CGTalk - compositing, rotoscoping, know how?

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Captain

01-08-2003, 03:05 AM

OK, so this is kind of a weird quiestion, its kinda theoretical. So lets say you have a live action shot that you want to incorporate a cg element into. Such as a CG car swirving behind a live action car or an object (like a bush) or a character running behind a box or desk. In the live action shot, the camera will be moving, (wether it is slightly or considerably seems irrlevant to me) how ould you do this. I thought I knew how, but recently, a teacher like figure pointed out to me that the way he would do it would require lots of post render touch up. He would render the cg element on top of the video file that was composited as the background, then he would bring it into photoshop with eachframe origional, and layer them. THen in PS he would do all the erasing for each fram when parts of the object wouldn't be visible. Now I have seen the results, and they do work. This is similar to what they do for wires in movies. THey way I saw it however, a way I seem to have seen backed up in several "making of documentarys" (one in particular being the cave troll scene from the first LOTR) where they would replicate the objects the cg enitiy would pass behind in 3d, then they would put them in their sceen with the movie images as the texture, that way when the character went behind it it would disapear, and no post production? The only problem i see with my method si that with low quility video, you can't be exact with the texture.....

Is there yet another way? Have any of you used the above ways? What do you recomend. THis is a chance for some good experienced people to give me some insight, just incase I tackle this in the near future (i hope to atleast!).

-Captain.

policarpo

01-08-2003, 03:15 AM

hey captain...sounds like you need a copy of the free Aura DV or download the demo of Aura 2.5 and learn to do this...

as far as objects passing behind elements in the scene...you basically block the shot in cg so that your composited elements will pass behind the object in the footage plate. this block is actually a mask which you will track to the item in the footage that is supposed to be in the foreground blocking your cg composite when it magically passes behind it ( you can do this in 2D with a package like AURA)...keep in mind that your CG camera will need to match your live footage camera plate for the best results...so it will take some tweaking.

there is a whole black art science to it all...but the easiest way to understand it is to start reading books on compositing and the like if you really want to get into it and download demo's of applications like the following:

discreet combustion
adobe after effects
commontion
eyeone digital fusion
newtek aura 2.5

and see which apps work the way you think...

this should be a good thread...i hope we all learn a thing or two about a thing or three.

:)

Tommi

01-08-2003, 08:33 AM

I wonder whether there is a way (post-processing plugin for rendering?) which overrides the texture with an alpha, also hiding the objects beyond?

Jayk2k

01-13-2003, 05:53 PM

I believe most 3D apps will have some sort of Matte material (I know max does for sure). There you can specify whether the object with the matte material has a transparent or solid alpha.

The biggest issues with dealing with mattes in 3D is that camera tracking and the like is not always as accurate as a 2D track. There are instances where 3D tracking is needed, but most often a 2D track in the composite app will be faster and easier.

The other advantage to a composite application is that most will provide tools to generate an appropriate matte rather than modeling.

For example, a bush in the foreground, with a 3D element that passes behind it. To model a cut out of the bush would be easy enough, but what about all the irregular edges, and the holes in the density? Now you have a lot of work in 3D to match these holes....

In a 2D or Composite app, you have tools called keyers, which allow you to pick a specific color component of the image, (ie green leaves, or green screen..) and then pull a matte based on that. Since the computer picks the edges for you, there is typically a lot less work involved.

Ron Brinkman's book has a lot of technical information on digital compositing, and it is well worth the purchase if you decide to move into this area of CG.

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