A Quick OpenGL Tutorial

OK, so now you (hopefully) know how to move stuff around using OpenGL, lets do one more thing: depth testing. This is the process of determining what falls in front of what…in other words, is the house in front of the dog or is the dog in front of the house? This may sound trivial at first, but in order to get realistic looking scenes we usually use something called a z-buffer, which tests each pixel to see if it should is unhidden and should therefore be drawn. This would normally be a big to-do, but OpenGL is nice enough to handle all the details for us. Be warned though: unless you have a video accelerator with a lot of video memory, z-buffering will slow down rendering immensely. Z-buffers can make for really cool effects (such as objects passing through other objects), but watch out for the performance hit. That being said, applying a z-buffer to your scene is easy; first, put a call to glEnable in your initialization, like so:

glEnable(GL_DEPTH_TEST);

Then, clear your z-buffer at the end of every frame. You can do this at the same time you clear your drawing window, like so:

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

Simple, eh? Well now we have depth-cued perspective motion… in other words, your resume is stacked and you're rearing to go! Lets continue on to the final example:

/**********************************************************************/

/********************************************************************/

/* Example 3: A fully interactive 3D world with OpenGL */

/********************************************************************/

#include

#include

#include

#include /* GLU extention library */

void init(void);

void display(void);

void keyboard(unsigned char, int, int);

void resize(int, int);

void drawcube(int, int, int);

int is_depth; /* depth testing flag */

int main (int argc, char **argv) {

glutInit(&argc, argv);

glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);

glutInitWindowSize(600, 600);

glutInitWindowPosition(40, 40);

glutCreateWindow("The Cube World");

init();

glutDisplayFunc(display);

glutKeyboardFunc(keyboard);

/* this time we're going to keep the aspect ratio constant by trapping the window resizes */

glutReshapeFunc(resize);

glutMainLoop();

return 0;

}

void init(void) {

glClearColor(0.0, 0.0, 0.0, 0.0);

glEnable(GL_DEPTH_TEST);

is_depth = 1;

glMatrixMode(GL_MODELVIEW);

}

void display(void) {

if (is_depth) glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

else glClear(GL_COLOR_BUFFER_BIT);

/* draw the floor */

glBegin(GL_QUADS);

glColor3f(0.2f, 0.2f, 0.2f);

glVertex3f(-100.0, 0.0, –100.0);

glColor3f(0.4f, 0.4f, 0.4f);

glVertex3f(-100.0, 0.0, 100.0);

glColor3f(0.6f, 0.6f, 0.6f);

glVertex3f(100.0, 0.0, 100.0);

glColor3f(0.8f, 0.8f, 0.8f);

glVertex3f(100.0, 0.0, –100.0);

glEnd();

/* draw 12 cubes with different colors */

drawcube(75, 57, 2);

drawcube(-65, –12, 3);

drawcube(50, –50, 1);

drawcube(-56, 17, 2);

drawcube(67, 12, 3);

drawcube(-87, 32, 1);

drawcube(-26, 75, 2);

drawcube(57, 82, 3);

drawcube(-3, 12, 1);

drawcube(46, 35, 2);

drawcube(37, –2, 3);

glutSwapBuffers();

}

void keyboard(unsigned char key, int x, int y) {

/* This time the controls are:

"a": move left

"d": move right

"w": move forward

"s": move back

"t": toggle depth-testing */

switch (key) {

case 'a':

case 'A':

glTranslatef(5.0, 0.0, 0.0);

break;

case 'd':

case 'D':

glTranslatef(-5.0, 0.0, 0.0);

break;

case 'w':

case 'W':

glTranslatef(0.0, 0.0, 5.0);

break;

case 's':

case 'S':

glTranslatef(0.0, 0.0, –5.0);

break;

case 't':

case 'T':

if (is_depth) {

is_depth = 0;

glDisable(GL_DEPTH_TEST);

} else {

is_depth = 1;

glEnable(GL_DEPTH_TEST);

}

}

display();

}

void resize(int width, int height) {

if (height == 0) height = 1;

glMatrixMode(GL_PROJECTION);

glLoadIdentity();

/* note we divide our width by our height to get the aspect ratio */

gluPerspective(45.0, width / height, 1.0, 400.0);

/* set initial position */

glTranslatef(0.0, –5.0, –150.0);

glMatrixMode(GL_MODELVIEW);

}

void drawcube(int x_offset, int z_offset, int color) {

/* this function draws a cube centerd at (x_offset, z_offset) x and z _big are the back and rightmost points, x and z _small are the front and leftmost points */

float x_big = (float)x_offset + 5;

float z_big = (float)z_offset + 5;

float x_small = (float)x_offset – 5;

float z_small = (float)z_offset – 5;

switch(color) {

case 1:

glColor3f(1.0,0.0,0.0);

break;

case 2:

glColor3f(0.0,1.0,0.0);

break;

case 3:

glColor3f(0.0,0.0,1.0);

break;

}

glBegin(GL_QUADS);

glVertex3f(x_small,10.0,z_big); /* front */

glVertex3f(x_small,0.0,z_big);

glVertex3f(x_big,0.0,z_big);

glVertex3f(x_big,10.0,z_big);

glVertex3f(x_big,10.0,z_small); /* back */

glVertex3f(x_big,0.0,z_small);

glVertex3f(x_small,0.0,z_small);

glVertex3f(x_small,10.0,z_small);

glVertex3f(x_big,10.0,z_big); /* right */

glVertex3f(x_big,0.0,z_big);

glVertex3f(x_big,0.0,z_small);

glVertex3f(x_big,10.0,z_small);

glVertex3f(x_small,10.0,z_small); /* left */

glVertex3f(x_small,0.0,z_small);

glVertex3f(x_small,0.0,z_big);

glVertex3f(x_small,10.0,z_big);

glVertex3f(x_small,10.0,z_big); /* top */

glVertex3f(x_big,10.0,z_big);

glVertex3f(x_big,10.0,z_small);

glVertex3f(x_small,10.0,z_small);

glVertex3f(x_small,0.0,z_small); /* bottom */