// -180.0 < angle <= 180.0の値を返します
// ベクトルbが反時計周りにあるとき、値は負の数を返します
public float SignedAngle(PointXY b)
{
float rad = (float)Math.Acos(InnerProduct(b) / (Length() * b.Length()));
if (OuterProduct(b) < 0.0F)
{
rad = -rad;
}
return(rad * 180.0F / (float)Math.PI);
}
// 0.0 <= angle <= 180.0の値を返します
public float Angle(PointXY b)
{
float rad = (float)Math.Acos(InnerProduct(b) / (Length() * b.Length()));
return(rad * 180.0F / (float)Math.PI);
}