Date   : Thu, 08 Dec 1994 11:53:34 -0600
From   : barnett@... (David Barnett)
Subject: Screen updates on Changes

Chris Lam replied to my suggestion:
>
>As for checking which bits of the screen have been updated, this is an
>overhead in itself. It's fiddly and again, i'm not sure the time
>saved justifies implementing the system.
>
The overhead is very small.  Suppose the screen is divided into a 4*4 or
8*8 Grid (or even into a number of horizontal bands).  A screen write would
mark the region on the changed list.

Even just marking the whole screen as changed (i.e. one region only) would
save significant time because there are many programs don't touch the
screen for long periods.

Even fast moving games do not change the whole screen every field.

>
>One idea I did toy with is to plot the top half/ bottom half of the screen
>alternately. If the visual result isn't too noticeable, my gut feeling is
>that this could save some significant time. It's not difficult to implement
>either. Mmm, I might just give it go...
>
This might be better than a slow refresh rate but it does not address the
fact that frequently the screen does not change for many fields so why
gratuitously recopy it to the host screen?

David Barnett