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docs: clarify effect of mask-bitmap alphas channel in dc<%> draw-bitmap

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Matthew Flatt 2010-12-16 19:35:23 -07:00
parent f8ec47cdeb
commit 37682242e5
1 changed files with 4 additions and 2 deletions
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collects/scribblings/draw/dc-intf.scrbl
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@ -112,8 +112,10 @@ If the @racket[mask] bitmap is monochrome, drawing occurs in the
If the @racket[mask] bitmap is grayscale, then the blackness of each
mask pixel controls the opacity of the drawn pixel (i.e., the mask
acts as an inverted alpha channel). If the @racket[mask] bitmap is
color, the component values of a given pixel are averaged to arrive
at an @racket[alpha] value for the pixel.
color, the color component values of a given pixel are averaged to arrive
at an @racket[alpha] value for the pixel. If the @racket[mask] bitmap
has an alpha channel, it is combined for masking with the alpha value that is
computed from the color components.
The current brush, current pen, and current text for the DC have no
effect on how the bitmap is drawn, but the bitmap is scaled if the DC