how to avoid clutch billiard balls? - c++

Question

I'm working on the simple behaviour of billiard balls in a collision with each other. All works normal, but there was a problem when facing a few easy balls is the effect of coupling balls and they're cool with each other. Tell me how to prevent this.
bool MGBilliard::CollisingBall(CCPoint curr_point, CCPoint next_point)
{
float dx = next_point.x - (curr_point.x + dvdt.x);
float dy = next_point.y - (curr_point.y - dvdt.y);
float d = dx*dx+dy*dy;
return d <= BALL_RADIUS * BALL_RADIUS;
}
double MGBilliard::angleCollisionBalls(Ball* current, Ball* next)
{
double na;
double dx = fabs(next->location.x - current->location.x);
double dy = fabs(next->location.y - current->location.y);
na = atan(fabs(dy/dx));
if(atan(fabs(current->location.y/current->location.x)) < atan(fabs(next->location.y/next->location.x)))
na = current->angle - na;
else if(atan(fabs(current->location.y/current->location.x)) > atan(fabs(next->location.y/next->location.x)))
na = current->angle + na;
return na;
}
for(unsigned int i = 0;i<BALL_COUNT;++i)
{
if(vBalls[i]->speed > 0){
vBalls[i]->speed += vBalls[i]->acceleration;
float dsdt = vBalls[i]->speed*dt;
dvdt.x = dsdt*cos(vBalls[i]->angle);
dvdt.y = dsdt*sin(vBalls[i]->angle);
vBalls[i]->location.x += dvdt.x;
vBalls[i]->location.y += dvdt.y;
for(unsigned int j = 1; j < BALL_COUNT; ++j)
{
if(i == j) continue;
if(CollisingBall(vBalls[i]->spriteBall->getPosition(),vBalls[j]->spriteBall->getPosition()))
{
vBalls[j]->speed = 600;
double angle;
angle = angleCollisionBalls(vBalls[i],vBalls[j]);
vBalls[i]->angle = (float)-angle;
vBalls[j]->angle = (float)angle;
}
}
}
}

Answer 1

There are two straightforward bugs that spring to my attention with a quick look at your code.
Firstly, this:
vBalls[i]->angle = (float)-angle;
vBalls[j]->angle = (float)angle;
is not the correct way to calculate opposing angles. For example, it will not do what you want it to do when angle is zero (or 180 degrees, for that matter).
Secondly, you iterate over your whole vBalls array multiple times, once with the index i and an inner loop with the index j. This means collisions will be calculated twice, and the speed of both balls would be set to 600! Changing your inner loop to be this:
for(unsigned int j = i + 1; j < BALL_COUNT; ++j)
should prevent this occurring.
There's also a more subtle bug. Your collision detection does not take into account time. Each ball moves a particular distance each iteration of your game loop. This means that if a collision does not occur in one 'tick', a ball could pass straight through another ball and then trigger the collision code on the far side of the ball. You cannot do a simple radius-based collision test in this situation, as if a ball moves more than (BALL_RADIUS * BALL_RADIUS) in a single step, your system will behave strangely or not work at all.
I personally would use vectors to describe speed and direction for each ball rather than angles and speeds, but refactoring your code to do this is a bit outside the scope of this question.