GLUT and OpenGL

by Robby T. Tan



1. GLUT window init

#include <GL/glut.h>

int main(int argc, char **argv)
{ 
   // #1: GLUT window initialization
   glutInit(&argc, argv);
   glutInitDisplayMode(GLUT_RGB|GLUT_DOUBLE);
   glutInitWindowSize(600, 600);
   glutInitWindowPosition(0, 0);
   glutCreateWindow("Window's Title");

   // #2: Registering callbacks
   
   // #3: GLUT main loop
   glutMainLoop();
   
   return 0;
}

2. Reshape + Display in 2D 

#include <GL/glut.h>

void reshape(int width, int height);
void display();

int main(int argc, char **argv)
{ 
   // #1: GLUT window initialization
   glutInit(&argc, argv);
   glutInitDisplayMode(GLUT_RGB|GLUT_DOUBLE);
   glutInitWindowSize(600, 600);
   glutInitWindowPosition(0, 0);
   glutCreateWindow("Window's Title");

   //glutFullScreen(); //optional

   // #2: Registering callbacks
   glutReshapeFunc(reshape);
   glutDisplayFunc(display);

   // #3: GLUT main loop
   glutMainLoop();
   
   return 0;
}

void reshape( int w, int h)
{
    glViewport(0, 0, w, h);		
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();    
    glOrtho( 0, w, 0, h , -1.0, 1.0 ); // the origin is at the bottom left!!
    glMatrixMode(GL_MODELVIEW);
}

void display()
{
    glClear( GL_COLOR_BUFFER_BIT);
    
    // to draw the 2D objects
    
    glutSwapBuffers();
}

3. Draw 2D point 

#include <GL/glut.h>

void reshape(int width, int height);
void display();
void draw_point(int x, int y);

int main(int argc, char **argv)
{ 
   // #1: GLUT window initialization
   glutInit(&argc, argv);
   glutInitDisplayMode(GLUT_RGB|GLUT_DOUBLE);
   glutInitWindowSize(600, 600);
   glutInitWindowPosition(0, 0);
   glutCreateWindow("Window's Title");

   //glutFullScreen(); //optional

   // #2: Registering callbacks
   glutReshapeFunc(reshape);
   glutDisplayFunc(display);

   // #3: GLUT main loop
   glutMainLoop();
   
   return 0;
}

void reshape( int w, int h)
{
    glViewport(0, 0, w, h);		
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();    
    glOrtho( 0, w, 0, h , -1.0, 1.0 ); // the origin is at the bottom left!!
    glMatrixMode(GL_MODELVIEW);
}

void display()
{
    glClear( GL_COLOR_BUFFER_BIT);
    
    // to draw a point
    draw_point(300,300);
    
    glutSwapBuffers();
}

void draw_point( int x, int y )
{
  // to set the color of the point
  GLfloat RED[]= { 1.0, 0.0, 0.0 };
  glColor3fv(RED);

  // to set the size of the point (or square, precisely)
  int point_size = 10;
  glPointSize(point_size);

  // to draw the point
  glBegin( GL_POINTS );
  glVertex2f( x,y );
  glEnd();
}

4. Draw 2D line 

#include <GL/glut.h>

void reshape(int width, int height);
void display();
void draw_closed_curve(int x, int y);
void draw_open_curve(int x, int y);

int main(int argc, char **argv)
{ 
   // #1: GLUT window initialization
   glutInit(&argc, argv);
   glutInitDisplayMode(GLUT_RGB|GLUT_DOUBLE);
   glutInitWindowSize(600, 600);
   glutInitWindowPosition(0, 0);
   glutCreateWindow("Window's Title");

   //glutFullScreen(); //optional

   // #2: Registering callbacks
   glutReshapeFunc(reshape);
   glutDisplayFunc(display);

   // #3: GLUT main loop
   glutMainLoop();
   
   return 0;
}

void reshape( int w, int h)
{
    glViewport(0, 0, w, h);		
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();    
    glOrtho( 0, w, 0, h , -1.0, 1.0 ); // the origin is at the bottom left!!
    glMatrixMode(GL_MODELVIEW);
}

void display()
{
    glClear( GL_COLOR_BUFFER_BIT);
    
