Green Screen Question

[FilmBoy77]

Question -- sometimes when I look at BTS photos and footage I see little plus signs on the green screen that are a different color like this http://www.idontlikeyouinthatway.com/pictures/20111027/Henry Cavill Man Of Steel/FNP_EW_0245627.html

Can anyone explain what they are used for?

 

[Paul F]

Reference marks like that allow you to stabilize the shot and/or track the shot so that the background can match the movement of the foreground. For a better understanding, read up on After Effects stabilizing and tracking features.

The clip below has both a background (first part of the clip) and foreground (greenscreen). The + on the stand is the reference mark used to first stabilize the foreground shot and then track the foreground to the back ground.

background clip followed by foreground clip - Green Screen Test Backdrop and Foreground

Composite clip - http://www.youtube.com/watch?v=qYmq73j7m2E

Note that both the foreground and background dolly shots have bumps in them and the that camera pans and tilts through the shot. The bumps in the foreground need to be removed. Then the foreground follows the bumps in the background to match the movement.

The reference point in the foreground was used to stabilize the shot. The reference point in the background was used to lock the foreground to the background.

 

[FilmBoy77]

Ahhh...makes perfect sense. Thanks.