Editing Hough transform (section)

===Using the gradient direction to reduce the number of votes===
An improvement suggested by O'Gorman and Clowes can be used to detect lines if one takes into account that the local [[gradient]] of the image intensity will necessarily be orthogonal to the edge. Since [[edge detection]] generally involves computing the intensity gradient magnitude, the gradient direction is often found as a side effect. If a given point of coordinates (''x,y'') happens to indeed be on a line, then the local direction of the gradient gives the ''θ'' parameter corresponding to said line, and the ''r'' parameter is then immediately obtained. (Shapiro and Stockman, 305) The gradient direction can be estimated to within 20°, which shortens the sinusoid trace from the full 180° to roughly 45°. This reduces the computation time and has the interesting effect of reducing the number of useless votes, thus enhancing the visibility of the spikes corresponding to real lines in the image.