Date   : Thu, 08 Dec 1994 15:06:16 +0000
From   : lamcw <lamcw@...>
Subject: Re: BBC screen emulation

D. Barnett wrote:

;>my graphics routine has 3 stages:-
;>
;> a) process beeb vram into pixel map (for Mac)
;>
;> b) this pixel map holds LOGICAL colours. convert to their PHYSICAL colours
;>
;> c) call the Mac OS routine to draw the pixel map.
;>
;>
;>Stage c) is the time-eater but there's not much that can be done about that.
;>
;
;Here's a scheme which may reduce the cpu time routinely expended on the screen:
;
;In b) store a palette code as part of the logical code and use the host's
;palate settings to translate this extended logical code to the displayed
;palate.  This would allow the flashing colours to be implemented by palette
;switching just as the BBC does it now.


Yes, you are right. I used the hardware palette method (called colour look up
tables or CLUTs on the Mac) initially and then dropped it, and I really don't
know why. I do still use CLUTs to emulate the flashing colours but yes, i can
use them for the beeb's palette. 

But to put this into perpective, I don't think it will save much time.

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.

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...


Chris Lam,
Aston Unversity,
U.K.