Saturday, 21 July 2012

Bluescreen or greenscreen lighting problems

Earlier this year I tried my first greenscreen superimposing on video - except it was a blue screen!

...and it went fairly OK, as you can see. It was a good try, for a first try!  But look on the left, and you can see the shadow I was casting on the blue screen. The idea of chroma key is that we select everything that's blue, reverse the selection so that we've got everything that isn't blue, and then paste it onto the main video. That's what the software will do, anyway.

Trouble with shadow is that that isn't blue either... it's a sort of murky blue-gray. So you have to increase your tolerance (threshhold) as to how you define blue, so that it grabs the blue-gray as well. And then the selection starts eating into areas you don't want - such as my gloved arm.

Compare my upraised arm in the original bluescreen view with how it is on the video. It's been nibbled at, basically because there isn't a lot of difference between dark blue-gray, and shiny black.

The professional way of avoiding that problem is to have a bigger blue screen, and stand around 10 feet away from it, and have angled lighting that illuminates all of the screen equally. You want to avoid hotspots and avoid shadow.

Huh. Easier said than done, when you aren't living in a Hollywood studio.