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000053_amos-request@svcs1.digex.net_Wed Sep 1 21:02:22 1993.msg
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From: Richard Barry Ling <u9147063@cs.uow.edu.au>
Message-Id: <199309020043.AA25434@topaz.cs.uow.edu.au>
Subject: Re: Jumping/flicker
To: amos-list@access.digex.net (AMOS User group)
Date: Thu, 2 Sep 1993 10:43:41 +1000 (EST)
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> From amos-request@svcs1.digex.net Wed Sep 1 21:58:12 1993 +1000
> Date: Wed, 01 Sep 1993 06:44:39 -0400 (EDT)
> From: sfmcnally@BIX.com
> Subject: Re: Jumping/flicker
> In-Reply-To: <9309010339.AA19739@elms.concept.com.au>
> To: Tom Plackowski <tom@elms.concept.com.au>
> Cc: amos-list@access.digex.net
> Message-Id: <9309010644.memo.74751@BIX.com>
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> X-Cosy-To: Tom Plackowski <tom@elms.concept.com.au>
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>
> 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.
> ;)
>
> Seumas
>
As far as I know, the correct order is
Do
... your drawing ...
Screen Swap
Wait Vbl
Loop
I've always used this for flicker-free drawing, and the majority of other
programs I have seen do as well. The Screen Swap command simply exchanges
the copper list pointers for the physical and logical screens. The copper
is already executing the list for the current physical screen, so you don't
actually interrupt the display at the exact instant you call Screen Swap:
the change only happens at the start of the next frame. However, drawing
commands are immediately affected - they are directed to the current logical
screen. So if you do a screen swap, then draw immediately, your display
will flicker like a bastard, since your logical screen is halfway through
being displayed! If you follow your Screen Swap with a Wait Vbl, you wait
until the current display is finished. Then the copper pointer is updated
from your new physical screen and your logical screen flips to the back, and
you can start drawing over again.
For the flicker-free bob animation, try this:
Screen open whatever
Double Buffer : Bob Update Off
For i = 1 to however many images
Bob 0, x, y, i
Update
Screen Swap
Wait Vbl
Next
I can't remember off the top of my head whether Update does a screen swap or
not. If it does try leaving out the screen swap command.
RL.
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