Thread: Collision Detection in OpenGL

1.)
2.) fairly
3.) did you search Google? edit: or NeHe?

When you say something like, "how is collision detection done in OpenGL" that's pretty vague. CD in OpenGL is the same as it would be in D3D except for the function calls. Do you mean 3D collisions? Because you can make 2D games in OpenGL and the collision detection is way different then in a 3D game. So, what kind of game are you wondering about?

here are 2 easy ways.

draw a box around every object.
if any of the boxes overlap then you have a collision.

draw a circule around every object.

d = distance between 2 objects.
a = radis of the circule on the first object.
b = radis of the circule on the second object.

if ( a + b >= d ) colission
else no colission

forgive the spelling.

Originally posted by Nor
Things

Would there be any particular functions you'd use?

Originally posted by MrWizard
So, what kind of game are you wondering about?

I want to learn how to detect collisions between two simple 2D rectangles. If I were going to start making a game, it'd be 2D.

>>Would there be any particular functions you'd use?

Of course! One you wrote, which does exactly what Nor just said. There isn't a function that you can just use to make every part of a game. You have to actually program things. Look at what Nor said. You can't get much more specific than that without writing the code.

draw a circule around every object.

d = distance between 2 objects.
a = radis of the circule on the first object.
b = radis of the circule on the second object.

if ( a + b >= d ) colission
else no colission

You want to click me...you can't wait to click me...oh yes, you can feel it now...yes...yes....

If your just talking about basic bounding box collision detection, then this or this may help you. Those are Allegro specific, I know, but you'll get the idea...

Code:

take your pick.
//#define twod_collision 0
/* or */
//#define threed_collision 0

bool detectcollision(int x1, int y1, int z1,int radis1, int x2, int y2, int z2, int radis2)
{
    int xd, yd, zd, Distance;
    
    //2d
    #ifdef twod_collision
	    xd = x2-x1;
	    yd = y2-y1;    	
	    Distance = SquareRoot(xd*xd + yd*yd)
    #endif
    
    //3d
    #ifdef threed_collision
	    xd = x2-x1;
	    yd = y2-y1;
	    zd = z2-z1;
	    Distance = SquareRoot(xd*xd + yd*yd + zd*zd);
    #endif
	
	if( radis1 + radis2 >= Distance)
	    return true; //collision
	return false;    //no collision    
}

Originally Posted by Krak

I want to learn how to detect collisions between two simple 2D rectangles. If I were going to start making a game, it'd be 2D.

Have you gotten a 2D rectangle to move in a straight line yet? The first collision you probably want to detect is the edge of the window...after that you will find it Pretty Damn Easy. Look:

Code:

#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glut.h>
#include <GL/glext.h>
#include <stdlib.h>

/* bouncing-blocks.c */

#define VOL 150
#define NP 7

struct square {
        int x;  // x position
        int y;  // y position
        int w;  // size
        int v;  // vertical velocity
        int vd; // vertical direction (1 or -1)
        int h;  // horizontal velocity
        int hd; // horizontal direction (1 or -1)
} *piec;

void scene() {
	glClear(GL_COLOR_BUFFER_BIT);
	glColor3f(0.0f, 1.0f, 0.0f);
	glRecti(piec[0].x,piec[0].y,piec[0].x+piec[0].w,piec[0].y-piec[0].w);
	glColor3f(0.0f, 0.5f, 0.0f);
	glRecti(piec[1].x,piec[1].y,piec[1].x+piec[1].w,piec[1].y-piec[1].w);
	glColor3f(0.0f, 0.0f, 0.5f);
	glRecti(piec[2].x,piec[2].y,piec[2].x+piec[2].w,piec[2].y-piec[2].w);
	glColor3f(1.0f, 1.0f, 0.0f);
	glRecti(piec[3].x,piec[3].y,piec[3].x+piec[3].w,piec[3].y-piec[3].w);
	glColor3f(0.5f, 0.0f, 0.0f);
	glRecti(piec[4].x,piec[4].y,piec[4].x+piec[4].w,piec[4].y-piec[4].w);
	glColor3f(0.0f, 0.0f, 0.0f);
	glRecti(piec[5].x,piec[5].y,piec[5].x+piec[5].w,piec[5].y-piec[5].w);
	glColor3f(1.0f, 1.0f, 1.0f);
	glRecti(piec[6].x,piec[6].y,piec[6].x+piec[6].w,piec[6].y-piec[6].w);
	glutSwapBuffers();
}

void setup(float R, float G, float B, GLdouble P) {
	GLdouble N=0-P;
	glClearColor(R,G,B,1.0f);
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	glOrtho(N,P,N,P,N,P);
	glMatrixMode(GL_MODELVIEW);
	glLoadIdentity();
}

