The document provides an explanation of the Digital Differential Analyzer (DDA) algorithm used in computer graphics to draw lines. It outlines the calculation of differences in x and y coordinates (dx and dy), the number of steps involved, and the incremental changes for x and y coordinates (x_inc and y_inc) during the line drawing process. Two examples illustrate how to apply the algorithm to determine the pixels that form the line between two points.

1. COMPUTER GRAPHICS
DDA Algorithm
By: Ms. Rachana Marathe

2. DDA algorithm
dx= x2-x1;
dy= y2-y1;
if (dx) > (dy) then steps = (dx)
else steps= (dy);
x_inc = dx/steps;
y_inc = dy/steps;
x=x1;
y=y1;
for= 1 to steps do
x= x+x_inc;
y =y+y_inc;

3. Example 1: x1,y1= 2,2 x2,y2= 4,4
Algorithm:
dx= x2-x1;
dy= y2-y1;
if (dx) > (dy)
then steps = (dx)
else steps= (dy);
x_inc = dx/steps;
y_inc = dy/steps;
x=x1;
y=y1;
for= 1 to steps do
x= x+x_inc;
y =y+y_inc;
Solution:
dx= 4-2 =2
dy= 4-2 =2
true
steps = 2
x_inc = 2/2= 1
y_inc = 2/2= 1
x=2;
y=2;
for= 1 to 2 do (for loop 2 times)
(first loop)
x= 2+1=3
y= 2+1=3

4. Therefore the pixels are
(2,2) (3,3) (4,4)

5. Example 2: x1, y1= 1, 2 x2, y2= 4, 6
dx= x2-x1;
dy= y2-y1;
if (dx) > (dy)
then steps = (dx)
else steps= (dy);
x_inc = dx/steps;
y_inc = dy/steps;
x=x1;
y=y1;
for= 1 to steps do
x= x+x_inc;
y =y+y_inc;
Solution:
dx= 4-1 =3
dy= 6-2 =4
false
steps = 4
x_inc = 3/4= 0.75
y_inc = 4/4= 1
x=1;
y=2;
for= 1 to 4 do (for loop 4 times)
(first loop)
x= 1+0.75= 1.75 = 2 (round off)

6. Therefore the pixels are
(1,2) (2,3) (3,4) (3,5) (4,6)

7. Reference
Hearn,Baker - Computer Graphics - C Version 2nd Ed
http://edu.uokufa.edu.iq/staff/dr.nidhal/compressed%20comp.book/H
earn,Baker%20-%20Computer%20Graphics%20-
%20C%20Version%202nd%20Ed.pdf