[racket] monochrome bitmaps

From: Harry Spier (vasishtha.spier at gmail.com)
Date: Thu Nov 1 17:38:38 EDT 2012

Matthew Flatt wrote:

> A monocrhome bitmap doesn't have an alpha channel, but it you ask for
> its pixels in `just-alpha?' mode, then while pixels generate a 0 alpha
> and black pixels generate a 255 alpha.

Thanks Matthew.  What confused me was that  for:
(send a-dc set-alpha opacity)
opacity is a value between 0 and 1 (not a value between 0 and 255)
and I didn't realize this method acts as a multiplier of each pixels
alpha channel and doesn't set the value of the alpha channel directly.
So I incorrectly assumed each pixels alpha channel has a value between
0 and 1 not between 0 and 255.

Harry Spier

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