[racket] erasing a line?

From: John Riedl (riedl at cs.umn.edu)
Date: Thu Jun 30 22:55:38 EDT 2011

>> Two interesting problems that I ran into are:
>> 1) The bitmap draw routines do not erase the rectangle they draw into.
>>  I don't know why.  I ended up erasing the rectangle "by hand" with
>> draw-rectangle with a transparent pen.  (I tried 'solid and 'opaque
>> styles.  My backing bitmap and canvas are all default for things like
>> monochrome? and alpha.)
> I believe the default bitmap is one with no alpha component. If you
> want one with an opaque background, then you have to initialize it
> that way instead.

Ahhh!  You put me onto the fix :).  The problem is that I had read the
docs for make-object bitmap, which makes a bitmap with no alpha
channel.  However, I was actually using make-bitmap, as recommended in
the docs.  It turns out that make-bitmap reverses the default to
create an alpha channel -- but set to zero, whence the transparent
behavior.  Once I changed the make-bitmap call to force no alpha
channel, I get the nice boring bitblt behavior I was expecting, so no
more need for drawing rectangles to erase the region first.  (I think
your note above has the semantics of no alpha backwards: I believe no
alpha channel causes boring semantics, which an alpha channel makes
transparency possible.)

In any case, this is a usability fail.  The two ways of making bitmaps
should have the same defaults, even if their designers disagree on
which is preferred :).  To whom do I mention that?

>> 2) The draw-rectangle draws a little too big of a rectangle,
>> presumably because of the outline it is drawing with the transparent
>> pen.  I had to make the draw-bitmap draw just a couple of extra pixels
>> to make the erase work.
> That doesn't sound right. Just to double check: coordinates are on the
> boundaries between pixels, not on the pixels (and drawing in aligned
> and smoothed and regular mode change some details here and there about
> how lines are drawn, so maybe that's also confusing things).

I'm not being that fancy.  I just was erasing a rectangle of a given
size, and then bitblting in exactly that size of a bitmap from my
backing bitmap to exactly that location.  The erase was erasing a
pixel more than the bitblt was bringing in, here and there.
Increasing the size of the bitblt by a pixel fixed the problem.  I
think the easiest explanation is that the rectangle was drawing a
little too big ...

Thanks again!  Nice to get such a fast response.


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