#include <gl/glut.h>  // Also included gl.h, glu.h
#include <Math.h>     // Needed for sin, cos
#define PI 3.14159265f
      
// Global variables
char title[] = "Bouncing Ball (2D)";  // Windowed mode's title
int windowWidth  = 640;     // Windowed mode's width
int windowHeight = 480;     // Windowed mode's height
int windowPosX   = 50;      // Windowed mode's top-left corner x
int windowPosY   = 50;      // Windowed mode's top-left corner y
   
GLfloat ballRadius = 0.5f;   // Radius of the bouncing ball
GLfloat xPos = 0.0f;         // Ball's (x, y) position
GLfloat yPos = 0.0f;
GLfloat xPosMax, xPosMin, yPosMax, yPosMin; // Ball's (x, y) bounds
GLdouble xLeft, xRight, yBottom, yTop;      // Projection clipping area
GLfloat xSpeed = 0.02f;      // Ball's speed in x and y directions
GLfloat ySpeed = 0.007f;
int refreshMillis = 30;      // Refresh period in milliseconds
      
// Initialize OpenGL Graphics
void initGL() {
   glClearColor(0.0, 0.0, 0.0, 1.0); // Set background (clear) color to black
}
   
// Callback handler for window re-paint event
void display() {
   glClear(GL_COLOR_BUFFER_BIT);    // Clear the color buffer
   
   glLoadIdentity();                // Reset model-view matrix
   glTranslatef(xPos, yPos, 0.0f);  // Translate to (xPos, yPos)
   // Use triangular segments to form a circle
   glBegin(GL_TRIANGLE_FAN);
      glColor3f(0.0f, 0.0f, 1.0f);  // Blue
      glVertex2f(0.0f, 0.0f);       // Center of circle
      int numSegments = 100;
      GLfloat angle;
      for (int i = 0; i < numSegments; i++) {
         angle = i * 2.0f * PI / numSegments;  // 360 deg for all segments
         glVertex2f(cos(angle) * ballRadius, sin(angle) * ballRadius);
      }
      glVertex2f(ballRadius, 0.0f);     // Last vertex
   glEnd();
   
   glutSwapBuffers();  // Swap front and back buffers (of double buffered mode)
   
   // Animation Control - compute the location for the next refresh
   xPos += xSpeed;
   yPos += ySpeed;
   // Check if the ball exceeds the edges
   if (xPos > xPosMax) {
      xPos = xPosMax;
      xSpeed = -xSpeed;
   } else if (xPos < xPosMin) {
      xPos = xPosMin;
      xSpeed = -xSpeed;
   }
   if (yPos > yPosMax) {
      yPos = yPosMax;
      ySpeed = -ySpeed;
   } else if (yPos < yPosMin) {
      yPos = yPosMin;
      ySpeed = -ySpeed;
   }
}
   
// Call back when the windows is re-sized.
void reshape(GLsizei weight, GLsizei height) {
   if (height == 0) height = 1;               // To prevent divide by 0
   GLfloat aspect = (GLfloat)weight / height; // Get aspect ratio
   
   // Set the viewport to cover the entire window
   glViewport(0, 0, weight, height);
   
   // Adjust the aspect ratio of clipping area to match the viewport
   glMatrixMode(GL_PROJECTION); // Select the Projection matrix
   glLoadIdentity();            // Reset
   if (weight <= height) {
      xLeft   = -1.0;
      xRight  = 1.0;
      yBottom = -1.0 / aspect;
      yTop    = 1.0 / aspect;
   } else {
      xLeft   = -1.0 * aspect;
      xRight  = 1.0 * aspect;
      yBottom = -1.0;
      yTop    = 1.0;
   }
   gluOrtho2D(xLeft, xRight, yBottom, yTop);
   xPosMin = xLeft + ballRadius;
   xPosMax = xRight - ballRadius;
   yPosMin = yBottom + ballRadius;
   yPosMax = yTop - ballRadius;
   
   // Reset the model-view matrix
   glMatrixMode(GL_MODELVIEW); // Select the model-view matrix
   glLoadIdentity();           // Reset
}
   
// Animation timer function
void Timer(int value) {
	glutPostRedisplay();    // Post a paint request to activate display()
	glutTimerFunc(refreshMillis, Timer, 0); // subsequent timer call at milliseconds
}
   
// main function - GLUT run as a Console Application
int main(int argc, char** argv) {
   glutInit(&argc, argv);            // Initialize GLUT
   glutInitDisplayMode(GLUT_DOUBLE); // Enable double buffered mode
   glutInitWindowSize(windowWidth, windowHeight);  // Initial window width and height
   glutInitWindowPosition(windowPosX, windowPosY); // Initial window top-left corner (x, y)
   glutCreateWindow(title);      // Create window with given title
   glutDisplayFunc(display);     // Register callback handler for window re-paint
   glutReshapeFunc(reshape);     // Register callback handler for window re-shape
   glutTimerFunc(0, Timer, 0);   // First timer call immediately
   initGL();                     // Our own OpenGL initialization
   glutMainLoop();               // Enter event-processing loop
   return 0;
}

if (weight <= height) {
      xLeft   = -1.0;
      xRight  = 1.0;
      yBottom = -1.0 / aspect;
      yTop    = 1.0 / aspect;
   } else {
      xLeft   = -1.0 * aspect;
      xRight  = 1.0 * aspect;
      yBottom = -1.0;
      yTop    = 1.0;
   }
   gluOrtho2D(xLeft, xRight, yBottom, yTop);
   xPosMin = xLeft + ballRadius;
   xPosMax = xRight - ballRadius;
   yPosMin = yBottom + ballRadius;
   yPosMax = yTop - ballRadius;