You know the centroid (xCentroid, yCentroid), and you know the slope (because you drew the line). The perpendicular slope is -1/mainSlope. The equation of a line is (y-y0) = slope * (x-x0). Or
perpSlope = -1 / mainSlope;
y = perpSlope * (x - x0) + y0;
Put in (x0,y0) of the point you want to take the cross section of - the point along the main slope. Then put in x = 1, for the left edge, and x = (number of columns in the image) for the right edge. That will get you two endpoints of the line. Then just call improfile() to get a line drawn across, and use find() to determine the x and y locations of where the line crosses the white region.
