I want to take some portrait shots and then replace the background. I don't have a green screen or anything I can use for chroma keying, so I had this idea that I could take a picture of a background, and then take another picture with people in front of it, and then separate the people from the background by doing a "difference" on the two pictures to create the mask.
Both pictures were taken from exactly the same position (using a tripod and a remote shutter release), so that isn't a problem. The problem is that despite having identical exposures, the lighting of the background is slightly different, so when I take the difference there is a lot of values in the range of 0-15 (on a 0-255 scale) making it impossible for Threshold to determine the mask.
Is there another technique I can use, or am I barking up the wrong tree here?