Arcball Rotation with Quaternions (using iOS GLKit

问题:

I'm looking for a simple implementation for arcball rotation on 3D models with quaternions, specifically using GLKit on iOS. So far, I have examined the following sources:

• Arcball rotation with GLKit
• How to rotate a 3D object with touches using OpenGL

I've also been trying to understand source code and maths from here and here. I can rotate my object but it keeps jumping around at certain angles, so I fear gimbal lock is at play. I'm using gesture recognizers to control the rotations (pan gestures affect roll and yaw, rotate gestures affect pitch). I'm attaching my code for the quaternion handling as well as the modelview matrix transformation.

Variables:

• GLKQuaternion rotationE;

Quaternion Handling:

- (void)rotateWithXY:(float)x and:(float)y
{
    const float rate = M_PI/360.0f;
    GLKVector3 up = GLKVector3Make(0.0f, 1.0f, 0.0f);
    GLKVector3 right = GLKVector3Make(1.0f, 0.0f, 0.0f);

    up = GLKQuaternionRotateVector3(GLKQuaternionInvert(self.rotationE), up);
    self.rotationE = GLKQuaternionMultiply(self.rotationE, GLKQuaternionMakeWithAngleAndVector3Axis(x*rate, up));

    right = GLKQuaternionRotateVector3(GLKQuaternionInvert(self.rotationE), right);
    self.rotationE = GLKQuaternionMultiply(self.rotationE, GLKQuaternionMakeWithAngleAndVector3Axis(y*rate, right));
}

- (void)rotateWithZ:(float)z
{
    GLKVector3 front = GLKVector3Make(0.0f, 0.0f, -1.0f);

    front = GLKQuaternionRotateVector3(GLKQuaternionInvert(self.rotationE), front);
    self.rotationE = GLKQuaternionMultiply(self.rotationE, GLKQuaternionMakeWithAngleAndVector3Axis(z, front));
}

Modelview Matrix Transformation (Inside Draw Loop):

// Get Quaternion Rotation
GLKVector3 rAxis = GLKQuaternionAxis(self.transformations.rotationE);
float rAngle = GLKQuaternionAngle(self.transformations.rotationE);

// Set Modelview Matrix
GLKMatrix4 modelviewMatrix = GLKMatrix4Identity;
modelviewMatrix = GLKMatrix4MakeTranslation(0.0f, 0.0f, -0.55f);
modelviewMatrix = GLKMatrix4Rotate(modelviewMatrix, rAngle, rAxis.x, rAxis.y, rAxis.z);
modelviewMatrix = GLKMatrix4Scale(modelviewMatrix, 0.5f, 0.5f, 0.5f);
glUniformMatrix4fv(self.sunShader.uModelviewMatrix, 1, 0, modelviewMatrix.m);

Any help is greatly appreciated, but I do want to keep it as simple as possible and stick to GLKit.

回答1:

There seem to be a few issues going on here.

1. You say that you're using [x,y] to pan, but it looks more like you're using them to pitch and yaw. To me, at least, panning is translation, not rotation.

2. Unless I'm missing something, it also looks like your replacing the entire rotation everytime you try to update it. You rotate a vector by the inverse of the current rotation and then create a quaternion from that vector and some angle. I believe that this is equivalent to creating the quaternion from the original vector and then rotating it by the current rotation inverse. So you have q_e'*q_up. Then you multiply that with the current rotation, which gives q_e*q_e'*q_up = q_up. The current rotation is canceled out. This doesn't seem like it's what you want.

   All you really need to do is create a new quaternion from axis-and-angle and then multiply it with the current quaternion. If the new quaternion is on the left, the orientation change will use the eye-local frame. If the new quaternion is on the right, the orientation change will be in the global frame. I think you want:

   self.rotationE =
     GLKQuaternionMultiply( 
       GLKQuaternionMakeWithAngleAndVector3Axis(x*rate, up),self.rotationE);
   

   Do this, without the pre-rotation by inverse for all three cases.

3. I've never used the GLKit, but it's uncommon to extract axis-angle when converting from Quaternion to Matrix. If the angle is zero, the axis is undefined. When it's near zero, you'll have numeric instability. It looks like you should be using GLKMatrix4MakeWithQuaternion and then multiplying the resulting matrix with your translation matrix and scale matrix:

   GLKMatrix4 modelviewMatrix = 
     GLKMatrix4Multiply( GLKMatrix4MakeTranslation(0.0f, 0.0f, -0.55f),
                         GLKMatrix4MakeWithQuaternion( self.rotationE ) );
   modelviewMatrix = GLKMatrix4Scale( modelviewMatrix, 0.5f, 0.5f, 0.5f );
   

回答2:

I was recently asked a bit more about my resulting implementation of this problem, so here it is!

- (void)rotate:(GLKVector3)r
{
    // Convert degrees to radians for maths calculations
    r.x = GLKMathDegreesToRadians(r.x);
    r.y = GLKMathDegreesToRadians(r.y);
    r.z = GLKMathDegreesToRadians(r.z);

    // Axis Vectors w/ Direction (x=right, y=up, z=front)
    // In OpenGL, negative z values go "into" the screen. In GLKit, positive z values go "into" the screen.
    GLKVector3 right = GLKVector3Make(1.0f, 0.0f, 0.0f);
    GLKVector3 up = GLKVector3Make(0.0f, 1.0f, 0.0f);
    GLKVector3 front = GLKVector3Make(0.0f, 0.0f, 1.0f);

    // Quaternion w/ Angle and Vector
    // Positive angles are counter-clockwise, so convert to negative for a clockwise rotation
    GLKQuaternion q = GLKQuaternionIdentity;
    q = GLKQuaternionMultiply(GLKQuaternionMakeWithAngleAndVector3Axis(-r.x, right), q);
    q = GLKQuaternionMultiply(GLKQuaternionMakeWithAngleAndVector3Axis(-r.y, up), q);
    q = GLKQuaternionMultiply(GLKQuaternionMakeWithAngleAndVector3Axis(-r.z, front), q);

    // ModelView Matrix
    GLKMatrix4 modelViewMatrix = GLKMatrix4Identity;
    modelViewMatrix = GLKMatrix4Multiply(modelViewMatrix, GLKMatrix4MakeWithQuaternion(q));
}

Hope you put it to good use :)