Fwd: Re: Window code issue

Tim Rowledge rowledge at interval.com
Thu Dec 31 18:16:07 UTC 1998

I forgot an important paragraph in my last note on this subject - and one
that might just help the BeOS folks work out what is slowing them down.

The reason that having the Display be in little-endian form and doing a
pixel reverse for any blit where the Display is source and/or target is a
poor choice is that the Display is involved in a _lot_ of blits. I forget
the precise details, but at one time I had to trace something  that lead me
to notice that something like 20-50 times more blits were to the screen
than there were display update cycles.

On Acorn and Windows (possibly X11, though I haven't read the code enough
to be sure) the Display is only copied to the glass when the appropriate OS
event is received. At that time, and that time only, is the Dispaly bitmap
pixel reversing done - which is why Andreas is quite right that the
overhead is fairly low, as long as the OS merges small rectangles etc
effectively and as long as we have to handle these events relatively rarely
(~100 times/sec max) all is ok.  If I understand the code correctly, the
Mac updates the glass every time.(?)

If you are doing a pixel reverse _every_ time you write to the Display, not
to mention actually forcing a copy to the glass, then performance
would probably suffer. Perhaps, just perhaps, this is what the BeOS code is


Useful random insult:- Born a day late and like that ever since.
Tim Rowledge:  rowledge at interval.com (w)  +1 (650) 842-6110 (w)
 tim at sumeru.stanford.edu (h)  <http://sumeru.stanford.edu/tim>

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