Line Drawing Algorithms

Line Algorithm
The basic algorithm works for lines which look like this:

      o-------                         |
     p1       --------                 | deltaY
                      -------      p2  |
                             -------o  |
      ----------------------------------
                  deltaX

where 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 = x1
y = y1
d = (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;