The latest version of my sample clone!

Here’s the last part of my multi-part series on programming your very own pong clone in c and opengl!  It’s been a long time coming, so I hope you enjoy.  There’s lots of improvements in this version including a completely custom old-style “LED Alarm Clock” scoreboard, flashes on bounce, funky changing colors, improved collision detection, and a round ball!

Here’s the code!

#include <stdlib.h>
#include <stdio.h>
#include <math.h>

#include <OpenAL/al.h>
#include <GLUT/glut.h>

#define PADDLE_MOVE_SPEED 350
#define CLEAR_COLOR_DECAY 2.0
#define BALL_RADIUS 10
#define BALL_FACETS 60
#define START_SPEED 200

int lastFrameTime = 0, windowWidth = 0, windowHeight = 0, now = 0;
float midWidth = 0, midHeight = 0;
float elapsedTime = 0, elapsedMilliSeconds = 0;
char player1Move = 0, player2Move = 0;
float player1V = 0, player2V = 0;

float PI = 3.14159265358979323846264;

//scores
int player1Score = 0, player2Score = 0;

//the following variables calculate a random starting projectile trajectory
float randomDirection;
long longMax = 0x7fffffff;	//max of longs 2^31-1 (2^(n-1)-1 for 2's complement) float trajectory[2] = {0, 0};

float projMove[2] = {0, 0};
double trajectory[2] = {0, 0};
short moveSpeed = START_SPEED;

//glitz and glamour
float player1ColorDirection[3];
float player2ColorDirection[3];
float player1Color[3];
float player2Color[3];
float colorMoveSpeed = 2.0;
double vectorLength1 = 0;
double vectorLength2 = 0;

float ballColorDirection[3];
float ballColor[3];
double ballVectorLength = 0;

//background explosion color
float clearColor[3];

int i = 0;

//scoreboard
char player1Num1[7] = {1, 1, 1, 0, 1, 1, 1};	//digital display set to zero, (each segment is enabled or disabled)
char player1Num2[7] = {1, 1, 1, 0, 1, 1, 1};
char player2Num1[7] = {1, 1, 1, 0, 1, 1, 1};	//digital display set to zero, (each segment is enabled or disabled)
char player2Num2[7] = {1, 1, 1, 0, 1, 1, 1};

//digit representations
char digit0[7] = {1, 1, 1, 0, 1, 1, 1};
char digit1[7] = {0, 0, 1, 0, 0, 0, 1};
char digit2[7] = {0, 1, 1, 1, 1, 1, 0};
char digit3[7] = {0, 1, 1, 1, 0, 1, 1};
char digit4[7] = {1, 0, 1, 1, 0, 0, 1};
char digit5[7] = {1, 1, 0, 1, 0, 1, 1};
char digit6[7] = {1, 1, 0, 1, 1, 1, 1};
char digit7[7] = {0, 1, 1, 0, 0, 0, 1};
char digit8[7] = {1, 1, 1, 1, 1, 1, 1};
char digit9[7] = {1, 1, 1, 1, 0, 1, 1};

void setSpaceEffect(void) {
for (i=0; i < 3; i++) {
player1ColorDirection[i] = (float) random() / (float) 0x7fffffff; //random vector
player2ColorDirection[i] = (float) random() / (float) 0x7fffffff;
ballColorDirection[i] = (float) random() / (float) 0x7fffffff;
}
//figure out vector lenngth
for (i=0; i < 3; i++) {
vectorLength1 += player1ColorDirection[i] * player1ColorDirection[i];
vectorLength2 += player2ColorDirection[i] * player2ColorDirection[i];
ballVectorLength += ballColorDirection[i] * ballColorDirection[i];
}
sqrt(vectorLength1);
sqrt(vectorLength2);
sqrt(ballVectorLength);
//normalize vector
for (i=0; i < 3; i++) {
player1ColorDirection[i] /= vectorLength1;
player2ColorDirection[i] /= vectorLength2;
ballColorDirection[i] /= ballVectorLength;
}
}

void calculateColor(float color[3], float colorDirection[3]) {
//caculate the next color based on the direction and elapsed time values
for (i=0; i < 3; i++) {
color[i] += colorDirection[i] * colorMoveSpeed * elapsedTime;
if (color[i] > 1) {
color[i] = 1;
colorDirection[i] *= -1;
}
else if (color[i] < 0) {
color[i] = 0;
colorDirection[i] *= -1;
}
}
}

void calculateClearColor(float color[3]) {
for (i=0; i < 3; i++) {
color[i] -= CLEAR_COLOR_DECAY * elapsedTime;
if(color[i] < 0)
color[i] = 0;
}
}

void setClearColor(float color[3]) {
for (i=0; i < 3; i++) {
color[i] = 1.0;
}
}

void resetProjectile(void) {
//set starting random trajectory
randomDirection = 2 * PI * (float) random() / (float) 0x7fffffff;
trajectory[0] = (float) cos(randomDirection);
trajectory[1] = (float) sin(randomDirection);
projMove[0] = midWidth;
projMove[1] = midHeight;
}

