Sheet forming processes measure performance data mostly through scanning sensors that traverse in the cross-direction (CD) as the sheet is formed in the machine direction (MD), thus creating a zigzag pattern of discrete data path. However, there are some rare applications that have the capability for full sheet measurement at each sampling time. Figure 10.1 shows the diﬀerence between the two and the resulting spatio-temporal form of process data. Representing the scanner generated data as a twodimensional full matrix Y(n, k) is a practical approximation that greatly simpliﬁes the necessary calculations for process performance tracking and evaluation. Nature of the process data Y(n, k) may be the thickness of the sheet, its moisture, basis weight (mass/area), brightness or any other pertinent measure of process performance or product value. The process itself may involve manufacturing of metal sheets, glass, plastic ﬁlm, fabrics or pulp and paper sheets.