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correct draw-bitmap docs

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Matthew Flatt 2010-12-16 20:41:54 -07:00
parent 37682242e5
commit 6f114b318b
1 changed files with 20 additions and 11 deletions
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31
collects/scribblings/draw/dc-intf.scrbl
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@ -104,18 +104,27 @@ If a @racket[mask] bitmap is supplied, it must have the same width and height
@scheme[bitmap-dc%] object, both bitmaps must be distinct from the
destination bitmap, otherwise @|MismatchExn|.
If the @racket[mask] bitmap is monochrome, drawing occurs in the
target @scheme[dc<%>] only where the mask bitmap contains black
pixels (independent of @racket[style], which controls how the white
pixels of a monochrome @racket[source] are handled).
The effect of @racket[mask] on drawing depends on the type of the
bitmap:
@;
@itemlist[
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 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.
@item{If the @racket[mask] bitmap is monochrome, drawing occurs in
the target @scheme[dc<%>] only where the mask bitmap contains
black pixels (independent of @racket[style], which controls how
the white pixels of a monochrome @racket[source] are handled).}
@item{If @racket[mask] is color with an alpha channel, the alpha
channel only is used as the mask for drawing @racket[source].}
@item{If the @racket[mask] bitmap is color without an alpha channel,
the color components of a given pixel are averaged to arrive at
an @racket[alpha] value for the pixel. In particular, 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).}
]
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