void Vcollision (int P) {
	int i;
	for (i=0;i<NP;i++) {
		if (i==P) continue;
		if ((piec[P].y==piec[i].y-piec[i].w) && (piec[P].x<=piec[i].x+piec[i].w) && 
			(piec[P].x+piec[P].w>=piec[i].x)) {piec[P].vd=-1;
			piec[P].y-=1;
			if (piec[i].vd==-1) piec[i].vd=1;
			if (piec[P].v>=piec[i].v) piec[i].v--;
			else {piec[P].v++;piec[i].v++;}}
		else if ((piec[P].y-piec[P].w==piec[i].y) && (piec[P].x<=piec[i].x+piec[i].w) && 
			(piec[P].x+piec[P].w>=piec[i].x)) {piec[P].vd=1;
			piec[P].y+=1;
			if (piec[i].vd==1) piec[i].vd=-1;
			if (piec[P].v>=piec[i].v) piec[i].v--;
			else {piec[P].v++;piec[i].v++;}}
	}
}

void Hcollision (int P) {
	int i;
	for (i=0;i<NP;i++) {
		if (i==P) continue;
		if ((piec[P].x==piec[i].x+piec[i].w) && (piec[P].y>=piec[i].y-piec[i].w) && 
			(piec[P].y-piec[P].w<=piec[i].y)) {piec[P].hd=1;
			piec[P].x+=1;
			if (piec[i].hd==1) piec[i].hd=-1;
			if (piec[P].h>=piec[i].h) piec[i].h--;
			else {piec[P].h++;piec[i].h++;}}
		else if ((piec[P].x+piec[P].w==piec[i].x) && (piec[P].y>=piec[i].y-piec[i].w) && 
			(piec[P].y-piec[P].w<=piec[i].y)) {piec[P].hd=-1;
			piec[P].x-=1;
			if (piec[i].hd==-1) piec[i].hd=1;
			if (piec[P].h>=piec[i].h) piec[i].h--;
			else {piec[P].h++;piec[i].h++;}}
	}
}

void move(int T) {
	int rendintrv, i;
	for (rendintrv=0; rendintrv<15; rendintrv++) {
		if (T==1000001) T=1;
		for (i=0; i<NP; i++) {
			if ((piec[i].v>0) && (T%piec[i].v==0)) {piec[i].y+=piec[i].vd;
				if (piec[i].y-piec[i].w<=-VOL) {piec[i].vd=1;
					piec[i].y=-VOL+piec[i].w+1; piec[i].v--;}
				else if (piec[i].y>=VOL) {piec[i].vd=-1;
					piec[i].y=VOL-1; piec[i].v--;}
				else Vcollision(i);
			}
			if ((piec[i].h>0) && (T%piec[i].h==0)) {piec[i].x+=piec[i].hd; 
				if (piec[i].x+piec[i].w>=VOL) {piec[i].hd=-1;
					piec[i].x=VOL-piec[i].w-1; piec[i].h--;}
				else if (piec[i].x<=-VOL) {piec[i].hd=1;
					piec[i].x=-VOL+1; piec[i].h--;}
				else Hcollision(i);
			}
			if (piec[i].v<=1) piec[i].v=2;		// min-max speeds
			else if (piec[i].v>30) piec[i].v=50;
			if (piec[i].h<=1) piec[i].h=2;
			else if (piec[i].h>30) piec[i].h=50;
		}
		T++;
	}
	scene();
	glutTimerFunc(1,move,T);
}	

	 
int main(int argc, char *argv[]) {
	glutInit(&argc,argv);
	glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
        glutInitWindowSize(900,900);
	glutCreateWindow("openGL test");
	glutDisplayFunc(scene);
	glutTimerFunc(1500,move,1);
	
	piec=malloc(NP*sizeof(*piec));
	piec[0].x=-100;piec[0].y=0; piec[0].w=30; piec[0].v=8;piec[0].vd=1; piec[0].h=4;piec[0].hd=1;
	piec[1].x=10;piec[1].y=50; piec[1].w=18; piec[1].v=4;piec[1].vd=-1; piec[1].h=8;piec[1].hd=-1;
	piec[2].x=50;piec[2].y=0; piec[2].w=14; piec[2].v=8;piec[2].vd=-1; piec[2].h=8;piec[2].hd=1;
	piec[3].x=75;piec[3].y=90; piec[3].w=22; piec[3].v=6;piec[3].vd=1; piec[3].h=6;piec[3].hd=-1;
	piec[4].x=-50;piec[4].y=0; piec[4].w=60; piec[4].v=5;piec[4].vd=-1; piec[4].h=5;piec[4].hd=1;
	piec[5].x=100;piec[5].y=-60; piec[5].w=45; piec[5].v=3;piec[5].vd=-1; piec[5].h=6;piec[5].hd=1;
	piec[6].x=-120;piec[6].y=-100; piec[6].w=27; piec[6].v=7;piec[6].vd=-1; piec[6].h=4;piec[6].hd=1;
	setup(1.0,0.0,1.0,VOL);

	glutMainLoop();
	return 0;
}

This was my first openGL program (for real) and I think I am managing things slightly differently now in 3D, but it works and it's all about collision and acceleration/deceleration. Seven brightly colored but differently sized squares bounce around and off walls perpetually, causing each other to reverse direction and speed up or slow down as circumstances dictate. Rated: G Running Time: infinite

Waking old thread time?

--
Mats

Originally Posted by matsp

Waking old thread time?

--
Mats

Oh I just noticed that! Silly bOMI and me -- at least I didn't write that for the thread. I must have watched bouncing blocks for like, hours.

But there's no point in waiting for Krack to come back, I guess.