Question

I’m working on a simple 2D game (birds eye view). I have projectiles, which are circles. There are barriers around the game level, which are all rectangles (axis aligned). I’d like the projectiles to bounce off the barriers when they collide. I’m not sure how to implement this in my game loop. Guessing it’d be something like this:

void gameLoop() {

    Projectile p = ..;

    p.updateLocation(p.getVelocity(), p.getDirection());

    Barrier intersected = p.intersects(barriers);
    if (intersected != null) {
        // We hit a barrier after moving on this time tick.
        // Figure out where our reflected location off the 
        // barrier should be now.
        Point ptNew = intersected.reflect(p.getLastPoint(), p.getVelocity(),
            p.getDirection());

        // Our new location after getting bounced off the barrier.
        ptNew.setLocation(ptNew);
    }
}

So after we move the projectile, we can check if we’re intersecting (or completely inside) one of the barriers. If so, do a computation to figure out where our reflected position should be off the barrier given our starting point and velocity/direction.

Thanks

———- Update ————————

A little more specific – given Erik’s comments, I do need to make sure the projectiles bounce properly, I can’t have them pass through a barrier if their velocity happens to be so fast they go right through on a single game loop iteration.

In that case, I guess I need to test a starting point, velocity, direction, against all barriers (possibly repeatedly) until no more intersections for the velocity are possible. Something like:

void gameLoop() {
    Projectile p = ...;

    Barrier barrier = moveProjectile(p, barriers);
    while (barrier != null && p.stillHasVelocityThisFrame()) {
        barrier = moveProjectile(p, barriers);
    }

    // at this point, the projectile is done moving / bouncing around.
}

void moveProjectile(Projectile projectile,
                    List<Barrier> barriers)
{
    for (Barrier barrier : barriers) {
        // tbd
        Barrier intersected = test2dVectorAgainstRectangle(
            projectile.getPosition(),
            projectile.get2dVector());
        // the above would test the projectile's vector
        // to see if it intersects any barrier, reflect
        // off of it if necessary, modify the projectile's
        // position, and reduce the available distance it
        // can still travel for this frame.
        if (intersected) {
            return intersected;
        }
    }

    // no intersections.
    return null;
}

Yeah this is already getting kind of tricky.

Thanks

Answer 1

You want to use vector math for your reflections and collision checks. It’s pretty straight forward to detect if you were on one side of a line (edge of world rectangle) on 1 tic and on the other side of that line next. The same check will also give you the point of collision with just a touch more math. Then you have a point from which to do a reflection, again using vectors. Plotting collisions with other objects is a bit tricker, but still can be done. Make a list of all collisionswithin a loop and try and find the first one that would have occured along their movement paths – again reusing some dot product magic. Or, if you detect multiple collisions, you can always back up each object to it’s position the tic before and apply collisions from there.

One thing you never want to do is allow an object to stay inside another object. It can cause them to get stuck on each other and act very erraticly – depending on how you do your collision code.