[racket] question about dc% and scale

Although pixel alignment can be confusing, I think this one is just a
bug.

The `draw-rectangle' method tries to adjust the width of an outline so
that in the simple case --- a width 0 or 1 pen in unsmoothed or aligned
mode with a bitmap target --- the rectangle outline is the given width
and height, instead of extending an extra pixel. That adjustment isn't
implemented correctly, and the problem is particularly clear with a
large scaling factor.

You can work around the problem by creating a rectangle `dc-path%' and
drawing it with `draw-path'. Meanwhile, I'll fix `draw-rectangle'.

At Thu, 10 May 2012 09:57:26 -0500, Robby Findler wrote:
> The problem is that the coordinates are "between" the pixels. There is an
> explanation of this in the context of another library, but the issues are
> the same:
> 
> 
> http://docs.racket-lang.org/teachpack/2htdpimage.html?q=2htdp/image#%28part._nitty-g
> ritty%29
> 
> hth,
> Robby
> 
> On Thu, May 10, 2012 at 8:29 AM, Garrett Mitchener <
> garrett.mitchener at gmail.com> wrote:
> 
> > Hi, I'm learning the racket GUI package, and I ran into something that
> > doesn't make sense to me:
> >
> > #lang racket/gui
> >
> > (define black-pen
> >   (new pen%
> >        [width 1]
> >        [color "black"]))
> >
> > (define blue-brush
> >   (new brush%
> >        [color "blue"]))
> >
> > (define (draw-test-2 dc)
> >   (send dc scale 10 10)
> >   (send dc set-pen black-pen)
> >   (send dc set-brush blue-brush)
> >   (send dc draw-rectangle 7 2 4 4))
> >
> > (define pic (make-bitmap 200 100))
> >
> > (define dc (new bitmap-dc% [bitmap pic]))
> >
> > (draw-test-2 dc)
> >
> > And at the end, pic looks like this:
> >
> > [image: Inline image 1]
> > The outline and the filling don't line up.  What am I doing wrong?  This
> > is with DrRacket 5.2.1 on linux.
> >
> > Thanks,
> > -- Garrett Mitchener
> >
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> >
> >
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