DESCRIPTION: This illustration indicates the principle of Hough Transform and a line detection method. In fact, en:Hough transform has very good illustrations. But, unfortunately, they do not meet the Commons' copyright requirements. So, I draw this and uploaded. Consider five points in a Image Space (left). The Hough space representations of various lines that lie on each point are shown in the right panel. The representing sine curves are coloured correspondingly to the points in the Image Space. To see how to build a sine curve in the Hough space, we use the green point for example (middle). For this single point, various lines that pass through it can be drawn (e.g. red, green, black, and blue lines). Then we calculate each line's direction (th) and distance (r) from the origin (0, 0). This procedure is accomplished by drawing perpendiculars (lines with arrow heads) from the origin to each line. Now we can plot length vs. angle of the lines with arrow heads. The plot gives us a sine curve like green one in the right panel; r1, r2, r3, and r4 are points corresponding to perpendiculars in the middle panel, respectively. Please note that the r0 point in the right panel is the crossing point of all sine curves. This point representing the line in the Image Space that lies on all five points, initially mentioned.