/*draw a digital (a la clock style) digit some place specified by x and y
*the enabled array is which segments of the display are enabled (counted starting from top left in a snake/8 fasion)
*
* 1 -
* 0| |2
* 3 -
* 4| |6
* 5 -
*/
void drawDigit(char enabled[7], float x, float y) {
glPushMatrix();
glTranslatef(x, y, 0);	//position digit to be drawn
//draw digital number (a la digital clock) segment by segment
glColor3f(1.0, 1.0, 1.0);
//draw segment 5
if(enabled[5]) {
glBegin(GL_QUADS);
glVertex2f(0,0);
glVertex2f(10,0);
glVertex2f(10,2);
glVertex2f(0,2);
glEnd();
}

//draw segment 3
if(enabled[3]) {
glBegin(GL_QUADS);
glVertex2f(0,9);
glVertex2f(10,9);
glVertex2f(10,11);
glVertex2f(0,11);
glEnd();
}

//draw segment 1
if(enabled[1]) {
glBegin(GL_QUADS);
glVertex2f(0,18);
glVertex2f(10,18);
glVertex2f(10,20);
glVertex2f(0,20);
glEnd();
}

//draw segment 4
if(enabled[4]) {
glBegin(GL_QUADS);
glVertex2f(0,2);
glVertex2f(2,2);
glVertex2f(2,9);
glVertex2f(0,9);
glEnd();
}

//draw segment 0
if(enabled[0]) {
glBegin(GL_QUADS);
glVertex2f(0,11);
glVertex2f(2,11);
glVertex2f(2,20);
glVertex2f(0,20);
glEnd();
}

//draw segment 6
if(enabled[6]) {
glBegin(GL_QUADS);
glVertex2f(8,2);
glVertex2f(10,2);
glVertex2f(10,9);
glVertex2f(8,9);
glEnd();
}

//draw segment 2
if(enabled[2]) {
glBegin(GL_QUADS);
glVertex2f(8,11);
glVertex2f(10,11);
glVertex2f(10,20);
glVertex2f(8,20);
glEnd();
}

glPopMatrix();
}

void setDigitEnablePattern(char enabled[7], char digit) {

switch(digit) {
case 0:
for(i=0; i<7; i++) {
enabled[i] = digit0[i];
}
break;
case 1:
for(i=0; i<7; i++) {
enabled[i] = digit1[i];
}
break;
case 2:
for(i=0; i<7; i++) {
enabled[i] = digit2[i];
}
break;
case 3:
for(i=0; i<7; i++) {
enabled[i] = digit3[i];
}
break;
case 4:
for(i=0; i<7; i++) {
enabled[i] = digit4[i];
}
break;
case 5:
for(i=0; i<7; i++) {
enabled[i] = digit5[i];
}
break;
case 6:
for(i=0; i<7; i++) {
enabled[i] = digit6[i];
}
break;
case 7:
for(i=0; i<7; i++) {
enabled[i] = digit7[i];
}
break;
case 8:
for(i=0; i<7; i++) {
enabled[i] = digit8[i];
}
break;
case 9:
for(i=0; i<7; i++) {
enabled[i] = digit9[i];
}
break;
}
}

void drawScore(char player1, char player2) {

if(player1 < 10) {
setDigitEnablePattern(player1Num1, 0);
setDigitEnablePattern(player1Num2, player1);
}
else if (player1 < 100) {
setDigitEnablePattern(player1Num1, player1 / 10);
setDigitEnablePattern(player1Num2, player1 - ((player1/10) * 10));
}
else {	//max out at 99
setDigitEnablePattern(player1Num1, 9);
setDigitEnablePattern(player1Num2, 9);
}

if(player2 < 10) {
setDigitEnablePattern(player2Num1, 0);
setDigitEnablePattern(player2Num2, player2);
}
else if (player2 < 100) {
setDigitEnablePattern(player2Num1, player2 / 10);
setDigitEnablePattern(player2Num2, player2 - ((player2/10) * 10));
}
else {	//max out at 99
setDigitEnablePattern(player2Num1, 9);
setDigitEnablePattern(player2Num2, 9);
}

drawDigit(player1Num1, (windowWidth/4)-15, 20);
drawDigit(player1Num2, (windowWidth/4)+5, 20);
drawDigit(player2Num1, ((windowWidth/4)*3)-15, 20);
drawDigit(player2Num2, ((windowWidth/4)*3)+5, 20);

}

void display(void)
{
//get window parameters
windowWidth = glutGet(GLUT_WINDOW_WIDTH);
windowHeight = glutGet(GLUT_WINDOW_HEIGHT);

midWidth = windowWidth/2;
midHeight = windowHeight/2;

