Static Clock using Bresenham's Circle Drawing in OpenGL

/* To draw a static clock using Bresenham's circle drawing algorithm */
#include <stdio.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);
  }
}

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

  int radius = 100;
  // Draw a circle
  bresenham_circle(radius);
  glBegin(GL_LINES);
  // Minute hand
  glVertex2i(xc, yc);
  glVertex2i(xc, yc+95);
  // Hour hand
  glVertex2i(xc, yc);
  glVertex2i(xc+30, yc+50);
  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("bresenham_circle");
  /* Initialize drawing colors */
  Init();
  /* Call the displaying function */
  glutDisplayFunc(static_clock);
  /* 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