I've created a potentially beautiful double buffered Canvas class
that has less flicker than others I've tried however there is one
major problem: I can't animate a canvas that has translucent masks
like a shadow. I'm looking for a work around before I release the
class into the wild.
Here's an example. Lets say you are animating a spinning beach ball
with a shadow.
Steps:
1) Assign a circular mask with translucent shadow to a new picture in
memory for buffering.
2) Copy State1 (a graphic) to the Buffer. (State1 is a beach ball
surrounded by black. The mask turns part of the black into a shadow.)
3) Draw the Buffer to the screen
(so far so good.)
4) Clear the buffer. Copy State2 to the Buffer.
5) Draw the Buffer to the screen.
Ooops! Here's where it goes wrong.
We are drawing State2 literally over top of the existing graphic.
The beach ball itself looks fine because it is entirely replaced.
However the translucent shadow is drawn over top of the existing
shadow adding them together. Each step darkens the shadow until it's
black. This is because we don't copy a square buffer to the screen.
It smartly copies only the masked images as it should.
Hmmm... What if we erased the image first?
Well, we could erase the canvas and then copy the buffer but that
creates flicker on Windows. We could mess with Refresh but that gives
the same result. Erasing the buffer first doesn't help because in
the end we always copy a masked buffer to the Canvas. (You can't
mask the canvas directly.)
Joe Strout gets away with this successfully in his famous Three Ways
to Animate
http://www.rblibrary.com/index.php?main_page=product_info&products_id=7
because he is animating over a known fixed background of stars and
copies the masked rocket ship into a non masked buffer that already
contains the fixed starry background. The non masked, rectangular
buffer then gets copied to the Canvas. Alas, this won't work if you
don't know what might be behind your canvas.
I seem to be a dead end and can only conclude it's not possible in RB
to do flicker-free canvas based animations with translucency against
an arbitrary background. I'd love to be proven wrong and welcome any
suggestions.
Cheers!
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