    // to draw lines
    draw_open_curve(400,400);
    draw_closed_curve(200,200);
    
    glutSwapBuffers();
}

void draw_open_curve(int x, int y)
{
  // set the color of the line:
  GLfloat GREEN[]= { 0.0, 1.0, 0.0 };
  glColor3fv(GREEN);

  int SQUARE_SIZE = 50;

  // draw an open curve:
  glBegin(GL_LINE_STRIP);
  glVertex2f( x - SQUARE_SIZE, y - SQUARE_SIZE);
  glVertex2f( x + SQUARE_SIZE, y - SQUARE_SIZE);
  glVertex2f( x + SQUARE_SIZE, y + SQUARE_SIZE);
  glVertex2f( x - SQUARE_SIZE, y + SQUARE_SIZE);
  glEnd( );
}

void draw_closed_curve(int x, int y)
{
  // set the color of the line:
  GLfloat GREEN[]= { 0.0, 1.0, 0.0 };
  glColor3fv(GREEN);

  int SQUARE_SIZE = 50;

  // draw a closed curve:
  glBegin(GL_LINE_LOOP);
  glVertex2f( x - SQUARE_SIZE, y - SQUARE_SIZE);
  glVertex2f( x + SQUARE_SIZE, y - SQUARE_SIZE);
  glVertex2f( x + SQUARE_SIZE, y + SQUARE_SIZE);
  glVertex2f( x - SQUARE_SIZE, y + SQUARE_SIZE);
  glEnd( );
}

5. Mouse



Note: pay attention to the reshape function: we change the origin position due to the coordinate system used by the mouse. 

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

using namespace std;

#define POINT_SIZE    20

// Declaring the functions:
void reshape(int width, int height);
void display();
void mouseClick(int button, int state, int x, int y);
void mouseMotion(int x, int y);
void draw_point(int x, int y);
void draw_all_points();
void modify_point(int j, int x, int y);
void add_point(int x, int y);
int doesPointExist(int x, int y);

// Declaring the global data structure
struct s_point{
  float x, y;
};

// Setting the global variables:
int m_modify;
vector<s_point> m_points;
GLfloat RED[] = { 1.0, 0.0, 0.0 };



int main(int argc, char **argv)
{ 
   // #1: GLUT window initialization
   glutInit(&argc, argv);
   glutInitDisplayMode(GLUT_RGB|GLUT_DOUBLE);
   glutInitWindowSize(600, 600);
   glutInitWindowPosition(0, 0);
   glutCreateWindow("Window's Title");

   //glutFullScreen(); //optional

   // #2: Registering callbacks
   glutReshapeFunc(reshape);
   glutDisplayFunc(display);
   glutMouseFunc(mouseClick);    // mouse click input
   glutMotionFunc(mouseMotion);  // mouse motion input

   // #3: GLUT main loop
   glutMainLoop();
   
   return 0;
}

void reshape(int w, int h)
{
   glViewport(0, 0, w, h);		
   glMatrixMode(GL_PROJECTION);
   glLoadIdentity();    
   // glOrtho( 0, w, 0, h , -1.0, 1.0 ); // the origin is at the bottom left!!

   // below, we change the origin to the top left,
   // since the mouse motion is based on the top-left origin 
   glOrtho( 0, w, h, 0 , -1.0, 1.0 ); 

   glMatrixMode(GL_MODELVIEW);
}

void display()
{
   glClear( GL_COLOR_BUFFER_BIT);
    
   // to draw points
   draw_all_points(); 
    
   glutSwapBuffers();
}

void mouseClick(int button, int state, int x, int y)
{
   m_modify = -1;

   if( button == GLUT_LEFT_BUTTON && state == GLUT_DOWN ){   
     
     // to check if the point exists:
     int j = doesPointExist(x,y);
     
     
     if(j<0) add_point(x,y);
     else m_modify = j;
     
