A density centrifuge blood processing system with automatic two-dimensional optical control of fluid separation by observing fluid characteristics in observation regions. The location of the regions is determined by monitoring an optical reference. Points representing edges of an optical reference are measured and lines are computed through the points. An error measurement is calculated for each line. If the error is too large, the image is abandoned. One of the lines is selected as a referent line. A new line is calculated orthogonal to the referent line. The error function is again computed for the dependant line. If the error exceeds a selected maximum, the frame is discarded. A transformation function translates data points from an (r, s) domain derived from measurements of the edges into an (x, y) domain used to identify pixels in the observation areas.