[racket] make only selected color to transparent

At Sun, 21 Nov 2010 10:55:22 +0900, 김태윤 wrote:
> 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.
> please~~!
> 
> #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)))
>     (super-new)))
> (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)

Is the problem that "arena.png" and "char4.png" are images with
transparency, but `draw-bitmap' draws them solid?

If so, you need to supply 'png/mask as the after the pathname when
loading the bitmap

   (define bm1 (make-object bitmap% "./src/arena.png" 'png/mask))

and then use the bitmap mask when drawing:

   (send base-dc draw-bitmap bm1 
                 0 0 solid 
                 (send the-color-database find-color "black")
                 (send bm1 get-loaded-mask))


It's much simpler in the current development version of Racket, where
transparency is more directly supported on bitmaps. In version
5.0.99.2,

  (send base-dc draw-bitmap (read-bitmap "./src/arena.png") 0 0)

automatically loads and uses transparency information.