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June 24, 2007

lsgl: another compositing strategy

by sven at 10:22 pm

Yesterday I ran into a tricky compositing issue... Which led to an interesting solution.

explorers with stand-in Elder Things

When combining footage of actors with a CG environment, it's critical that the camera angle from the real world and the camera angle in the CG world match.

One way to find that magic angle is to set up CG stand-in explorers. This works best when the actors are only moving from left to right (or right to left) across the screen.

Shots where there's depth -- where the actors are walking toward or away from the camera -- are harder. For those, you can create a sort of CG ruler. You make the ruler the same height as your actor, and the same depth as one stride. That allows you to match up the ball of the foot and the top of the head for the actor at point A and point B.

...But in the shot above? Neither of these strategies work. There's depth involved -- but I can't see the actor's feet. And, to make things even worse, the actors are crouching -- so I can't easily gauge their height.

click on image to play clip (1.2 MB)

After a lot of frustration, it occurred to me that I could maybe get the angle from footage I took with the same camera set up, but which isn't actually going into the film. I found a shot where you can see me taking a full step that seemed like it might work... (I include the actual clip here so you can laugh at my "directing" style.)

Before I got too far into that approach, though, I realized that I was staring at an even better ruler: the living room wall!

CG model of the living room wall

I got out my tape measure... That wall is 9' tall, 13'9" wide. Being just a flat panel, it was the easiest thing in the world to model.

living room wall and CG wall superimposed

Over in LightWave, I fussed and fiddled with the virtual camera until I got the CG wall to match up with the real one.

Then, I removed the CG wall, set up some stand-in Elder Things, and rendered out a test clip with the explorers reacting to them. Did it look right this time...?


posted by sven | June 24, 2007 10:22 PM | categories: let sleeping gods lie