How to use multiplication operation in game render delta time?

Question

We are using delta time like this:

private static final float SPEED = 200f; // Moving pixels in one second

@Override
public void render(float delta) {
    myObject.vx += SPEED * delta;
    myObject.vy += SPEED * delta;

    myObject.move();
}

So I am using this update method to apply friction on my object:

private static final float FRICTION = 0.98f;

@Override
public void render(float delta) {
    //...

    myObject.vx *= FRICTION;
    myObject.vx *= FRICTION;

    myObject.move();
}

I know this is wrong:
myObject.vx *= FRICTION * delta

I want to use friction with "multiplication operation" not "minus".

So how can I implement the delta time to my FRICTION variable?

Answer 1

If my assumption that you want your speed to lose 2% of its value every second, then this is a perfect opportunity to use the exponential rate of decay expression, which looks like this:

A=Pe^(rt)

A is the final amount you want (so, after 1 second, .98(vx)) and P is the initial amount. t is just time, so since you want the velocity to only be 98% of its value from the second before you can just plug in 1 for t, .98(vx) for A, and vx for P and solve for r.

This will get you about a value of about -.0202, but you can always store something more accurate.

So what your friction application code should look like is this:

myObject.vx *= Math.pow(Math.E, delta * -.0202);

If you did not know already, decimal numbers that omit the f are automatically initialized as doubles. Math.pow also uses and returns doubles, since they are more accurate. So you may want to have your velocity measured as a double. This will not likely make a very noticeable difference, but if you want accurate simulations, I recommend using doubles.

I have not tested this, so please leave a comment if you notice a problem, but I do believe this is the best way to do what you want.