## Wednesday, December 2, 2009

### Line Drawing Algorithms

Line Algorithm
The basic algorithm works for lines which look like this:
`      o-------                         |     p1       --------                 | deltaY                      -------      p2  |                             -------o  |      ----------------------------------                  deltaXwhere p1 = (x1,y1),      p2 = (x2, y2),      x and y are both increasing from p1 to p2,      deltaX = x2 - x1,      deltaY = y2 - y1 and      deltaX >= deltay.`
All other types of lines can be derived from this type. I'll get to this bit later.
First you need to perform the following intialisation:
`x = x1y = y1d = (2 * deltaY) - deltaX`
x is the current x location, you will add 1 to this variable after every pixel you draw until all pixels have been drawn. y is the current y location. The decision variable is used to determine when to add 1 to this value. d is the decision variable which will be used to keep a track of what to do.
Now you loop across the screen from x1 to x2 and for each loop perform the following operations for each pixel :
`PutPixel(x, y);  { Draw a pixel at the current point }if d < 0 then    d := d + (2 * deltaY)else  begin    d := d + 2 * (deltaY - deltaX);    y := y + 1;  end;x := x + 1;`