   }
   
   glutPostRedisplay();
}

void mouseMotion(int x, int y)
{
  if(m_modify>=0) 
     modify_point(m_modify, x, y);

  glutPostRedisplay();
}

int doesPointExist(int x, int y)
{
  for(int i = 0 ; i < m_points.size();i++){
    if( (x > m_points.at(i).x - POINT_SIZE) &&
	(x < m_points.at(i).x + POINT_SIZE) &&
	(y > m_points.at(i).y - POINT_SIZE) &&
	(y < m_points.at(i).y + POINT_SIZE))
      return i;
  }
  return -1;
}

void draw_point(int x, int y)
{
  glColor3fv(RED);
  
  glPointSize(POINT_SIZE);
  glBegin(GL_POINTS);
  glVertex2f(x,y);
  glEnd();
}

void draw_all_points()
{
  for(int i = 0; i< m_points.size(); i++){
    draw_point(m_points.at(i).x, m_points.at(i).y);
  }
}

void modify_point(int j, int x, int y)
{
  m_points.at(j).x = x;
  m_points.at(j).y = y;
}

void add_point(int x, int y)
{
  s_point np;
  np.x = x;
  np.y = y;

  m_points.push_back(np);
}

6. Keyboard 

#include <GL/glut.h>

// Declaring the functions:
void reshape(int width, int height);
void display();
void keyboard(unsigned char key, int x, int y);

// Declaring the global variables:
int m_glwindow;

int main(int argc, char **argv)
{ 
   // #1: GLUT window initialization
  glutInit(&argc, argv);
  glutInitDisplayMode(GLUT_RGB|GLUT_DOUBLE);
  glutInitWindowSize(600, 600);
  glutInitWindowPosition(0, 0);
  m_glwindow = glutCreateWindow("Window's Title");
  
  //glutFullScreen(); //optional
  
  // #2: Registering callbacks
  glutReshapeFunc(reshape);
  glutDisplayFunc(display);
  glutKeyboardFunc(keyboard);
  
  // #3: GLUT main loop
  glutMainLoop();
  
  return 0;
}

void reshape(int w, int h)
{
  glViewport(0, 0, w, h);		
  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();    
  glOrtho( 0, w, 0, h , -1.0, 1.0 ); // the origin is at the bottom left!!
  glMatrixMode(GL_MODELVIEW);
}

void display()
{
  glClear( GL_COLOR_BUFFER_BIT);
  
  // to draw points
  
  glutSwapBuffers();
}

void keyboard(unsigned char key, int x, int y) 
{
  if (key == 27 || key == 'q'){  // 27 = esc
    glutDestroyWindow(m_glwindow); 
    exit(0);                   
  }
}

7. Reshape + Display in 3D



gluLookAt(camx, camy, camz, posx, posy, posz,ux, uy, uz):


glulookat

  1. [camx, camy, camz] = the position of the camera or eye.
  2. [posx, posy, posz] = the position of the target point.
  3. [ux, uy, uz] = the camera up vector. It basically allows the camera to be tilted or rotated even though it keeps its position and its gazing direction. Imagine if we look at a point and then tilt our head. The up vector can define this tilting condition. Normally, it is set to [0,0,1] if the z-axis is the vertical axis (as you do not often tilt your head). Or, [0,1,0] if the y-axis is the vertical axis.
    upvector

gluPerspective(fovy,w/h,near,far):

gluPerspective 

#include <GL/glut.h>


void reshape(int width, int height);
void display();

float zoom = 1.0;

int main(int argc, char **argv)
{ 
   // #1: GLUT window initialization
   glutInit(&argc, argv);
   glutInitDisplayMode(GLUT_RGB|GLUT_DOUBLE);
   glutInitWindowSize(600, 600);
   glutInitWindowPosition(0, 0);
   glutCreateWindow("Window's Title");

   glutFullScreen(); //optional

   // #2: Registering callbacks
   glutReshapeFunc(reshape);
   glutDisplayFunc(display);

