Using angle_difference to calculate shortest turn to target [Gamemaker]

Question

I've tried, but I cannot understand angle_difference.

I have an object, oTurretDetTurret, which turns to face its Target, which lies at the angle DirectionOfTarget from it.

It does this by updating its direction thus:

direction = direction + SpinDirection

SpinDirection can be either +3 or -3, meaning it can turn in either direction at a rate of 3 degrees per step.

What determines the direction it turns in is the angle between its current direction and the `DirectionToTarget. I want the turret to turn in the direction with the shortest number of degrees to travel.

I've been using this:

if angle_difference(direction, DirectionToTarget) > 0 {
direction = direction + 3
} else if angle_difference(direction, DirectionToTarget <= 0 {
direction = direction - 3
}

However, in game this just means that the turret faces the opposite direction to its target at all times, oscillating back and forth slightly.

In my understanding, angle_difference returns the smaller of the two angles between two directions.

For example, if the turret's direction is 300 degrees and the DirectionToTarget is 90 degrees, angle_difference should return +150, the smaller of the two angles, and thus my above code should move the turret counter-clockwise towards 90 degrees.

Can anybody help me?

Answer 1

angle_difference is calculated with atan2 function which returns -PI to +PI (-180 deg to +180 deg). That is useful because if angle_difference > 0 you know it is clockwise from you and < 0 counterclockwise. your else if statement with <= should only be <, which is causing your oscillation.

Answer 2

I think probably your directToTarget is off by 180 degrees somehow. Maybe post that code. That could explain why your turret is always pointing away from the target. The next thing to consider is the oscillation. This is probably happening because you increment by 3 even if the difference between the angle and the desired angle is only 1 - so you'll get some overshoot over and over again. So instead, write in some logic that basically says if difference is more than 3, increment by 3, else increment by the difference