Midpoint Line Drawing

#include <windows.h>
#include <GL/glut.h>
#include <stdlib.h>
#include <stdio.h>
#include<math.h>

float Color1=1, Color2=0, Color3=0;
int r,p,q;


void keyboard(unsigned char key, int x, int y)
{
  switch (key)
  {
  case 27:             // ESCAPE key
	  exit (0);
	  break;

  case 'r':
	 Color1=1.0, Color2=0.0, Color3=0.0;
	 break;
  case 'g':
	 Color1=0.0, Color2=1.0, Color3=0.0;
     break;
  case 'b':
      Color1=0.0, Color2=0.0, Color3=1.0;
	  break;
  }
  glutPostRedisplay();

}

void Init(int w, int h)
{
    glClearColor(1.0, 1.0, 1.0, 1.0);
    glViewport(0,0, (GLsizei)w,(GLsizei)h);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D( (GLdouble)w/-2,(GLdouble)w/2, (GLdouble)h/-2, (GLdouble)h/2);

}

static void display(void)
{

    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    int i=0;

    glPointSize(1);
    glLineWidth(2);
    glColor3f(Color1,Color2, Color3);

    glBegin(GL_LINES);
        glVertex2f(0, -240);
        glVertex2f(0, 240);
        glVertex2f(-320, 0);
        glVertex2f(320, 0);
    glEnd();

    int j=-240,k=-320,l=0;

    glBegin(GL_POINTS);
     while(j<=240){
        for( i=-320;i<=320;i++)
        glVertex2f(i,j);
        j+=20;
     }
     while(k<=320){
        for( l=-240;l<=240;l++)
        glVertex2f(k,l);
        k+=20;
     }
    glEnd();

    //1 double x1=60,y1=40,x2=160,y2=50;
    //2 double x1=20,y1=40,x2=40,y2=150;
    //3 double x1=-20,y1=40,x2=-100,y2=150;
    //4 double x1=-160,y1=20,x2=-220,y2=100;
    //5 double x1=-100,y1=-20,x2=-200,y2=-70;
    //6 double x1=-40,y1=-79,x2=-100,y2=-200;
    //7 double x1=20,y1=-40,x2=80,y2=-120;
    double x1=60,y1=-20,x2=220,y2=-60;
    double m=(y2-y1)/(x2-x1);
    //1st
    if(x1>0 && x2>0 && y1>0 && y2>0 && m<1){
        glBegin(GL_POINTS);
        double dy = y2-y1, dx = x2-x1;
        double d = (2*dy)-dx;
        while(x1<=x2){
            glVertex2f(x1,y1);
            if(d<=0){
                d = (2*dy)+d;
            }
            else{
                d = (2*dy)-(2*dx)+d;
                y1++;
            }
            x1++;
        }
        glEnd();
    }
    //2nd
    if(x1>0 && x2>0 && y1>0 && y2>0 && m>1){
        glBegin(GL_POINTS);
        double dy = y2-y1, dx = x2-x1;
        double d = dy-(2*dx);
        while(y1<=y2){
            glVertex2f(x1,y1);
            if(d<=0){
                d = -(2*dx)+d;
            }
            else{
                d = (2*dy)+(2*dx)+d;
                x1++;
            }
            y1++;
        }
        glEnd();
    }
    //3rd
    if(x1<0 && x2<0 && y1>0 && y2>0 && m<-1){
        glBegin(GL_POINTS);
        double dy = y2-y1, dx = x2-x1;
        double d = -dy-(2*dx);
        while(y1<=y2){
            glVertex2f(x1,y1);
            if(d<=0){
                d = -(2*dx)+d;
            }
            else{
                d = -(2*dy)-(2*dx)+d;
                x1--;
            }
            y1++;
        }
        glEnd();
    }
    //4th
    if(x1<0 && x2<0 && y1>0 && y2>0 && m>-1){
        glBegin(GL_POINTS);
        double dy = y2-y1, dx = x2-x1;
        double d = -(2*dy)- dx;
        while(x1>=x2){
            glVertex2f(x1,y1);
            if(d<=0){
                d = -(2*dy)+d;
            }
            else{
                d = -(2*dy)-(2*dx)+d;
                y1++;
            }
            x1--;
        }
        glEnd();
    }
    //5th
    if(x1<0 && x2<0 && y1<0 && y2<0 && m<1){
        glBegin(GL_POINTS);
        double dy = y2-y1, dx = x2-x1;
        double d = -(2*dy)+dx;
        while(x1>=x2){
            glVertex2f(x1,y1);
            if(d<=0){
                d = -(2*dy)+d;
            }
            else{
                d = -(2*dy)+(2*dx)+d;
                y1--;
            }
            x1--;
        }
        glEnd();
    }
    //6th
    if(x1<0 && x2<0 && y1<0 && y2<0 && m>1){
        glBegin(GL_POINTS);
        double dy = y2-y1, dx = x2-x1;
        double d = -dy+(2*dx);
        while(y1>=y2){
            glVertex2f(x1,y1);
            if(d<=0){
                d = (2*dx)+d;
            }
            else{
                d = -(2*dy)+(2*dx)+d;
                x1--;
            }
            y1--;
        }
        glEnd();
    }
    //7th
    if(x1>0 && x2>0 && y1<0 && y2<0 && m<-1){
        glBegin(GL_POINTS);
        double dy = y2-y1, dx = x2-x1;
        double d = (2*dx)+dy;
        while(y1>=y2){
            glVertex2f(x1,y1);
            if(d<=0){
                d = (2*dx)+d;
            }
            else{
                d = (2*dy)+(2*dx)+d;
                x1++;
            }
            y1--;
        }
        glEnd();
    }
    //8th
    if(x1>0 && x2>0 && y1<0 && y2<0 && m>-1){
        glBegin(GL_POINTS);
        double dy = y2-y1, dx = x2-x1;
        double d = (2*dy)+dx;
        while(x1<=x2){
            glVertex2f(x1,y1);
            if(d<=0){
                d = (2*dy)+d;
            }
            else{
                d = (2*dy)+(2*dx)+d;
                y1--;
            }
            x1++;
        }
        glEnd();
    }



    glFlush();
}


int main(int argc, char *argv[])
{

    glutInit(&argc, argv);
    glutInitWindowSize(640,480);
    glutInitWindowPosition(400,200);
    glutInitDisplayMode(GLUT_RGB);

    glutCreateWindow("Polygon drawing");

    Init(640, 480);
    glutKeyboardFunc(keyboard);
    glutDisplayFunc(display);


    glutMainLoop();

    return 0;
}