   // #3: GLUT main loop
   glutMainLoop();
   
   return 0;
}

void reshape(int w, int h)
{
  glViewport(0, 0, w, h);		
  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();

  // to prevent a divide by zero 
  if (h==0) h=1;

  gluPerspective(zoom*45.0f,-(GLfloat)w/(GLfloat)h,0.1f,100.0f);
  glMatrixMode(GL_MODELVIEW);
}


void display()
{
  glClear(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);	
  glLoadIdentity();				

  gluLookAt(0,50,0, // eye position
	    0,0,0,  // target point's position
	    0,0,1); // up vector

  // #: to draw coordinates
  float length = 10.0f;
  
  // Z axis
  glColor3f(1,1,0); // yellow
  glBegin(GL_LINES);
  glVertex3f(0.0,0.0,length);
  glVertex3f(0.0,0.0,0);
  glEnd();

  // Y axis
  glColor3f(0,0,1); // blue
  glBegin(GL_LINES);
  glVertex3f(0.0,length,0.0);
  glVertex3f(0.0,0.0,0.0);
  glEnd();

  // X axis
  glColor3f(1,0,0); // red
  glBegin(GL_LINES);
  glVertex3f(length,0.0,0.0);
  glVertex3f(0.0,0.0,0.0);
  glEnd();

  // swap the buffers to display, since double buffering is used.
  glutSwapBuffers();
}

8. Idle 

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

void reshape(int width, int height);
void display();
void idle();

const float rad = 0.01745;
float move=0;
float camerarotationx = 2 * sin(rad * move);
float camerarotationy = 2 * cos(rad * move);
float zoom = 1.0;

int main(int argc, char **argv)
{ 
   // #1: GLUT window initialization
   glutInit(&argc, argv);
   glutInitDisplayMode(GLUT_RGB|GLUT_DOUBLE);
   glutInitWindowSize(600, 600);
   glutInitWindowPosition(0, 0);
   glutCreateWindow("Window's Title");

   //glutFullScreen(); //optional

   // #2: Registering callbacks
   glutReshapeFunc(reshape);
   glutDisplayFunc(display);
   glutIdleFunc(idle);


   // #3: GLUT main loop
   glutMainLoop();
   
   return 0;
}

void reshape(int w, int h)
{
  glViewport(0, 0, w, h);		
  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();

  // to prevent a divide by zero 
  if (h==0) h=1;

  gluPerspective(zoom*45.0f,(GLfloat)w/(GLfloat)h,0.1f,1000.0f);
  glMatrixMode(GL_MODELVIEW);
}


void display()
{
  glClear(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);	
  glLoadIdentity();				


  gluLookAt(100*camerarotationx,100*camerarotationy,0,  // eye position
	    0,0,0,                                      // target point's position   
	    0,0,1);				        // the up vector



  // #: to draw coordinates
  float length = 30.0f;
  
  // Z axis
  glColor3f(1,1,0);
  glBegin(GL_LINES);
  glVertex3f(0.0,0.0,length);
  glVertex3f(0.0,0.0,0);
  glEnd();
  // Y axis
  glColor3f(0,0,1);
  glBegin(GL_LINES);
  glVertex3f(0.0,length,0.0);
  glVertex3f(0.0,0.0,0.0);
  glEnd();
  // X axis
  glColor3f(1,0,0);
  glBegin(GL_LINES);
  glVertex3f(length,0.0,0.0);
  glVertex3f(0.0,0.0,0.0);
  glEnd();

  // swap the buffers to display, since double buffering is used.
  glutSwapBuffers();
}

void idle()
{
  // when idle, change the viewing position orbiting the origin:
  move += 1.1f;
  if( move>1000) move=0; 

  camerarotationx = sin(rad * move); 
  camerarotationy = cos(rad * move);

  glutPostRedisplay();
}

Additional Resources:
  1. http://www.cs.uoi.gr/~fudos/opengl-tutorial/search.htm
  2. http://fly.cc.fer.hr/~unreal/theredbook/
  3. OpenGL examples
  4. GLUT: an introduction