[feat.request] Add RotationBetweenVectors in GLM_GTX_quaternion

[Calvin1602]

I couldn't find in glm a function to rotate a direction into another one.
Here's one :

quat RotationBetweenVectors(vec3 start, vec3 dest){
    start = normalize(start);
    dest = normalize(dest);

    float cosTheta = dot(start, dest);
    vec3 rotationAxis;

    if (cosTheta < -1 + 0.001f){
        // special case when vectors in opposite directions :
        // there is no "ideal" rotation axis
        // So guess one; any will do as long as it's perpendicular to start
        // This implementation favors a rotation around the Up axis (Y),
        // since it's often what you want to do.
        rotationAxis = cross(vec3(0.0f, 0.0f, 1.0f), start);
        if (gtx::norm::length2(rotationAxis) < 0.01 ) // bad luck, they were parallel, try again!
            rotationAxis = cross(vec3(1.0f, 0.0f, 0.0f), start);

        rotationAxis = normalize(rotationAxis);
        return gtx::quaternion::angleAxis(180.0f, rotationAxis);
    }

    // Implementation from Stan Melax's Game Programming Gems 1 article
    rotationAxis = cross(start, dest);

    float s = sqrt( (1+cosTheta)*2 );
    float invs = 1 / s;

    return quat(
        s * 0.5f, 
        rotationAxis.x * invs,
        rotationAxis.y * invs,
        rotationAxis.z * invs
    );


}

Ogre's version enables the user to choose the fallback axis instead of forcing the Y one; but when not provided, it favours the Z axis, which I find weird : https://bitbucket.org/sinbad/ogre/src/3cbd67467fab3fef44d1b32bc42ccf4fb1ccfdd0/OgreMain/include/OgreVector3.h?at=default#cl-651

[Groovounet]

I am a bit surprize to see:
"return gtx::quaternion::angleAxis(180.0f, rotationAxis);"
in the proposed code.
angleAxis is supposed to take radians...

Thanks,
Christophe

[Groovounet]

This feature request has been implemented. This is just one change, by default the function assume that the inputs are already normalized.

Thanks for contributing!
Christophe