The program is working just like you designed it. You are subtracting the background image from the chair image. Where the background is showing, they cancel out and you get a difference of zero. Where the chair is, it subtracts the background from the chair (actually chair from background, I think). Since the chair is a solid color, it is essentially subtracting a constant value from the background. In the chair area, all that is left is a darker version of the background.
What you probably want is to only have the image of the chair, which is a little more complicated. I would first use absolute difference on the two images. The resulting image will be zero (or very close) wherever the background matches. You should get a fairly solid image where the chair is. Threshold this, and use binary fill holes to fill in any holes in the chair. Then apply this image as a mask to the original chair image. It will keep the masked area (chair) and the rest will be black.
Bruce
Bruce Ammons
Ammons Engineering
