#define RADIANS_PER_PIXEL (M_PI / 320.f)
 
/**
 * do this in viewDidLoad
 *
 * these are class variables:
 
	CGPoint iniLocation;
	GLKQuaternion quarternion;
 
 */
- (void) initArcBall
{
	quarternion = GLKQuaternionMake(0.f, 0.f, 0.f, 1.f);
	iniLocation = CGPointMake(0.f, 0.f);
}
 
/**
 * do this in update
 
 // skip auto rotation
 //baseModelViewMatrix = GLKMatrix4Rotate(baseModelViewMatrix, _rotation, 0.0f, 1.0f, 0.0f);
 
 // rotate with quaternion instead
 [self rotateMatrixWithArcBall:&baseModelViewMatrix];
 
 */
- (void) rotateMatrixWithArcBall:(GLKMatrix4 *)matrix
{
	GLKVector3 axis = GLKQuaternionAxis(quarternion);
	float angle = GLKQuaternionAngle(quarternion);
	if( angle != 0.f )
		*matrix = GLKMatrix4Rotate(*matrix, angle, axis.x, axis.y, axis.z);
}
 
// ------------------------------------------------------------------------------------------
 
- (void) rotateQuaternionWithVector:(CGPoint)delta
{
	GLKVector3 up = GLKVector3Make(0.f, 1.f, 0.f);
	GLKVector3 right = GLKVector3Make(-1.f, 0.f, 0.f);
 
	up = GLKQuaternionRotateVector3( GLKQuaternionInvert(quarternion), up );
	quarternion = GLKQuaternionMultiply(quarternion, GLKQuaternionMakeWithAngleAndVector3Axis(delta.x * RADIANS_PER_PIXEL, up));
 
	right = GLKQuaternionRotateVector3( GLKQuaternionInvert(quarternion), right );
	quarternion = GLKQuaternionMultiply(quarternion, GLKQuaternionMakeWithAngleAndVector3Axis(delta.y * RADIANS_PER_PIXEL, right));
}
 
- (void) touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
	UITouch *touch = [touches anyObject];
	CGPoint location = [touch locationInView:self.view];
 
	iniLocation = location;	
}
 
- (void) touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
	UITouch *touch = [touches anyObject];
	CGPoint location = [touch locationInView:self.view];
 
	// get touch delta
	CGPoint delta = CGPointMake(location.x - iniLocation.x, -(location.y - iniLocation.y));
	iniLocation = location;
 
	// rotate
	[self rotateQuaternionWithVector:delta];
}