//start timing code
if (lastFrameTime == 0) {
lastFrameTime = glutGet(GLUT_ELAPSED_TIME);
resetProjectile();	//since this is the fist time, this is actually a set 
player1V = midHeight;
player2V = midHeight;
//initialize the space effect colors
setSpaceEffect();
glClearColor(0.0, 0.0, 0.0, 0.0);	//set clear color to black
}

now = glutGet(GLUT_ELAPSED_TIME);
elapsedMilliSeconds = now - lastFrameTime;
elapsedTime = elapsedMilliSeconds/1000.0f;
lastFrameTime = now;

calculateClearColor(clearColor);
glClearColor(clearColor[0], clearColor[1], clearColor[2], 0.0);
glClear(GL_COLOR_BUFFER_BIT);
glShadeModel(GL_FLAT);

//draw score
drawScore(player1Score, player2Score);

//calculate move distances
switch (player1Move) {
case 1:		//move up
player1V += PADDLE_MOVE_SPEED * elapsedTime;
break;
case -1:	//move up
player1V -= PADDLE_MOVE_SPEED * elapsedTime;
break;
default:	//do nothing
break;
}
//check out of bounds
if (player1V > windowHeight-50) {
//bounce back
player1Move = -1;
}
if (player1V < 0+50) {
//bounce back other way
player1Move = 1;
}

switch (player2Move) {
case 1:		//move up
player2V += PADDLE_MOVE_SPEED * elapsedTime;
break;
case -1:	//move up
player2V -= PADDLE_MOVE_SPEED * elapsedTime;
break;
default:	//do nothing
break;
}
//check out of bounds
if (player2V > windowHeight-50) {
//bounce back
player2Move = -1;
}
if (player2V < 0+50) {
//bounce back other way
player2Move = 1;
}

//create and position player 1
glPushMatrix();

calculateColor(player1Color, player1ColorDirection);
glColor3f(player1Color[0], player1Color[1], player1Color[2]);	//player 1 color

glTranslatef(30.0f, player1V, 0.0f);

glBegin(GL_QUADS);
glVertex2f(-10, -50);
glVertex2f(-10, 50);
glVertex2f(10, 50);
glVertex2f(10, -50);
glEnd();

glPopMatrix();

//create and position player 2
glPushMatrix();

calculateColor(player2Color, player2ColorDirection);
glColor3f(player2Color[0], player2Color[1], player2Color[2]);	//player 2 color

glTranslatef(windowWidth-30, player2V, 0.0f);

glBegin(GL_QUADS);
glVertex2f(-10, -50);
glVertex2f(-10, 50);
glVertex2f(10, 50);
glVertex2f(10, -50);
glEnd();

glPopMatrix();

/***************************************
*	Projectile CODE!!!
*
***************************************/

//calculate move distances
projMove[0] += moveSpeed * trajectory[0] * elapsedTime;
projMove[1] += moveSpeed * trajectory[1] * elapsedTime;

//create and move projectile
glPushMatrix();

calculateColor(ballColor, ballColorDirection);
glColor3f(ballColor[0], ballColor[1], ballColor[2]);	//player 1 color

glTranslatef(projMove[0], projMove[1], 0.0f);

glBegin(GL_TRIANGLE_FAN);
for(i=0; i < 360; i += 360 / BALL_FACETS) {
glVertex2f(sin(i * PI / 180) * BALL_RADIUS, cos(i * PI / 180) * BALL_RADIUS);
}
glEnd();

glPopMatrix();

//set bounds for projectile
if (projMove[0] > (windowWidth)) {	//player 1 scored - out of bounds right
player1Score++;
moveSpeed = START_SPEED;
printf("Player 1 Scored!  Score: %f ", player1Score);
resetProjectile();
}

else if (projMove[0] < (0)) {		//player 2 scored - out of bounds left
player2Score++;
moveSpeed = START_SPEED;
resetProjectile();
}

else if (projMove[1] > (windowHeight-10)) {	//out of bounds top
trajectory[1] *= -1;
projMove[1] -= 0.5;
}

else if (projMove[1] < (10)) {	//out of bounds bottom
trajectory[1] *= -1;
projMove[1] += 0.5;
}

//collision detection code
else if (projMove[0] < 50 &amp;amp;amp;amp;amp;amp;amp;&amp;amp;amp;amp;amp;amp;amp; projMove[0] > 49) {		//intersecting with line of left paddle
if (projMove[1] <= player1V + 50 &amp;amp;amp;amp;amp;amp;amp;&amp;amp;amp;amp;amp;amp;amp; projMove[1] >= player1V-50) {
trajectory[0] *= -1;
moveSpeed += 30;
setClearColor(clearColor);
projMove[0] += 0.5;
}
}

else if (projMove[0] > windowWidth-50 &amp;amp;amp;amp;amp;amp;amp;&amp;amp;amp;amp;amp;amp;amp; projMove[0] < windowWidth-49) {		//intersecting with line of right paddle
if (projMove[1] <= player2V + 50 &amp;amp;amp;amp;amp;amp;amp;&amp;amp;amp;amp;amp;amp;amp; projMove[1] >= player2V-50) {
trajectory[0] *= -1;
moveSpeed += 30;
setClearColor(clearColor);
projMove[0] -= 0.5;
}
}

/******************************
*  END PROJECTILE CODE
*
******************************/

glutSwapBuffers();

}

void reshape(int width, int height)
{
glViewport(0, 0, width, height);

