Algorithm

Code in c++ using Glut

#include<GL/glut.h>
#include<math.h>
#include<iostream>
using namespace std;

float x_1,y_1,x_2,y_2;

void Line(){

    float dy, dx, m , P;
    dy = y_2 - y_1;
    dx = x_2 - x_1;

    m = dy/dx;

    P = 2*dy - dx;

    int x = x_1, y = y_1;
    if(m<1){

        for(int i=0; i<=dx;i++){

            glBegin(GL_POINTS);
                glVertex2i(x,y);
            glEnd();

            if(P<0){

                x = x +1;
                y =y;
                P = P + 2*dy;
            }
            else{

                x= x+1;
                y = y+1;
                P = P + 2*dy - 2*dx;

            }
        }
    }
    else{
        for(int i=0;i<=dy;i++){
            glBegin(GL_POINTS);
                glVertex2i(x,y);
            glEnd();
            if(P<0){

                x = x;
                y =y+1;
                P = P + 2*dx;
            }
            else{

                x= x+1;
                y = y+1;
                P = P + 2*dx - 2*dy;

            }    
        }
    }

    glFlush();
}


void init(){

    glClearColor(1,1,1,0);
    glColor3f(1,0,0);
    gluOrtho2D(0,640,0,400);
    glClear(GL_COLOR_BUFFER_BIT);
}


int main(int arcg, char** argv){    

    cout<<"\nEnter x1,y1 ";
    cin>>x_1>>y_1;
    cout<<"\nEnter x2, y2 ";
    cin>>x_2>>y_2;

    glutInit(&arcg, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutInitWindowSize(640,400);
    glutCreateWindow("Batch B CGL B Line");
    init();
    glutDisplayFunc(Line);

    glutMainLoop();
}

Output

g++ filename.cpp -lGL -lGLU -lglut
./a.out