Systematic ROI selection. Let M represents the ROI and pt(i, j) the pixel intensity at position (i, j) of a frame taken at t heart beats. Initially, all pixels are considered in the ROI, i.e. M(i, j) = 1 for all pixels. [1] Set M(i, j) = 0 if there exists a frame taken at PI ≤2 hb which intensity at (i, j) exceeds K1. [2] Set M(i, j) = 0 if among all frames, the intensity at (i, j) exceeds K2 at least once. The resulting M is the ROI.