callback needs access to multiple topics - how?

edit

I'm trying to do some processing on images published by a kinect but this is a general camera-related question.

I would like to use the image_geometry functions contained in the PinholeCameraModel for converting 3d rays to pixels and 2d pixels to 3d rays, while making use of the calibration parameters published under /camera/rgb/camera_info.

The documentation gives an example like this:

image_geometry::PinholeCameraModel model_;

void imageCb(const sensor_msgs::ImageConstPtr& raw_image,
             const sensor_msgs::CameraInfoConstPtr& cam_info)
{
  // Update the camera model (usually a no-op)
  model_.fromCameraInfo(cam_info);

  // Do processing...
}

Defining a subscriber like this doesn't work though:

ros::Subscriber sub = n.subscribe("/camera/rgb/image_rect /camera/rgb/camera_info", 1, &imageCb);

I'm guessing that I have to do something to group the image with the camera_info into one single message, perhaps with image_transport(?), but I don't have any idea how to do this. I suppose i could create a separate callback for the camera info and update some global variable that the other callback could access but I think that's a less than ideal solution in general (in this case I'm sure it works, since the calibration parameters are constant).