Smiley using Bresenham's Circle Drawing in OpenGL

#include <stdio.h>
#include <GL/glut.h>

// Center of the cicle = (320, 240)
int xc = 320, yc = 240;

// Plot two points on the lower quadrants 
// using circle's symmetrical property
void plot_point(int x, int y)
{
  glBegin(GL_POINTS);
  glVertex2i(xc+x, yc-y);
  glVertex2i(xc-x, yc-y);
  glEnd();
}

// Function to draw a circle using bresenham's
// circle drawing algorithm
void bresenham_circle(int r)
{
  int x=0,y=r;
  float pk=(5.0/4.0)-r;

  /* Plot the points */
  plot_point(x,y);
  int k;
  /* Find all vertices till x=y */
  while(x < y)
  {
    x = x + 1;
    if(pk < 0)
      pk = pk + 2*x+1;
    else
    {
      y = y - 1;
      pk = pk + 2*(x - y) + 1;
    }
    plot_point(x,y);
  }
}

// Function to draw a smiley
void smiley(void)
{
  /* Clears buffers to preset values */
  glClear(GL_COLOR_BUFFER_BIT);

  int radius = 100;
  // Draw an arc
  bresenham_circle(radius);

  // Draw a vertical line
  glBegin(GL_LINES);
  glVertex2i(xc, yc);
  glVertex2i(xc, yc-60);
  glEnd();

  // Draw 2 points on either side 
  // of the line
  glBegin(GL_POINTS);
  // A big dot made of 4 small dots
  // to the left of the line
  glVertex2i(xc-20, yc-20);
  glVertex2i(xc-19, yc-20);
  glVertex2i(xc-20, yc-19);
  glVertex2i(xc-19, yc-19);

  // A big dot made of 4 small dots
  // to the right of the line
  glVertex2i(xc+20, yc-20);
  glVertex2i(xc+21, yc-20);
  glVertex2i(xc+21, yc-19);
  glVertex2i(xc+20, yc-19);
  glEnd();
  glFlush();
}

void Init()
{
  /* Set clear color to black */
  glClearColor(0.0,0.0,0.0,0);
  /* Set fill color to white*/
  glColor3f(1.0,1.0,1.0);
  gluOrtho2D(0 , 640 , 0 , 480);
}

void main(int argc, char **argv)
{
  /* Initialise GLUT library */
  glutInit(&argc,argv);
  /* Set the initial display mode */
  glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
  /* Set the initial window position and size */
  glutInitWindowPosition(0,0);
  glutInitWindowSize(640,480);
  /* Create the window with title "DDA_Line" */
  glutCreateWindow("Smiley :-)");
  /* Initialize drawing colors */
  Init();
  /* Call the displaying function */
  glutDisplayFunc(smiley);
  /* Keep displaying untill the program is closed */
  glutMainLoop();
}

Output
=====

Written by Munia Balayil

Bresenham's Circle Drawing Algorithm using OpenGL

This program is to draw two concentric circles using bresenham's circle drawing algorithm with center (320, 240) and radii of circles as 100 and 200.

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

// Center of the cicle = (320, 240)
int xc = 320, yc = 240;

// Plot eight points using circle's symmetrical property
void plot_point(int x, int y)
{
  glBegin(GL_POINTS);
  glVertex2i(xc+x, yc+y);
  glVertex2i(xc+x, yc-y);
  glVertex2i(xc+y, yc+x);
  glVertex2i(xc+y, yc-x);
  glVertex2i(xc-x, yc-y);
  glVertex2i(xc-y, yc-x);
  glVertex2i(xc-x, yc+y);
  glVertex2i(xc-y, yc+x);
  glEnd();
}

// Function to draw a circle using bresenham's
// circle drawing algorithm
void bresenham_circle(int r)
{
  int x=0,y=r;
  float pk=(5.0/4.0)-r;

  /* Plot the points */
  /* Plot the first point */
  plot_point(x,y);
  int k;
  /* Find all vertices till x=y */
  while(x < y)
  {
    x = x + 1;
    if(pk < 0)
      pk = pk + 2*x+1;
    else
    {
      y = y - 1;
      pk = pk + 2*(x - y) + 1;
    }
    plot_point(x,y);
  }
  glFlush();
}

