Jarmund - 5 months ago 28

Java Question

I have a 2D side-view shooter-thingy that I need some fresh eyes on. I'm currently working on the aiming, and it partially works: muzzle velocity is known, gravity is known, and x,y distance to target is also known. Using various sources both here on SE and wikipedia, I've come up with the following method for calculating starting vector for projectile speed:

`public Vector2 calculateAim(Vector2 pos, Unit target) {`

// Check if previous calculation can be recycled

if (target.equals(lastTarget))

return lastAim.cpy();

lastTarget = target;

double v = RifleBullet.projectileSpeed; // Muzzle velocity

double g = Settings.gravity.y; // Gravity, positive value

double x = offset.x; // x distance to target. Negative if towards left

double y = offset.y; // y distance to target. Negative if lower

double theta = Math.atan(((v * v) - Math.sqrt((v*v*v*v) - (g*(g*(x*x) + 2*y*(v*v))))) / (g * x));

Vector2 aim = new Vector2(-(float)Math.cos(theta), (float)Math.sin(theta));

aim.nor();

aim.scl((float)RifleBullet.projectileSpeed);

lastAim = aim.cpy();

return aim;

}

The above partially works: The rightmost guy will aim correctly. However, the guy standing to the left is consistently firing in the wrong direction. It looks like the Y component is correct, but the X component is quite simply reversed. If I add this after calculating theta:

`if (side.contains("left"))`

angleInRadians += Math.PI;

...they both fire correctly, but it's not viable in the long run as there will be more units on the battleground, and it's bound to cause problems when I get to that point.

I believe the source of the problem could be the

`Math.atan()`

How should I refine my method so that it works properly regardless of who's left/right/up/down?

Answer

What Tenfour04 said is quite helpful. You're probably going to want to use `Math.atan2(y,x)`

instead of `Math.atan(y/x)`

Notice that instead of doing the division yourself, you give the y and x as separate arguments. This is so atan2 knows the difference between things like 5/2 and -5/-2. To normal atan, they both look like 2.5