(xc,yc) was the centroid of the contour, (x,y) was the coordinate of
the contour pixel, (xs,ys) was the coordinate of the shrunk contour
pixel, N is the number of pixels of the contour, and d0 was the
shrunk distance. The masks for the ROI of marbling standards were
thereby obtained by setting pixels inside the shrunk contour open
and pixels outside the shrunk contour close.