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0, width, 0, height);
glMatrixMode(GL_MODELVIEW);
}

void idle(void)
{
glutPostRedisplay();
}

void keyboard(unsigned char key, int x, int y) {
switch (key) {
case 'w':
//do something
player1Move=1;
glutPostRedisplay();
break;
case 's':
//do something
player1Move=-1;
glutPostRedisplay();
break;
case 'o':
//do something
player2Move=1;
glutPostRedisplay();
break;
case 'l':
//do something
player2Move=-1;
glutPostRedisplay();
break;
default:
//some other key
break;
}

}

void keyboardup(unsigned char key, int x, int y) {
switch (key) {
case 'w':
//do something
player1Move=0;
glutPostRedisplay();
break;
case 's':
player1Move=0;
glutPostRedisplay();
break;
case 'o':
player2Move=0;
glutPostRedisplay();
break;
case 'l':
player2Move=0;
glutPostRedisplay();
break;
default:
//some other key
break;
}

}

int main(int argc, char** argv)
{
glutInit(&amp;amp;amp;amp;amp;amp;amp;argc, argv);

glutInitDisplayMode(GLUT_RGBA | GLUT_DOUBLE | GLUT_DEPTH);
glutInitWindowSize(500, 500);

glutCreateWindow("Tim GL");

glutDisplayFunc(display);
glutReshapeFunc(reshape);
glutKeyboardFunc(keyboard);
glutKeyboardUpFunc(keyboardup);
glutIdleFunc(idle);

glutMainLoop();
return EXIT_SUCCESS;
}

I delayed so long because I wanted to refactor this code to better separate components and functions, but I didn’t get the time to do so. (That’s why I should do it that way from the start!) You can get a direct copy of the binary here.

Enjoy!

In interest of actually finally getting this post out the door, I’m just going to post the code!  This is actually a snapshot of the code from a month or so back and I apologize for not tidying it up.  The reason I’m using a snapshot from a while back is because I have a newer, more glitzy version with a scoreboard and some simple effects that aren’t really necessary to get the basic game down.  I will post that code in a future post though when I’ve polished it up a bit more!  This is also most definitely not up to proper coding standards, but it gets the job done.  It was a late night hack-fest, so please disregard the lack of neatness!

A word on use..

You’re free to use the code however you like.  If you use it though, I would most definitely appreciate it if you post a link to Last Stop on your website or blog and a mention in the credits of the application.  Please also credit the references to Timothy M. Rodriguez.  You don’t have to do this, but it’d be nice.  I’d also very much love an e-mail or comment letting me know about your project!

On to the Code!

#include <stdlib.h>
#include <stdio.h>
#include <math.h>

#include <GLUT/glut.h>

static int lastFrameTime = 0, windowWidth = 0, windowHeight = 0, now = 0;
static float midWidth = 0, midHeight = 0;
static float elapsedTime = 0, elapsedMilliSeconds = 0;
static char player1Move = 0, player2Move = 0;
static float player1V = 0, player2V = 0;

static float PI = 3.14159265358979323846264;

//scores
static int player1Score = 0, player2Score = 0;

//the following variables calculate a random starting projectile trajectory
static float randomDirection;
static long longMax = 0x7fffffff;    //max of longs 2^31-1 (2^(n-1)-1 for 2′s complement) static float trajectory[2] = {0, 0};

static float projMove[2] = {0, 0};
static double trajectory[2] = {0, 0};
static char moveSpeed = 70;

void resetProjectile(void) {
//set starting random trajectory
randomDirection = 2 * PI * (float) random() / (float) 0x7fffffff;
trajectory[0] = (float) cos(randomDirection);
trajectory[1] = (float) sin(randomDirection);
projMove[0] = midWidth;
projMove[1] = midHeight;
}

void display(void)
{
//get window parameters
windowWidth = glutGet(GLUT_WINDOW_WIDTH);
windowHeight = glutGet(GLUT_WINDOW_HEIGHT);

midWidth = windowWidth/2;
midHeight = windowHeight/2;

//start timing code
if (lastFrameTime == 0) {
lastFrameTime = glutGet(GLUT_ELAPSED_TIME);
resetProjectile();    //since this is the fist time, this is actually a set
}

now = glutGet(GLUT_ELAPSED_TIME);
elapsedMilliSeconds = now – lastFrameTime;
elapsedTime = elapsedMilliSeconds/1000.0f;
lastFrameTime = now;

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glShadeModel(GL_FLAT);

//calculate move distances
switch (player1Move) {
case 1:        //move up
player1V += 150 * elapsedTime;
break;
case -1:    //move up
player1V -= 150 * elapsedTime;
break;
default:    //do nothing
break;
}
//check out of bounds
if (player1V > (windowHeight/2)-50) {
//bounce back
player1Move = -1;
}
if (player1V < -(windowHeight/2)+50) {
//bounce back other way
player1Move = 1;
}

switch (player2Move) {
case 1:        //move up
player2V += 150 * elapsedTime;
break;
case -1:    //move up
player2V -= 150 * elapsedTime;
break;
default:    //do nothing
break;
}
//check out of bounds
if (player2V > (windowHeight/2)-50) {
//bounce back
player2Move = -1;
}
if (player2V < -(windowHeight/2)+50) {
//bounc back other way
player2Move = 1;
}

