I am trying to use image_proc to rectify an image which is just a
camera image that has a border added around. The purpose is to make a
rectified image that includes all of the image (including what does
not strictly fit into a square), not just the centermost part. To do
this I made a new node image_letterboxer (included source) to add the
border and to adjust the camera calibration cx and cy params to
reflect the new center of the image. This has worked when not using
image_proc to do the undistort.

However, when I try to use this node with image_proc, it does not work
at all. I dug a little into the source and found that the image
transport is subscribing to the new image topics correctly, and that
image transport is registering that subscription, but no callbacks are
being called. My image_letterboxer node works well enough so that
image_view can display the new image_raw topic with the letterboxed
image, but image_proc does not work, though it does work on the camera
itself.

I assume that the problem is in the image_letterboxer node, but I
cannot figure out what I did wrong. Is there some flaw in the way that
I'm using image_transport to publish the images? Or is there something
wrong with image_proc?

Thanks in advance,
-Dave