# OpenGL program to draw a line using dda algorithm

Below is the OpenGL Computer Graphics program to draw a line using dda algorithm (digital differential analyzer algorithm).

void glBegin(Glenum mode);
// delimit the vertices of a primitive or a group of like primitives

// delimit the vertices of a primitive or a group of like primitives

// clear preset values of buffer

// specifies vertex

// force execution of GL commands in finite time

// Used to define orthographic projection of 2D matrix

Program:
#include <stdio.h>
#include <math.h>
#include <GL/glut.h>

double P1, Q1, P2, Q2;

float round_value(float rv)
{
return floor(rv + 0.5);
}
void LineDDA(void)
{
double dx=(P2-P1);
double dy=(Q2-Q1);
double steps;
float pInc,qInc,x=P1,y=Q1;
/* Find out which to increment x or y */
steps=(abs(dx)>abs(dy))?(abs(dx)):(abs(dy));
pInc=dx/(float)steps;
qInc=dy/(float)steps;

/* Clears preset values of buffer */
glClear(GL_COLOR_BUFFER_BIT);

/* Plot points */
glBegin(GL_POINTS);
/* Plot first point */
glVertex2d(x,y);
int k;
/* find intermediate vertex with every steps*/
for(k=0;k<steps;k++)
{
x+=pInc;
y+=qInc;
/* printf("%0.6lf %0.6lf\n",floor(x), floor(y)); */
glVertex2d(round_value(x), round_value(y));
}
glEnd();

glFlush();
}
void Init()
{
/* clear white color buffer */
glClearColor(1.0,1.0,1.0,0);
/* clear black color buffer */
glColor3f(0.0,0.0,0.0);
gluOrtho2D(0 , 640 , 0 , 480);
}
void main(int argc, char **argv)
{
printf("Enter 2 end points of the line to draw :\n");
printf("\n************************************");
printf("\nEnter Point 1 ( P1 , Q1):\n");
scanf("%lf%lf",&P1,&Q1);
printf("\n************************************");
printf("\nEnter Point 1 ( P2 , Q2):\n");
scanf("%lf%lf",&P2,&Q2);

/* Initialize GLUT library */
glutInit(&argc,argv);
/* Initialize display mode */
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
/* Initialize window position and size */
glutInitWindowPosition(0,0);
glutInitWindowSize(640,480);
/* Create new window with title "DDA_Line" */
glutCreateWindow("DDA_Line");
/* Initialize drawing colors */
Init();
/* Call the displaying function */
glutDisplayFunc(LineDDA);
/* Keep looping untill program is closed */
glutMainLoop();
}