//create and position player 1
glPushMatrix();

glColor3ub(255, 0, 0);    //red

glTranslatef(0.0f, player1V, 0.0f);

glBegin(GL_QUADS);
glVertex2f(20.0f, (midHeight-50));
glVertex2f(20.0f, (midHeight+50));
glVertex2f(50.0f, (midHeight+50));
glVertex2f(50.0f, (midHeight-50));
glEnd();

glPopMatrix();

//create and position player 2
glPushMatrix();

glColor3ub(0, 0, 255);    //blue

glTranslatef(0.0f, player2V, 0.0f);

glBegin(GL_QUADS);
glVertex2f(windowWidth-20.0f, (midHeight-50));
glVertex2f(windowWidth-20.0f, (midHeight+50));
glVertex2f(windowWidth-50.0f, (midHeight+50));
glVertex2f(windowWidth-50.0f, (midHeight-50));
glEnd();

glPopMatrix();

/***************************************
*    Projectile CODE!!!
*
***************************************/

//calculate move distances
projMove[0] += moveSpeed * trajectory[0] * elapsedTime;
projMove[1] += moveSpeed * trajectory[1] * elapsedTime;

//create and move projectile
glPushMatrix();

glColor3ub(255, 255, 255);

glTranslatef(projMove[0], projMove[1], 0.0f);

glBegin(GL_QUADS);
glVertex2f(-10, -10);
glVertex2f(10, -10);
glVertex2f(10, 10);
glVertex2f(-10, 10);
glEnd();

glPopMatrix();

//set bounds for projectile
if (projMove[0] > (windowWidth)) {    //player 1 scored – out of bounds right
player1Score++;
printf(“Player 1 Scored!  Score: %f “, player1Score);
printf(“projMove[0] %f projMove[1] %f “, projMove[0], projMove[1]);
resetProjectile();
}

if (projMove[0] < (0)) {        //player 2 scored – out of bounds left
player2Score++;
resetProjectile();
}

if (projMove[1] > (windowHeight)) {    //out of bounds top
resetProjectile();
}

if (projMove[1] < (0)) {    //out of bounds bottom
resetProjectile();
}

//collision detection code

/******************************
*  END PROJECTILE CODE
*
******************************/

glutSwapBuffers();

}

void reshape(int width, int height)
{
glViewport(0, 0, width, height);

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0, width, 0, height);
glMatrixMode(GL_MODELVIEW);
}

void idle(void)
{
glutPostRedisplay();
}

void keyboard(unsigned char key, int x, int y) {
switch (key) {
case ‘w’:
//do something
player1Move=1;
glutPostRedisplay();
break;
case ‘s’:
//do something
player1Move=-1;
glutPostRedisplay();
break;
case ‘a’:
player1Move=0;
glutPostRedisplay();
break;
case ‘o’:
//do something
player2Move=1;
glutPostRedisplay();
break;
case ‘l’:
//do something
player2Move=-1;
glutPostRedisplay();
break;
case ‘k’:
player2Move=0;
glutPostRedisplay();
break;
default:
//some other key
break;
}

}

void keyboardup(unsigned char key, int x, int y) {
switch (key) {
case ‘i’:
//do something
player1Move=0;
glutPostRedisplay();
break;
case ‘u’:
player2Move=0;
glutPostRedisplay();
break;
default:
//some other key
break;
}

}

int main(int argc, char** argv)
{
glutInit(&argc, argv);

glutInitDisplayMode(GLUT_RGBA | GLUT_DOUBLE | GLUT_DEPTH);
glutInitWindowSize(500, 500);

glutCreateWindow(“Tim GL”);

glutDisplayFunc(display);
glutReshapeFunc(reshape);
glutKeyboardFunc(keyboard);
glutKeyboardUpFunc(keyboardup);
glutIdleFunc(idle);

glutMainLoop();
return EXIT_SUCCESS;
}

Well that’s a lot of code, but don’t be intimidated!  If you read through it (maybe a couple of times), you’ll start to understand it.  The main loop is short and sweet.  We start glut, create a window with it, and then register callback functions for displaying the game, reshaping (what to do if someone changes the size of the window), and for keyboard downpress and release events (keyboard func and keyboardupfunct).  Our display function then pretty much does all the work, and I use a bunch of global variables to keep track of state, such as where the projectile and the paddles are.  There’s also some brute force vanilla collision detection, which I hope you can appreciate.  The funny part is that most real collision detection systems use this concept, called the bounding box.  In pong, the bounding box of the paddle is the paddle.

You can get a compiled Mac OS X version here, but if you have your IDE setup right a straight copy and paste should work.  (If you had to make any changes, please let me know.)

Let any and all questions loose in the comments..

