Camera flip problem

Question

I'm programming a game in C# using the XNA3.1 engine. However I'm having a small issue with my camera, basically my camera tends to "flip" when it rotates more than 180 degrees on its roll (when the camera reaches 180 degrees, it seems to flip back to 0 degrees). The code for obtaining the view matrix is as follows:

Globals.g_GameProcessingInfo.camera.viewMat = Matrix.CreateLookAt(Globals.g_GameProcessingInfo.camera.target.pos, Globals.g_GameProcessingInfo.camera.LookAt, up);                //Calculate the view matrix

The Globals.g_GameProcessingInfo.camera.LookAt variable the position 1 unit directly in front of the camera, relative to the rotation of the camera, and the "up" variable is obtained with the following function:

static Vector3 GetUp()      //Get the up Vector of the camera
{
    Vector3 up = Vector3.Zero;
    Quaternion quat = Quaternion.Identity;
    Quaternion.CreateFromYawPitchRoll(Globals.g_GameProcessingInfo.camera.target.rot.Y, Globals.g_GameProcessingInfo.camera.target.rot.X, Globals.g_GameProcessingInfo.camera.target.rot.Z, out quat);

    up.X = 2 * quat.X * quat.Y - 2 * quat.W * quat.Z;       //Set the up x-value based on the orientation of the camera
    up.Y = 1 - 2 * quat.X * quat.Z - 2 * quat.Z * quat.Z;   //Set the up y-value based on the orientation of the camera
    up.Z = 2 * quat.Z * quat.Y + 2 * quat.W * quat.X;       //Set the up z-value based on the orientation of the camera
    return up;      //Return the up Vector3
}

Answer 1

meh was hoping to see a tan in there somewhere.

can you link to where you got your equation from please? (am at work and really don;t want to sit down myself and derive it)

how are you setting your camera rotation? are you sure nothing is going on there?

Answer 2

I'm a bit unsure about the math in your GetUp method. Could you elaborate on the math behind it?

In my lookat camera I initialize my up-vector once and then rotate that vector using a quaternion. This removes the possibility of trying to do a cross-product on parallel vectors in order to calculate the up vector.

Some semicode to clarify perhaps:

var up = Vector3.Up;
var target = <some point in space>;
var rotation = <current rotation quaternion>;

var forward = target - position;
forward = Vector3.Transform(forward, rotation);

var updatedPosition = target - forward;
var updatedUp = Vector3.Transform(up, rotation);

var view = Matrix.CreateLookAt(updatedPosition, target, updatedUp);

Answer 3

This is probably slower but the only way I know to do with would be the with the rotation matrix for 3D. Wikipedia Link

Where

and U = (Camera.position - Camera.lookat).norm

... Now, I believe that would give you the rotation part of the view matrix. However, I'm not 100% on it. I'm still looking into this though.

Answer 4

I got same problems in OpenGL with gluLookAt. I fixed that problem with my own camera class:

void Camera::ComputeVectors()
{
    Matrix4x4 rotX, rotZ;
    Quaternion q_x, q_y, q_z;
    Quaternion q_yx, q_yz;
    q_x.FromAngleAxis(radians.x, startAxisX);
    q_y.FromAngleAxis(radians.y, startAxisY);
    q_z.FromAngleAxis(radians.z, startAxisZ);
    q_yx = q_y * q_x;
    q_yx.ToMatrix(rotZ);
    q_yz = q_y * q_z;
    q_yz.ToMatrix(rotX);
    axisX = startAxisX;
    axisZ = startAxisZ; 
    axisX.Transform(rotX);
    axisZ.Transform(rotZ);
    axisY = axisX.Cross(axisZ);

    position = startPosition;
    position -= center;
    position.Transform(q_yx);
    position += center;
}

It is maybe overcomplicated, but working. axisY is your up vector. Full code listing is at: http://github.com/filipkunc/opengl-editor-cocoa/blob/master/PureCpp/MathCore/Camera.cpp

Hope it helps.