PVector.angleDistance() often returns NaN when finding the distance between equal vectors. At first I thought it was my rusty trig, but digging in, it seems to be a rounding error. The method uses PVector.mag() (which returns a float), and passes it to Math.acos(), which the Java APIs say returns NaN if the argument's absolute value is > 1.  

I wrote a simple sketch to verify, and was able to get past the NaN results by using a version of mag() which returns a double:

void setup() {
 // some offending vectors
 PVector[] vs = new PVector[4];
 vs[0] = new PVector(-1, -1);
 vs[1] = new PVector(1 , -1);
 vs[2] = new PVector(1 ,  1);
 vs[3] = new PVector(-1,  1);
 
 for (int i = 0; i < vs.length; i++) {
   println(vs[i] + "\t default: " + PVector.angleBetween(vs[i], vs[i]));
   println(vs[i] + "\t doubles: " + angleBetween(vs[i], vs[i]));
   println();
 }
}

double mag(PVector v) {
 return Math.sqrt(v.x*v.x + v.y*v.y);
}

float angleBetween(PVector v1, PVector v2) {
 double dot = v1.dot(v2);
 float theta = (float) Math.acos(dot / (mag(v1) * mag(v2)));
 return theta;
}

It prints:

[ -1.0, -1.0, 0.0 ]       default: NaN
[ -1.0, -1.0, 0.0 ]       doubles: 2.1073424E-8

[ 1.0, -1.0, 0.0 ]       default: NaN
[ 1.0, -1.0, 0.0 ]       doubles: 2.1073424E-8

[ 1.0, 1.0, 0.0 ]       default: NaN
[ 1.0, 1.0, 0.0 ]       doubles: 2.1073424E-8

[ -1.0, 1.0, 0.0 ]       default: NaN
[ -1.0, 1.0, 0.0 ]       doubles: 2.1073424E-8



Should this be fixed? Could PVector.angleDistance() use a private version of mag(), which returns a double? I plan on pulling down the code and building, but haven't quite yet...

Done: Bug 1316.  Let me know if there's anything I can do to help fix it.

Thanks!