and simply create a garbage matte around the part of the arm that you don't want to use. Ta-Done! That way you still have your animation and tracking reference, but you don't have to deal with the headache of pulling a clean key. Garbage mattes are fast and easy, and you can apply the exact same tracking data to the matte so that it moves the same way as the CG arm; two for one!
That is how you do this. No green paint, or green screen required. If you take my advice you will save hours or days on your compositing time. If you're compositor thinks I'm wrong, then don't pay him for all the extra hours of needless work he is creating.
Ok, so... sorry if this seems a little harsh. This subject just really burns me! No offense intended to anyone, I'm just sick of people creating hours of needless work for me, and I want to save other people from this terrible fate! Best of luck on your film. Please let me know how it goes!

Adam Benson
Visual Effects Artist/Musician/Sound Engineer/Editor
http://www.sleepdeprivedproductions.com


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