Re: HMI and Green screen

As one who both shoots...and then has to pull greenscreen composites, there is a couple things that I've concluded.

1. When pulling a screen from a color background, the fact that it's lit over or under doesn't seem to be the primary issue...the key factor is that it's different...over or under. The FX Guide TV guys did a test some time ago with green screens on a Viper camera, lighting at various levels relative to the foreground, and the easiest extraction was pulled from a shot where the green background was lit so brightly that it appeared to be incredibly desaturated. It was simply the shade that was the most distinct from the foreground palette.

2. Daylight lighting seems to help compositing, particularly with skin tones in my experience.

Blue is very nearly across the vectorscope from human skintone and the color of a typical blue screen wall can be moved even more directly opposing if lit with daylight as opposed to tungsten (and this shift becomes even more important with green, as the angle to green is only about 90 degrees) allowing standard spill suppression in most keying applications to work most efficiently.

The other factor to consider is that any video camera is noisier when balanced for tungsten than when balanced for daylight as blue needs far more gain applied to balance with most tungsten sources than red needs with most daylight sources.







TimK,
Director, Consultant
Kolb Productions,