Description: DDA

 Digital Differential Analyzer
 Walk through the line, starting at (x0,y0) 
 Constrain x, y increments to values in [0,1] range
 Case a: x is incrementing faster (m < 1)
 Step in x=1 increments, compute and round y
 Case b: y is incrementing faster (m > 1)
 Step in y=1 increments, compute and round x

 A line algorithm based on calculating either Δy or Δx using the above equations.
 There are two cases:
 1)Negative slope

 1)DDA Algorithm (Positive Slope)

 If  m ≤ 1 then take Δx = 1
 Compute successive y by
 yk+1 = yk + m -----(1)
 Subscript k takes integer values starting from 1, for the first point, and increases by 1 
 until the final end point is reached.
 If  m > 1, reverse the role of x and y and take Δy = 1, calculate successive x from
 xk+1 = xk + 1/m ------(2)

 In this case, each computed x value is rounded to the nearest integer pixel position.
 The above equations are based on the assumption that lines are to be processed from left
 endpoint to right endpoint.

 2)DDA Algorithm (Negative Slope)

 In case the line is processed from Right endpoint  to Left endpoint,
 then
 Δx = −1, yk+1 = yk − m for m ≤ 1 ---------(3)
 or
 Δy = −1, xk+1 = xk −1/m for  m > 1 ---------(4)

 Now,

 If  m < 1, 
 use(1) [provided line is calculated from left to right] and
 use(3) [provided line is calculated from right to left].
 If  m ≥ 1
 use (2) or (4).

 Algorithm:DDA 
 Procedure lineDDA(xa,ya,xb,yb:integer);
 Var
	dx,dy,steps,k:integer
	xIncrement,yIncrement,x,y:real;
 begin
	dx:=xb-xa;
	dy:=yb-ya;
	if abs(dx)>abs(dy) then steps:=abs(dx)
	else steps:=abs(dy);
	xIncrement:=dx/steps;
	yIncrement:=dy/steps;
	x:=xa;
	y:=ya;
	setPixel(round(x),round(y),1);
	for k:=1 to steps do
		begin
			x:=x+xIncrement;
			y:=y+yIncrement;
			setPixel(round(x),round(y),1)
		end
 end; {lineDDA}




C Program For DDA Line Drawing Algorithm 

Source Code:

 #include <graphics.h>
 #include <stdio.h>
 #include <math.h>
 #include <conio.h>

 void ddaLine(int x1,int y1,int x2,int y2)
 {
 float x=x1,y=y1,dx,dy;
 int length,i;

 putpixel(x1,y1,WHITE);

 if(abs(x2-x1)>=abs(y2-y1))
  length=abs(x2-x1);
 else
  length=abs(y2-y1);
 dx=(float)(x2-x1)/length;
 dy=(float)(y2-y1)/length;
 for(i=1;i<=length;i++)
 {
  x=x+dx;
  y=y+dy;
  putpixel((int)x,(int)y,WHITE);
 }
}

 void main()
 {
 int x1,y1,x2,y2;
 int gdriver = DETECT, gmode;
 initgraph(&gdriver, &gmode, "c:/tc/bgi");

 printf("Enter the starting co-ordinates: ");
 scanf("%d %d",&x1,&y1);
 printf("Enter the ending co-ordinates:   ");
 scanf("%d %d",&x2,&y2);

 ddaLine(x1,y1,x2,y2);
 getch();
 }