Also, the ball compresses the cushion and slides down the rail during rebound. That's why I started the line with the OB off the cushion a bit. That results in a better measure of the actual curve after rebound. Another thing we need to be careful with is the effect of the airborne CB on camera perspective. If the camera angle is oblique and the CB hops, the 3D perspective might fool us into thinking the ball is curving more (or less) than it actually is.Dr. Dave's line is a more accurate representation of how much the object ball curved imo.
The angle of the picture does not allow for accuracy when you make the line follow the path through the ball at rail and the ball in the first frame because the object ball jumps off the rail.
I will try to shoot some video tomorrow with two cameras ... one a view similar to John's, and another looking straight down the rebound path. I will also try different angles and speeds (but mostly fast) to see what is possible on my equipment.
Regards,
Dave
PS: I would have shot the video today, but I decided to dedicate the day to improving my BU Exam II score. I managed to improve my PR by one point, but I was still 4 points shy of a perfect score. I won't rest until I get a perfect score ... I think it is quite possible on the Masters-level Exam II. I think a perfect score on the Doctorate-level exam would be a lot, lot tougher to achieve, even for a top pro. I'll post the video on the AZB BU sticky thread after I get it edited and uploaded.