Placing UIView over image target - minor misalignment

Hi, I am working on placing an UIView over the image target. Starting from the ImageTarget example and timlai's class (http://pastebin.com/kmeVZ7jy) I use the following code in the render() method to create a CATransform3D structure from the pose matrix that later I can assign to the CALayer of UIView:

[code=c]

    const QCAR::ImageTarget* imageTarget = (const QCAR::ImageTarget*)trackable;    QCAR::Vec2F size = imageTarget->getSize();

    QCAR::Matrix44F modelViewMatrix = QCAR::Tool::convertPose2GLMatrix(imageTarget->getPose());    CGFloat ScreenScale = [[UIScreen mainScreen] scale];    float xscl = qUtils.viewport.sizeX/ScreenScale/2;    float yscl = qUtils.viewport.sizeY/ScreenScale/2;    QCAR::Matrix44F projectedTransform = {1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1};    QCAR::Matrix44F qcarTransform = {1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1};    ShaderUtils::translatePoseMatrix(size.data[0] / 2, size.data[1] / 2, 0, projectedTransform.data);    ShaderUtils::scalePoseMatrix(xscl, -yscl, 1.0, projectedTransform.data); // flip along y axis    ShaderUtils::multiplyMatrix(projectedTransform.data, qUtils.projectionMatrix.data, qcarTransform.data);    ShaderUtils::multiplyMatrix(qcarTransform.data, modelViewMatrix.data, qcarTransform.data);    CATransform3D transform = *((CATransform3D*)qcarTransform.data); // both are array[16] of float    // UIView* myView = ...    myView.frame = CGRectMake(0, 0, size.data[0], size.data[1]);    myView.layer.transform = transform;

[/code]

Now this code works almost as I expect, the view is positioned more or less over the image target. But unfortunately the position is inaccurate: if I walk around the image target the UVIew moves around the center of the target in a small circle with a radius of approxiamtely 5-10% of the width of the target and the view. The view seems to be placed somewhat closer to the camera then it should be, however I am unsure whether it is a misalignment in the z coordinate or an error in the transformation matrix calculation. Any hint on what could cause this error?