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000047_amos-request@svcs1.digex.net_Wed Sep 1 15:30:45 1993.msg
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Message-Id: <199309011833.AA16889@access.digex.net>
Date: 1 Sep 93 14:33:00 EST
From: "Andrew Church" <95ACHURCH@vax.mbhs.edu>
Subject: Re: Jumping/flicker
To: "amos-list" <amos-list@access.digex.net>
Status: RO
>>If you're using Screen Offset to scroll, you have to remember that it will
>>move the screen the MOMENT it's called.
>>Try autoview off and do a view somewhere around the screen swap.
Or have XOFS and YOFS variables, and a _WAIT_VBL procedure that does the
Screen Offset right after the Wait Vbl. This is similar to what I do in a C
program I have that needs to scroll a screen around, except I use the Vbl
interrupt. I still get a bit of flicker, but little enough that I don't
worry abot it.
>The scrolling is done using Scroll.
This can get ugly in a single-buffered screen, because a Scroll is
basically a Screen Copy. You could get part of the old picture and part of
the new one displayed.
>I didn't realise that the Double Buffer had to appear after the Open Screen.
That does help. :-)
>Why does the manual always say
> Screen Swap
> Wait Vbl
>in that order?
>
>Does Screen Swap do nothing until VBL? Or will this cause possible flicker on
>the lower half of the screen?
Seumas said:
>Ahh, try the wait vbl before the screen swap.
>You have to get the scan line to the vbl before you swap the screens, or parts
>will probly be different as they are swapped in the middle of the screen
>update.
Screen Swap switches the bitplane pointers in the Screen Base structure with
the bitplanes that aren't currently displayed, and updates the copper list.
However, the new bitmap is not displayed until after the vertical blanking
period, as messing with an active copper list can result in some impressive
fireworks displays. So if you do two Screen Swap commands before a vertical
blank, you'll wind up with the original display staying there.
It doesn't really matter whether you do Wait Vbl or Screen Swap first, as
long as you have them both (or you have some other way to make sure you only
do one Screen Swap per Vbl).
--Andy Church