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# Relative angles with dot product (2D)

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4 replies to this topic

### #1ViniGodoy  Members

Posted 05 August 2011 - 01:09 PM

Hi.

I recently found a dot product relative angle calculation for unit vectors as follows:

float Vector2D::angleBetween(const Vector2D& other) const
{
float dp = dot(other);

if(dp >= 1.0) dp = 1.0f;
else if(dp <=-1.0) dp = -1.0f;

float angPi = (float)acos(dp);

//side test
return y*other.x > x*other.y ? -angPi : angPi;
}

But i could not figure out where this "side test" came from.
Does it work? And how?

I usually use the traditional atan2 - atan2 calculation, but if this solution is really correct I could use in machines where this atan2 is not implemented.

### #2ViniGodoy  Members

Posted 05 August 2011 - 01:31 PM

Sorry, just now I realised that this is a disguised test of the perp dot product signal.

### #3Dirk Gregorius  Members

Posted 05 August 2011 - 02:00 PM

You could simply do this:

angle = atan2( abs( cross( v2, v1 ), dot( v2, v1 ) );

### #4Álvaro  Members

Posted 05 August 2011 - 02:51 PM

You could simply do this:

angle = atan2( abs( cross( v2, v1 ), dot( v2, v1 ) );

Man, you should read the question before giving out some canned answer.

### #5Emergent  Members

Posted 07 August 2011 - 03:20 PM

Also, just FYI, that" canned answer" is a bit wrong. Although the norm of the cross product gives you the magnitude of the sine of the angle, it doesn't give the sign, which is really what matters. Without that, you have no more information than the cosine gives you. You need the full determinant, sign included. I liked Vini's description as "perp dot product."

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