#! /usr/bin/perl -w

use OpenGL qw(:all);    # Use the OpenGL module
use strict;             # Use strict typechecking

# ASCII constant for the escape key
use constant ESCAPE => 27;

# Global variable for our window
my $window;

# A general GL initialization function 
# Called right after our OpenGL window is created
# Sets all of the initial parameters
sub InitGL {              

    # Shift the width and height off of @_, in that order
    my ($width, $height) = @_;

    # Set the background "clearing color" to black
    glClearColor(0.0, 0.0, 0.0, 0.0);

    # Enables clearing of the Depth buffer 
    glClearDepth(1.0);                    

    # The type of depth test to do
    glDepthFunc(GL_LESS);         

    # Enables depth testing with that type
    glEnable(GL_DEPTH_TEST);              
    
    # Enables smooth color shading
    glShadeModel(GL_SMOOTH);      

    # Reset the projection matrix
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity;

    # Calculate the aspect ratio of the Window
    gluPerspective(45.0, $width/$height, 0.1, 100.0);

    # Reset the modelview matrix
    glMatrixMode(GL_MODELVIEW);
}


# The function called when our window is resized 
# This shouldn't happen, because we're fullscreen
sub ReSizeGLScene {

    # Shift width and height off of @_, in that order
    my ($width, $height) = @_;

    # Prevent divide by zero error if window is too small
    if ($height == 0) { $height = 1; }

    # Reset the current viewport and perspective transformation
    glViewport(0, 0, $width, $height);              

    # Re-initialize the window (same lines from InitGL)
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity;
    gluPerspective(45.0, $width/$height, 0.1, 100.0);
    glMatrixMode(GL_MODELVIEW);
}

# The main drawing function.
sub DrawGLScene {

    # Clear the screen and the depth buffer
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);  

    # Reset the view
    glLoadIdentity;

    # Since this is double buffered, swap the buffers.
    # This will display what just got drawn.
    glutSwapBuffers;
}

# The function called whenever a key is pressed. 
sub keyPressed {
    # Shift the unsigned char key, and the x,y placement off @_, in
    # that order.
    my ($key, $x, $y) = @_;
    
    # Avoid thrashing this procedure
    # Note Perl does not support usleep
    # For finer resolution sleep than seconds, try:
    #    'select undef, undef, undef, 0.1;'
    # to sleep for (at least) 0.1 seconds
    sleep(100);

    # If escape is pressed, kill everything.
    if ($key == ESCAPE) 
    { 
        # Shut down our window 
        glutDestroyWindow($window); 
        
        # Exit the program...normal termination.
        exit(0);                   
    }
}

# --- Main program ---

# Initialize GLUT state
glutInit;  

# Select type of Display mode:   
# Double buffer 
# RGB color (Also try GLUT RGBA)
# Alpha components removed (try GLUT_ALPHA) 
# Depth buffer */  
glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE | GLUT_DEPTH);  

# Get a 640 x 480 window
glutInitWindowSize(300, 200);  

# The window starts at the upper left corner of the screen
glutInitWindowPosition(0, 0);  

# Open the window  
$window = glutCreateWindow("Lesson01");
# Register the function to do all our OpenGL drawing.
glutDisplayFunc(\&DrawGLScene);  

# Go fullscreen.  This is as soon as possible. 
#glutFullScreen;

# Even if there are no events, redraw our gl scene.
glutIdleFunc(\&DrawGLScene);

# Register the function called when our window is resized. 
glutReshapeFunc(\&ReSizeGLScene);

# Register the function called when the keyboard is pressed.
glutKeyboardFunc(\&keyPressed);

# Initialize our window.
InitGL(300, 200);
  
# Start Event Processing Engine
glutMainLoop;  

return 1;
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