Lines

A line feature is a straight edge in an image that usually denotes the boundary of an object. It sounds fairly straightforward, but the calculations involved for identifying lines can be a bit complex. The reason is because an edge is really a list of (x, y) coordinates, and any two coordinates could possibly be connected by a straight line. For instance, Figure 8-9 shows four coordinates and two different examples of line segments that might connect those four points. It’s hard to say which one is right—or if either of them are, since there are also other possible solutions. The way this problem is handled behind-the-scenes is by using the Hough transform technique. This technique effectively looks at all of the possible lines for the points and then figures out which lines show up the most often. The more frequent a line is, the more likely the line is an actual feature.