OpenGL is the Open Graphics Library and it is maintained by the Kronos Group, which is a consortium of top-tier companies that include board members such as ATI nVidia, Apple, IBM, and more. Microsoft used to be on the consortium, but left and decided to make DirectX, a competing closed graphics library that only runs on Windows (at least without some sort of emulation layer or wrapper like Wine or Cedega).

First off, OpenGL is a huge thing to master (and I am nowhere near mastery). I can’t explain everything about it in one post (or even in hundreds), but I strongly encourage you to get the OpenGL “Red Book” to start out. The “Red Book” is the official and definitive guide to OpenGL and is a must have for any graphics programmer. I also recommend the K&R C Programming book and, if you’re interested in the back-end math, the Real-Time Rendering book.

That being said, in our pong clone, we only use a few OpenGL functions. I’m going to list them and tell you what they do, but it may take until the next post to really understand how they work.

First, a word about primitives. OpenGL has only a few key primitives that you can draw with, but that’s okay, because you can use the primitives to draw more complex shapes!

The OpenGL primitives are:

• Points – a simple point in space
• Lines – a 2-dimension line (although it can occupy 3-dimensional space)
• Polygons – a polygon is a simple 2-dimensional shape comprised of 3 or more points. Triangles, or 3-point polygons, deserve special attention, because OpenGL will break up any other polygon into a combination of triangles. For example, a square can be broken up into two triangles along its diagonal. Just about every object you see in OpenGL is comprised of triangles!

So let’s draw a simple quadrilateral! In OpenGL, you draw polygons by calling a function whenever you want to draw a polygon and then creating a bunch of points in space to create that polygon. Once you’re done, you call another function to say you’re done.

Here’s an example for drawing a rectangle (our pong paddle):

glBegin(GL_QUADS);

glVertex2f(-10, -50);

glVertex2f(-10, 50);

glVertex2f(10, 50);

glVertex2f(10, -50);

glEnd();

We start by calling glBegin() and sending it GL_QUADS to indicate we want to draw a quadrilateral. This basically tells OpenGL take every four points I send you and make a quad out of it. We then call glVertex2f() and send it the coordinates of a two-dimensional point. the 2f indicates that this is the glVertex function which takes 2 floating point values as its arguments (or inputs). We then call glVertex2f() once for all four of each of our points and then call glEnd() when we’re done.

We could have optionally set a color before we began to draw that quadrilateral (the default is black on black) by calling this function:

glColor3ub(255, 0, 0); //red

Again we see the OpenGL naming convention of starting off a function with gl and then ending it with letters and numbers that indicate its arguments. In this case the 3ub means 3 unsigned bytes. Each unsigned byte is positive (hence unsigned) and can go from 0 to 255. The order of the bytes indicate the amount of Red, Green, and Blue respectively that will mix to give you your final color. (Color theory is another big subject matter I can post more about in another post, so let me know in the comments!)

Two other functions that are usually called at the beginning of every display function are:

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

glShadeModel(GL_FLAT);

glClear(), quite simply is a function that clears the screen and uses the values specified as its argument for doing so.

glShadeModel() specifies the shading model for how vertexes (points) are shaded or lit. In this case, we’re going for a simple flat colored 2-d look so we use GL_FLAT. This paints everything on screen with whatever colors specified and doesn’t do anything fancy. It’s sort of like painting in mspaint on Windows.

glPushMatrix();

glPopMatrix();

glTranslatef(30.0f, 10.0f, 0.0f);

glPush and Pop Matrix() are important functions. They allow you to draw something as if you are starting from scratch with a new coordinate system. Think about our pong paddle. When you want to draw it some place else, do you really want to respecify the coordinates for how it’s drawn? You could instead push a matrix (a set of points) on the stack, then translate (or move), to a new location and draw the object as you would anywhere else, and then finally pop the matrix which will draw the set of points translated (moved) by the glTranslatef() function. This is a little hard to understand (and explain), so I’ll give an example.

In Part II we talked about drawing a box that moved across the screen. We could draw our box as a point in a 2-dimensional plane. For example, let’s make a square that is 4 across centered at the origin (0,0). So it’s points are, going counter-clockwise, (1,1), (-1,1), (-1,-1), (1,-1). Rather than figuring out how to move it along for every frame. We can do this.

glPushMatrix();

glColor3ub(255, 0, 0); //red

glTranslatef(x, 0.0f, 0.0f);

glBegin(GL_QUADS);

glVertex2f(1,1);

glVertex2f(-1,1);

glVertex2f(-1,-1);

glVertex2f(1,-1);

glEnd();

glPopMatrix();

So we push a matrix (a set of points) on the stack, switch the drawing color to red, then Translate x units in the x-direction (horizontal) and then set the drawing mode to GL_QUADS and push the points of our square as it is centered at the origin, and finally pop the matrix! This has OpenGL do all the work of moving our square x units to the right.

Okay, that’s enough for now. I encourage you to scour the web and read up more before we get to our next post, where I’ll finally post the code and go through it!

For reference, you probably want to start out with Part’s II and I here and here.

Last we left off I said I’d get into some of the basic mechanics of GLUT and OpenGL, so let’s hit the ground running.

