Explain in detail about scan line method. Justify that it is better than plane equation method.

a) Line drawing is accomplished by calculating intermediate positions along the line path between two specified end-point positions.

b) Basic parameters required to draw line is the end points of line segment i.e. A(x1, y1) and B (x2, y2).

c) The Cartesian slope-intercept equation for a straight line is y = mx + b

d) A random scan system stores point plotting instructions in the display list and co- ordinate values in the instructions are converted into deflection voltages that position the electron beam at the screen location to be plotted during each reflect refresh cycle. Then a straight line can be drawn smoothly from one end point to the other.

e) A raster scan system displays a straight-line segment by plotting discrete points between two end points. The intensity value is then loaded into the frame buffer. The video-controller, then displays the pixels. Pixel positions are referenced with integer values and therefore the plotted positions approximate the actual line positions between the specified end points.

Requirements of Good Line Drawing Algorithm

(1) Lines should appear straight.

(2) Lines should terminate accurately.

(3) Lines should have constant density i.e. Number of pixels per unit length should be always same.

(4) Lines density should be independent of length and angle. i.e. Number of pixels per unit length should be always same for vertical line, horizontal line etc. It should not depend on the length of the line segment and angle made by line with horizontal axis.

(5) Lines should be drawn rapidly i.e. line drawing algorithm should perform fast calculations to calculate next pixel position along the line path