I need a clue
 

Hello All,

I need a clue about what I see on some green screen set-ups. On some green screens setups I see little X’s some might call them T’s all over the green screen. Why are they there and what purpose do they serve?

Matt

They're tracking markers.
This way, the camera is free to move and the background composited in can be animated to follow the markers.

Most compositing applications can hand 2D tracking natively (where you're just tracking a flat screen in the background) and there are specialized programs for "match moving" that can calculate 3D camera movement based on the motion of the markers in the frame.

Google Matchmover pro or Boujou.....
The software tracks pixel's using crazy mathematical algorithms, thus creating a 3D camera in a 3D program. you should read up on 3D tracking its very cool. The markers are there cause the green screen has no distinguishing features to track.

i have always wondered how you take out the "X's". Say you key out the green, it would leave the black X's. Do you have to mask those out?

Usually its a good idea to make ur markers darker green or blue so they can also be keyed out. Keying is horrible on DV so I usually just have to mask the markers. Its not much of a hassle in slow moving footage.

It helps to put the markers in a part of the screen that can be manually added to a "garbage matte". Don't make the markers pass behind someone's hair, for example.

yeah that is what I figured but I have seen where in music videos they put them all over the place and I thought how much of a pain it would be to have those intersecting with hair and stuff.

For one, you only need the markers at all if you're moving the camera (or the set in relation to the camera). If the video is to be all locked-off camera shots ... don't use markers at all.

If you're moving the camera all over the place ... it's far easier to use the markers to track the background into the shot and handle the occasions that they pass behind hair with roto than it is to manually try to animate the new background without markers. :)