[racket] processing images...

Hello all,  

I'm having some trouble processing a block of pixels from an image. I want to grab some portion of image, do something to it, and save it to a file.  In this example, I grab a portion of an image and save it to a file but the image in the file is corrupted.  Evidently I'm doing something wrong.  I've spent a number of hours trying to figure it out, but I'm blind to the problem.  I would be very grateful for any help someone can give me.

Sincerely,

Dan Anderson


#lang racket/gui
; define the portion of the image I want to process
(define x 0)
(define y 0)
(define w 10)
(define h 10)


(define bm (make-object bitmap% "4colors.jpg"))


(define pixels (make-bytes (* w h 4) 0)) ;each pixel has four numbers 
(send bm get-argb-pixels x y w h pixels true) ; get the block of pixels

pixels ;check that there is information in pixels 
;make a new bitmap of the section
(define nbm (make-object bitmap% pixels w h))
(send nbm save-file "test.png" 'png)