2D Line Segment Generation 

A digitally plotted line is basically an approximation of infinite number of points on an abstract line segment by only a finite number of points on a computer display.  This is needed because the display devices can only plot a finite number of points, however large the resolution of the device may be.  So, the key concept of any line drawing algorithm is to provide an efficient way of mapping a continuous abstract line into a discrete plane of computer display. 

This process is called rasterization or scan conversion.  These algorithms basically approximate a real valued line by calculating pixel coordinates to provide an illusion of line segment.  Since the pixels are sufficiently small, the approximation gives a good illusion of a continuous line segment to the human eyes.  To understand what is meant by rasterization, we plot a line segment on a pixel grid as shown in Fig (a).  The segment points are scan converted and approximated by a single shaded pixel as shown in Fig (b). Here we have shown a pixel by a square, but you know that a pixel actually has a disc shape with the boundary marked as the visible portion of the dot formed by the striking electron gun. The pixel shown here is the bounding rectangle of that dot.