Accessing image pixels

Hi,

I am banging my head on the following problem since a couple of days 
now, maybe someone else knows what I am missing:

I try to read a bmp image from a file and do some simple image 
processing on it. For this I took examples/capi/graphics/images.lisp as 
a basis. This displays a bmp just fine. However, how can I access 
individual pixels of this image (I mean how can I get RGB or some other 
color information for a pixel)? I tried the following:

(defun calculate-outline (interface)
   (with-slots (viewer image) interface
     (when image
       (let ((access (gp:make-image-access viewer image)))
          ...
         (unwind-protect
             (progn
               (gp:image-access-transfer-from-image access)
               (dotimes (i (gp:image-access-height access))
                 (dotimes (j (gp:image-access-width access))
                   (format t "~A~%" (gp:image-access-pixel access i 
j)))))
           (progn
             (gp:free-image-access access)))
         ...
         ))))

but all I get error "Image access not supported for this image" in the 
Cocoa event loop (I use Personal edition for Mac). I did not find 
anything in the documentation on gp:make-image-access and friends, so I 
am lost here.

All input is welcome!

Thanks,
Thomas