Cube rotated by quaternion based trackball

[Boondock_Saint]

I'm trying to implement a trackball camera controller according to the OpenGL wiki:

http://www.opengl.org/wiki/Trackball

Now I think I'm almost there. The cube rotates a little bit, but it keeps snapping back to it's original position. The idea is to store the complete rotation in the member quaternion (m_cCurrentRot) of the trackball. Is this the wrong approach? I saw other approaches keeping the current rotation in the Modelview matrix. However, when I combine that approach with a translation, it gets messy.

Here are the relevant code snippets. Let me know if you need more.

void Trackball::move(int iOldX, int iOldY, int iNewX, int iNewY, Matrix4x4& cRotMat) {
	// see if there is a rotation
	if(iOldX == iNewX && iOldY == iNewY)
		return;
 
	// obtaining trackball coordinates, v1 is new, v2 is old
 
	Vector<GLfloat, 3> v1;
	v1[0] = iNewX - (m_uiWidth / 2);
	v1[1] = iNewY - (m_uiHeight / 2);
	v1[2] = projectOnSphere(v1[0], v1[1]);
	v1.normalize();
 
	Vector<GLfloat, 3> v2;
	v2[0] = iOldX - (m_uiWidth / 2);
	v2[1] = iOldY - (m_uiHeight / 2);
	v2[2] = projectOnSphere(v2[0], v2[1]);
	v2.normalize();
 
	// axis of rotation
	Vector<GLfloat, 3> axis = cross(v1, v2);
 
	// angle of rotation
	double theta = std::acos(v1 * v2);
 
	// quaternion representing the rotation
	Quaternion<GLfloat> cQuaternion;
	cQuaternion[0] = std::cos(0.5 * theta);
	cQuaternion[1] = std::sin(0.5 * theta) * axis[0];
	cQuaternion[2] = std::sin(0.5 * theta) * axis[1];
	cQuaternion[3] = std::sin(0.5 * theta) * axis[2];
	cQuaternion.normalize();
 
	// adding rotation to existing rotations via multiplication of quaternions
	m_cCurrentRot = cQuaternion * m_cCurrentRot;
	m_cCurrentRot.normalize();
	m_cCurrentRot.print();
	buildRotMatrix(cRotMat, m_cCurrentRot);
}
 
void Trackball::buildRotMatrix(Matrix4x4&amp; cNewRot, const Quaternion<GLfloat>&amp; cQ) {
 
	// [url]http://en.wikipedia.org/wiki/Quaternions_and_spatial_rotation#Conversion_to_and_from_the_matrix_representation[/url]
	cNewRot.set(
			// row 0
			cQ[0] * cQ[0] + cQ[1] * cQ[1] - cQ[2] * cQ[2] - cQ[3] * cQ[3],
			2.0f * (cQ[1] * cQ[2] - cQ[0] * cQ[3]),
			2.0 * (cQ[1] * cQ[3] + cQ[0] * cQ[2]),
			0.0f,
 
			// row 1
			2.0f * (cQ[1] * cQ[2] + cQ[0] * cQ[3]),
			cQ[0] * cQ[0] - cQ[1] * cQ[1] + cQ[2] * cQ[2] - cQ[3] * cQ[3],
			2.0f * (cQ[2] * cQ[3] - cQ[0] * cQ[1]),
			0.0f,
 
			// row 2
			2.0f * (cQ[1] * cQ[3] - cQ[0] * cQ[2]),
			2.0f * (cQ[2] * cQ[3] + cQ[0] * cQ[1]),
			cQ[0] * cQ[0] - cQ[1] * cQ[1] - cQ[2] * cQ[2] + cQ[3] * cQ[3],
			0.0f,
 
			// row 3
			0.0f,
			0.0f,
			0.0f,
			1.0f
	);
 
//	cNewRot.print();
}
 
GLfloat Trackball::projectOnSphere(int iX, int iY) {
	GLfloat glfZ;
	GLfloat glfXxYy = iX * iX + iY * iY;
 
	if(glfXxYy <= (m_glfRadius * m_glfRadius * 0.5)) {
		// inside sphere
		glfZ = sqrt(m_glfRadius * m_glfRadius - glfXxYy);
	}
	else {
		// outside of sphere, using hyperbolic-sheet
		glfZ = (0.5 * m_glfRadius * m_glfRadius) / sqrt(glfXxYy);
	}
 
	return glfZ;
}

And here the drawing and mouse functions:

// windows width, height and trackball radius
Trackball cTrackball(800, 800, 400.0f);
Matrix4x4 cRotMat(
		1.0f, 0.0f, 0.0f, 0.0f,
		0.0f, 1.0f, 0.0f, 0.0f,
		0.0f, 0.0f, 1.0f, 0.0f,
		0.0f, 0.0f, 0.0f, 1.0f
);
 
void drawGL() {
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
	glLoadIdentity();
 
	glTranslatef(0.0f, 0.0f, -10.0f);
 
	glMultMatrixf(cRotMat);
 
	renderCube(1.0);
 
	glutSwapBuffers();
}
 
void mouseEvent(int iButton, int iState, int iX, int iY) {
	iOldX = iX;
	iOldY = iY;
}
 
void mouseMotion(int iX, int iY) {
 
	cTrackball.move(iOldX, iOldY, iX, iY, cRotMat);
 
	glutPostRedisplay();
}

[Boondock_Saint]

For future readers: Yes, this is the way to go, apart from a few adjustments

1) I had a bug in Quaternion:perator* (not shown above)
2) Coordinates were scaled to [-1, 1] and a trackball size of 0.8 was used.
3) In the mouseMotion function the old coordinates need to be updated:

void RenWin::mouseMotion(int iX, int iY) {
	m_cTrackball.move(m_iOldX, m_iOldY, iX, iY, m_cRotMat);
	m_iOldX = iX;
	m_iOldY = iY;
	glutPostRedisplay();
}

Another tip: Make sure your default constructor initializes the quaternion to (1, 0, 0, 0), which means no rotation.