GLUT

GLUT, or the Graphics Library Utility Toolkit, is a cross-platform C library for generating windows and handling IO events. Basically, GLUT was written so you can get to learning OpenGL very quickly without having to spend enormous amounts of time learning how to handle mouse or keyboard actions in your current OS, or worse, learn how to setup a simple GUI just so you can see your work. On top of making these tasks simpler, GLUT is, as I mentioned, cross-platform. This means that you can take your code and recompile it on another OS as long as you have the appropriate GLUT library referenced in your IDE. There are currently versions of GLUT created for Linux, OS X, Windows, and probably even more operating systems.

So how do we create a simple window?

Here’s a snippet of code for how to do just that!

int main(int argc, char** argv)
{
glutInit(&argc, argv);

glutInitDisplayMode(GLUT_RGBA | GLUT_DOUBLE | GLUT_DEPTH);
glutInitWindowSize(500, 500);

glutCreateWindow(“Tim GL”);

glutDisplayFunc(display);
glutReshapeFunc(reshape);
glutKeyboardFunc(keyboard);
glutKeyboardUpFunc(keyboardup);
glutIdleFunc(idle);

glutMainLoop();
return EXIT_SUCCESS;
}

You start off with your regular main function in C. Then you call the glutInit function and pass it is passed any parameters you may have passed to main. For this tutorial you won’t be needing any, so don’t worry about it. If you need to find out more, check out the GLUT Documentation.

Next, you pass the glutInitDisplayMode() function a “logically OR-ed” value of GLUT_RGBA, GLUT_DOUBLE, and GLUT_DEPTH. If you don’t know what logically OR-ed means, just think of it as smooshing all these flags together to set the display mode with all of these properties. (If you want an in-depth explanation of this, just ask and I’ll put up a post about it!).

After this, we set the pixel dimensions (size) of the window to 500×500 pixels with the line “glutInitWindowSize(500,500);”. We next set the window’s title bar with “glutCreateWindow(”Tim GL”);” You can set either of these last two functions to whatever value you wish!

Next, is the pretty interesting part. And to understand it you need to understand a little bit about Call-Back functions. Call-Back functions are functions that hold a pointer to another function and call it when some event happens. (This is also why Call-Back functions are usually regarded as “Event-Driven Programming”.) To explain this it’s best to show you by example. I’ll start with the glutKeyboardFunc(keyboard). Whenever a key is pressed down, we have set glutKeyboardFunc() to call ANOTHER function called keyboard that WE have defined. The function is not show here, but we will get to that in the next post! Suffice it to say, our keyboard function will handle certain things like “what to do when the up arrow is pressed” in our Pong clone.

It is important to understand that we can’t just write a function that constantly checks the keyboard like this

while(TRUE) {

whatIsTheKeyboardDoing();

}

because we’d be in an infinite loop and never be able to do anything else. The Call-Back function allows us to be “called back” whenever someone presses the keyboard.

Now that we’ve explained that, the rest of the functions are easy to describe:

• glutDisplayFunc() is called most of the time and is the function that paints our screen with whatever we tell it to with OpenGl
• glutReshapeFunc() is what is called whenever the user resizes the window (In case we need to do anything special if the user all of a sudden changes the screen size on us.)
• glutKeyboardFuncUp() is a tricky one, and it’s what is called whenever a key is “depressed”, or let go. This is a subtle point, but it is useful to know when someone presses a key and let’s go of one. Without this, you wouldn’t be able to tell when someone was holding a key.
• glutIdleFunc() this is the function that is called whenever there is nothing to do!

So that’s it! That’s about all you need to know for GLUT for now! Check out the next post for OpenGL.

Before I start with anything, we have to think about simple animation. In fact, animation is just simply a series of static images, with each image slightly displaced from the last.

So what does that mean?

For example, if I’m going to draw animate a ball falling from mid-air, I would first start with an image of the ball in mid-air, precisely as it would look the moment it began to fall. The next frame, or image, would be exactly that, but with the ball a little bit lower down. I would then continue to draw sequential images of the ball, with each image showing it a little bit further than the last. This is how every moving image you see works (well, just about). A series of images is drawn in front of you, quick enough that you can’t tell.

So the next question is, what’s quick enough? It turns out, this isn’t such a simple question. In fact, the answer is different for different animals! But for humans, it turns out, about 30fps, or 30 frames-per-second, is what it takes to fool us. The movies you see in the theatre are (at-the-moment anyways) displayed at 24fps, but most computer screens require 60fps to prevent you from seeing the flicker (some, like myself, need an 85Hz (fps) refresh rate, others up to 120!). The reasons for these differences has a lot to do with the way the image is drawn and the way your eye views things. It’s a pretty involved topic, but if you guys would like me to go into it more, let me know in the comments.

