Willytmailboy_at_aol.com (Willytmailboy_at_aol.com) wrote:
> I can't find my book, to check, but I seem to remember that the
windows/apps
> in the alto's OS (smalltalk?) could not be overlapped, ie move one so it
> covered some of all of another.
As far as I know, the issue wasn't that windows couldn't overlap, but
that windows partially hidden behind other windows couldn't be updated.
If a program wanted to write to a background window, it would have to
wait until you brought it to the front again. Apple's engineers
implemented the "region", an arbitrary set of pixels, and its
corresponding operators for creating the union, intersection, and
difference of two regions. Regardless of how complicated the shape of the
visible part of a window was, it could always be defined as a set of
pixels, i.e. a region. Drawing to a window was masked by the window's
visible region, so you could write to the window, just like every bit of
it was visible, and the OS would take care of preserving the pixels that
were actually hidden by other windows.
- Michael
Michael J. Hussmann
E-mail: michael_at_michael-hussmann.de
WWW: http://michael-hussmann.de
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