>> >> 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. Best, John _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
