[racket] make only selected color to transparent

From: 김태윤 (kty1104 at gmail.com)
Date: Sat Nov 20 20:55:22 EST 2010

it's about 10 hours since I stuck at overlaying several images
(make only selected color to transparent)
could somebody give me an little advice?
just little hint about big picture would be very helpful for me.

#lang racket/gui
(define base-dc (make-object bitmap-dc% (make-object bitmap% 320 320 #f) ))
(define m-canvas%
  (class canvas%
    (override on-paint)
    (define on-paint (λ () (send (send this get-dc) draw-bitmap (send
base-dc get-bitmap) 0 0)))
(define color (make-object color% "red"))
(define f (new frame% (label "a")))
(define c (new m-canvas% (parent f)(min-width 320) (min-height 320)))
(send base-dc draw-bitmap (make-object bitmap% "./src/arena.png") 0 0 )
(send base-dc set-argb-pixels 0 0 288 256 (make-bytes (* 4 288 256)))
(send base-dc draw-bitmap (make-object bitmap% "./src/char4.png") 0 0 )
(send f show #t)
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