Physics movement prediction is way off [closed]

Question

I'm making a simple, non-networked game. There are two players and a ball. In implementing the AI for the second player, I've run into a huge problem predicting where the ball will hit the ground. Either I've made a mistake in the physics engine or in implementing the prediction itself. I wrote the physics engine, and while it works visually as I expect it to, I won't rule it out as a suspect. First I'd like to make sure I haven't made a mistake in the prediction code. Here's what I have found so far:

I print the predicted time to impact and the accumulated time in each frame. The time predicted for impact is way off.

tti: 0.678233
total time: 0
.
.
tti: 0.0108339
total time: 0.94969
[ball] ground collision

The code for predicting is in two functions.

double nextBallCollisionWithGround(Ball *ball, double groundHeight) {
    double timeToImpact = quadraticSolver(kGravity, ball->velocity().y, ball->position().y - ball->radius() - groundHeight);
    std::cout << " tti: " << timeToImpact << endl;
    double groundPos = -3.;
    if (timeToImpact > 0) {
        groundPos = ball->velocity().x * timeToImpact + ball->position().x;
    }
    return groundPos;
}

double quadraticSolver(double a, double b, double c) {
    double ans = -1.;
    double det = (b*b) - (4.*a*c);
    double t1, t2;
    if (det > 0.) {
        t1 = (-b + sqrt(det)) / (2. * a);
        t2 = (-b - sqrt(det)) / (2. * a);
        if (t1 > 0.)
            ans = t1;
        if (t2 > 0.)
            ans = t2;
    }
    return ans;
}

If there's no problem in these functions, I can include the relevant parts of the physics engine to see if the problem is there.

Answer 1

I don't know what kGravity is, but it should be half the gravitational acceleration.

Because the formula is:

h(t) = t * velo_y + 0.5 * -gravity * t^2

Where, h(t) is the height in function of time, t is the time and velo_y is the initial y velocity. And gravity is for example 9.81 m/s^2.

So, as you can see, the a of the quadratic equation is 0.5 * -gravity.

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Some remarks:

• Little imperfection to your quadratic solve method: the determinant can be zero as well. You will have two identical roots.

• In order to get the correct root for this equation, you should make sure you are taking the smallest positive root for your collision prediction.