A Simple Animation Loop

Taking what we just learned, let’s think about how to draw a box moving across the screen. Let’s pretend we have a function that draws a box centered at a point we specify on the screen (in 2-D Space). It’s C function prototype would look something like this:

void drawbox(int x, int y);

So if we were going to draw a box that just infinitely moved to the right (or at least until it was off screen). We might decide to create a loop that drew it a little bit further each time. That might look something like this:

#define TRUE 1

int x=0;

while(TRUE) {

drawbox(0+x, 0);

x++;

}

This little snippet of code, runs an infinite loop and draws a box at a point 0+x, 0 and then increments x. Let’s look a little further. First it draws a box at point(0,0) since x is 0 on first count. Then x is incremented and the loop goes on again. Next, it draws a box at (1,0) and increments x. And so on. Congratulations, you just made your first animation loop!

Marching Onward

In the beginning, video games were hand-coded in assembly directly to a particular hardware. This had a lot of benefits, primarily speed, which was very important when hardware was relatively expensive. But as computers progressed and the market expanded, it became necessary to program for a multitude of different hardwares. And this is when a nasty problem reared it’s head. You see, when you decide to have the computer draw something on the screen, it doesn’t just do it instantaneously. The computer has to go and draw each individual pixel on the screen (also a topic for another discussion).

This

takes

time.

Moreover, the time it takes on your brand-spanking new ultra-super processor is not the same time it takes your friend down the street who isn’t as enthusiastic as you are. This is a problem, because if you set up all your animations expecting the processor to draw 30 frames per second on your machine. It’s going to run at less than that on your friends machine. (In fact, there’s some math that shows it will be a fraction/multiple of their refresh rate). Great, so your fast-action shoot-em-up just got stuck in the mud.

What’s a game programmer to do?

There’s a really simple answer to all of this–use our friend (archenemy) time. You see, rather than making your animation speed depend on the animation loop (which depends on your processor speed), have the your animation be a fixed function of time.

Take another look at our first example. The movement speed of the box can really be expressed as 1 unit in the x direction per loop iteration. Really what we want is for the box to move some number of units in the x direction per second (our hour if you like). To do this we need to express our box movement as a function of time and then know how long its been since the computer last drew the box. Essentially, we want our animation loop to look like this.

#define TRUE 1

int speed = 1; //1 unit per second

int x = 0;

startTime = 0;

elapsedTime = 0;

while(TRUE) {

startTime = getTime();

x += speed * elapsedTime;

drawbox(0+x, 0)

elapsedTime = getTime() – startTime;

}

As you can see, this simple example becomes a lot more complicated once we add timing to the mix. First, we defined a universal speed for our box. We still kept the x value, as this represents the boxes point in space, not it’s speed. Then we created variables for keep track when we started our animation loop and how long we were in it. We’re also assuming we have a function that gives us the time, called getTime() here, and that it returns us the current time in seconds.

First we enter our loop and fetch our start time from the getTime() function. Next we set our x position based on how long it’s been since our last loop. We multiply our elapsedTime by the speed of our box and add that distance to our x-position. Then, we draw our box at it’s new position. Finally, we figure out how long it took us to draw that box by setting our elapsed time equal to the current time minus the time we started drawing the box. Now, no matter the computer, our box will move along the screen at the same speed. Note, this does not mean the animation is fluid! If your friend’s computer is really slow (and I mean really if it can’t draw a box) it may only get to draw a box once every 5 seconds. By then, our box may already be off the screen!

So I hope that’s enough for you to digest for a while. Stay tuned for my next post on some of the basic mechanics of GLUT and OpenGL. Until then, it might be useful to brush up on your C programming on the interwebs!

If you ever wanted to program your own video game, this is a good place to start. I remember when I started learning programming it was surprisingly difficult to find out how to make an honest and simple game. In fact, after reading most programming books, the only type of game you could make would be a Zork clone.

You are in a dank cave and you can see nothing, but you hear a walrus in the distance.

What do you do?

1. Look for the walrus.
2. Watch The Big Lebowski
3. Decide that the walrus is Paul.

This is not to knock Zork in any way! Zork is a clever, well written, and extremely well thought out game (even if my pseudo-quote isn’t). But in a world where we’re spoiled by moving objects, you want to know how to move and interact with something on screen. The problem is it takes a lot of different knowledge sets to get the job done. But in this post, you’ll get started learning how to make your very own Pong clone.

A Few Notes..

First, a few notes about setting up. I’m writing this tutorial from the Mac OS X perspective. However, everything we are about to code is, essentially, cross-platform. This is because, it is written using plaing old C, OpenGL, and GLUT. All of these things are themselves cross-platform, and therefore, can be compiled anywhere you have a C compiler (OpenGL and GLUT are just C libraries).

Setting up your environment

Mac OS X

For some reason, this always seems to be the hardest part whenever learning something new in programming. Fortunately, I found a great blog post that makes setting up XCode for compiling an app using OpenGL and GLUT very easy. Read all about setting up your XCode.

Windows

The process in Windows is actually a bit easier (gasp!) than in Mac OS X and there’s plenty of resources available on Google for getting setup. Here’s one in particular.

Linux

Here’s something unusual, you guys get a video.

Congratulations, the hardest part is done! In my next blog post. We’ll start to get knee deep in game programming concepts.

Update!

Part II is Up!