> If you have a background picture, you have all the info you need to identify your subject - just plain subtraction It's not really "just plain subtraction",

> If you have a background picture, you have all the info you need to identify your subject - just plain subtraction It's not really "just plain subtraction", it's keying. Which AIUI basically means setting the alpha according to the difference between the image and the reference. Green screen works well for this because, excepting Zoe Soldana, people tend to hang out around the opposite side of the colour wheel, so there tends to be a good distance between foreground colour and background colour. If you're trying to do this against arbitrary backgrounds, you seemingly need to augment keying with additional techniques like image segmentation to get good results.