// Function to draw two concentric circles
void concentric_circles(void)
{
  /* Clears buffers to preset values */
  glClear(GL_COLOR_BUFFER_BIT);

  int radius1 = 100, radius2 = 200;
  bresenham_circle(radius1);
  bresenham_circle(radius2);
}

void Init()
{
  /* Set clear color to white */
  glClearColor(1.0,1.0,1.0,0);
  /* Set fill color to black */
  glColor3f(0.0,0.0,0.0);
  /* glViewport(0 , 0 , 640 , 480); */
  /* glMatrixMode(GL_PROJECTION); */
  /* glLoadIdentity(); */
  gluOrtho2D(0 , 640 , 0 , 480);
}

void main(int argc, char **argv)
{
  /* Initialise GLUT library */
  glutInit(&argc,argv);
  /* Set the initial display mode */
  glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
  /* Set the initial window position and size */
  glutInitWindowPosition(0,0);
  glutInitWindowSize(640,480);
  /* Create the window with title "DDA_Line" */
  glutCreateWindow("bresenham_circle");
  /* Initialize drawing colors */
  Init();
  /* Call the displaying function */
  glutDisplayFunc(concentric_circles);
  /* Keep displaying untill the program is closed */
  glutMainLoop();
}

Written by Munia Balayil

Generalized Bresenham's Line Drawing Algorithm using OpenGL

Bresenham's line algorithm is an algorithm that determines which points in an n-dimensional raster should be plotted in order to form a close approximation to a straight line between two given points. It is commonly used to draw lines on a computer screen, as it uses only integer addition, subtraction and bit shifting, all of which are very cheap operations in standard computer architectures[source]. For more visit this website.

#include<stdio.h>
#include<GL/gl.h>
#include<GL/glut.h>

/* Function that returns -1,0,1 depending on whether x */
/* is <0, =0, >0 respectively */
#define sign(x) ((x>0)?1:((x<0)?-1:0))

/* Function to plot a point */
void setPixel(GLint x, GLint y) {
  glBegin(GL_POINTS);
  glVertex2i(x,y);
  glEnd();
}

/* Generalized Bresenham's Algorithm */
void bres_general(int x1, int y1, int x2, int y2)
{
  int dx, dy, x, y, d, s1, s2, swap=0, temp;

  dx = abs(x2 - x1);
  dy = abs(y2 - y1);
  s1 = sign(x2-x1);
  s2 = sign(y2-y1);

  /* Check if dx or dy has a greater range */
  /* if dy has a greater range than dx swap dx and dy */
  if(dy > dx){temp = dx; dx = dy; dy = temp; swap = 1;}

  /* Set the initial decision parameter and the initial point */
  d = 2 * dy - dx;
  x = x1;
  y = y1;

  int i;
  for(i = 1; i <= dx; i++)
  {
    setPixel(x,y);
    
    while(d >= 0) 
    {
      if(swap) x = x + s1;
      else 
      {
        y = y + s2;
        d = d - 2* dx;
      }
    }
    if(swap) y = y + s2;
    else x = x + s1;
    d = d + 2 * dy;
  }
  glFlush();
}

/* Function to draw a rhombus inscribed in a rectangle and roll */
/* number printed in it */
void draw(void)
{
  glClear(GL_COLOR_BUFFER_BIT);
 
  /* Draw rectangle */
  bres_general(20,40,620,40);
  bres_general(620,40,620,440);
  bres_general(620,440,20,440);
  bres_general(20,440,20,40);

  /* Draw rhombus */
  bres_general(320,440,20,240);
  bres_general(20,240,320,40);
  bres_general(320,40,620,240);
  bres_general(620,240,320,440);

  /* 1 */
  bres_general(250,150,250,250);
  /* 0 */
  bres_general(300,150,300,250);
  bres_general(300,250,400,250);
  bres_general(400,250,400,150);
  bres_general(400,150,300,150);

  glFlush();
}

void init() {  
  glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
  glutInitWindowPosition(0,0);
  glutInitWindowSize(640, 480);
  glutCreateWindow("Green Window");
  glClearColor(0.0,0.0,0.0,0);
  glColor3f(1.0,1.0,1.0);
  gluOrtho2D(0,640,0,480);
  
}

int main(int argc, char **argv) 
{
  glutInit(&argc, argv);
  init();
  glutDisplayFunc(draw);
  glutMainLoop();
  return 0;
}

Written by Munia Balayil