Question

I have a data values which vary from +PI to -PI radian.

I need to get minimum rotation (in radians) required to go from old value to new value like:

float rotationRequired(float oldValue, float newValue){
      return newValue - oldValue;
}

but simply subtracting won’t do, because to go from -179 degree to +179 degree one does not need to rotate full circle around, just 2 degree clockwise. Because -PI = +PI in a circle are technically same rotation. Also the values can be in any range, i.e 740 = 360 + 360 + 20, hence only 20.

I’m thinking about breaking values into sin and cos values, subtract and then atan :

double oldY =  Math.sin(oldValue);
double oldX =  Math.cos(oldValue);

double newY =  Math.sin(newValue);
double newX =  Math.cos(newValue);

float delta = (float) Math.atan2( (newY - oldY),(newX - oldX) );

But still its not giving correct results, can anyone suggest another method ?

Answer 1

I converted angles to degrees and used this method to suggest what minimum rotation is required and in what direction:

public static int suggestRotation(int o, int n){
    //--convert to +0 to +360 range--
    o = normalize(o);
    n = normalize(n);

    //-- required angle change --
    int d1 = n - o;

    //---other (360 - abs d1 ) angle change in reverse (opp to d1) direction--
    int d2 = d1 == 0 ? 0 : Math.abs(360 - Math.abs(d1))*(d1/Math.abs(d1))*-1;

    //--give whichever has minimum rotation--
    if(Math.abs(d1) < Math.abs(d2)){
        return d1;
    }else {
        return d2;
    }

}

private static int normalize(int i){
    //--find effective angle--
    int d = Math.abs(i) % 360;

    if(i < 0){
    //--return positive equivalent--
        return 360 - d;
    }else {
        return d;
    }
}