quaternion inverts vector

Question

I'm using glm::quaternions to rotate a vector around a certain axis. But the vector inverts the axis everytime it is called.

Here is my code to rotate my object around the side axis:

void PlayerController::rotateForward(float angle) {
    angle = (angle*M_PI) / 180.0f;
    fquat rot = fquat(angle, playerObject->getVecSide());
    normalize(rot);
    vec4 newUpVec = rot * vec4(playerObject->getVecUp(), 1.0f);
    normalize(newUpVec);
    vec3 upVec3 = normalize(vec3(newUpVec.x ,newUpVec.y, newUpVec.z));

    playerObject->setVecUp(upVec3);
    playerObject->setVecForward(-normalize(cross(playerObject->getVecUp(), playerObject->getVecSide())));
    vec3 newPlayerPos = currentPlanet->getPosition() + (playerObject->getVecUp() * radius);
    playerObject->setPosition(newPlayerPos);
}

everytime I call this method, my up vector is turned around the axis, but also inverted. I can work around this by using:

    vec4 newUpVec = rot * -vec4(playerObject->getVecUp(), 1.0f);

but this is more like treating symptoms instead of finding the cause. Maybe someone around here can help me in understanding what the quaternion does here.

minime minime · Accepted Answer · 2012-06-06T00:35:50

well here is the answer, the quaternion initialization was wrong...

these lines were missing:

rotateAround = rotateAround * sinf(angle/2.0f);
angle = cosf(angle/2.0f);

so the correct version of the method looks like this:

vec3 GameObjectRotator::rotateVector(float angle, vec3 toRotate, vec3 rotateAround) {
    angle = (angle*M_PI) / 180.0f;

    rotateAround = rotateAround * sinf(angle/2.0f);
    angle = cosf(angle/2.0f);

    fquat rot = fquat(angle, rotateAround);
    normalize(rot);
    vec4 rotated =  rot * vec4(toRotate, 1.0f);
    normalize(rotated);
    return normalize(vec3(rotated));
}

(the class has gone through quite a bit refactoring since the first version, but the idea should be clear anyways)

quaternion inverts vector

1 Answers