Re: [9fans] Re: moving things in a window

[Theo Honohan <theoh@chiark.greenend.org.uk>]

In message <Pine.LNX.4.10.10009261109230.13135-100000@neon.myriadgate.net>, and
rey mirtchovski writes:
> 
> We start with a black square.
> Then we draw a white circle where the old circle was.
> Then we draw a black circle where the new circle will be.
> 
> The result is a black image with a sliver of white corresponding to
> the area of the old circle that is outside the new circle.
> 
> Interpreted as a mask, black is transparent; white opaque.
> So the mask says ``only draw where the old circle was
> but the new circle is not''.
> The final draw puts white down in those places.
> --- end paste --
> 

That matches my reading of Russ's code.  I left the masking with the
"old circle" out, in order to keep things simple.  As far as I can
see, it's really an optimisation, rather than part of the semantics of
the operation (unless switching "trails" on and off on the fly is
important to you; in general, I guessed that it wasn't).

> the explanation may be a bit too high level for you though :)

Well, I was trying to reason about a more general case, with full
RGBA objects.  

(Though I guess bringing the semantics of draw() into the argument was
lowering the *tone* a bit, groan)