This is what I have The trouble is acos returns between 0 and PI so my check is never true; so how do I get the angle between 0 and 2pi ?

The answer is simple: you can't. The angle between two vectors can never be larger than 180°.

Quote:Original post by aarbron
This is what I have

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The trouble is acos returns between 0 and PI so my check is never true; so how do I get the angle between 0 and 2pi ?

Without a frame of reference, you can only compute the unsigned angle between two vectors in 3-d.

Here is a function that computes the signed angle between two 3-d vectors, using a third vector as a reference to determine the sign:

float signed_angle(const vec3& v1, const vec3& v2, const vec3& reference){    vec3 c = cross(v1, v2);    float angle = std::atan2(length(c), dot(v1, v2));    return dot(c, reference) < 0.f ? -angle : angle;}

Post back if you have any questions about how (or why) it works.

Thanks, here's how it looks now

I suppose the actual value of the ref vector doesn't really matter ?

Quote:I suppose the actual value of the ref vector doesn't really matter ?

Well, it only matters if it matters, if you get my meaning :)

In other words, whether or not it matters depends on the context, so that's really up to you to determine. I will say though that this problem often comes up in the context of solving an essentially 2-d AI problem in a 3-d environment, in which case you can just compute the signed angle between two 2-d vectors, like this:

float angle = atan2(perp_dot(v1,v2), dot(v1,v2));

Note that for both the 2-d and 3-d versions of this algorithm, you don't need to normalize the vectors prior to finding the angle. Also, atan2() returns an angle in the range [-pi, pi] (in case it matters for what you're using it for).