From:	"jakub" <marushka AT ozemail DOT com DOT au>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Sprite "ghost" help please.
Date:	Tue, 21 Jul 1998 13:40:02 +1000
Organization:	OzEmail Ltd.
Lines:	54
Message-ID:	<6p12m0$r7u$1@reader1.reader.news.ozemail.net>
References:	<002901bdb03d$955c7500$784e08c3 AT arthur>
NNTP-Posting-Host:	slsdn2p08.ozemail.com.au
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

I've noticed that the allegro demo game has the same problem on my system.
I select page flipping mode, with a 640x480 screen.

When you move your shift left-right, I can see two ships slightly appart,
like a ghost image on a bad tv set. It seems I am seeing the last position
of the ship sprite.
Is this normal?

Arthur wrote in message <002901bdb03d$955c7500$784e08c3 AT arthur>...
>>I have a sprite that moves horizontally across the screen controlled by
the
>>keyboard.
>>
>>Am using Allegro, with a 640x480 screen, and page flipping.
>>The problem is that when the ship moves, I can see it 'flickering' at the
>>edges.
>
>
>You are using two "screens" do draw sprites. It may seem a stupid question,
>but are you drawing and erasing the same sprites from the same screen each
>loop. It happens.

Think I am doing it the right way, I posted a bit of my source in my
original message. I just copied the page flipping idea from one of the
allegro examples.

>Also, are you checking for a vertical blank before you
>swap the screens?


I just added this to the source but it has not made any difference.

>
>The correct order (although this can vary depending on what graphics mode
>you're in or what graphics card you have) is:
>
>1) erase old sprites on hidden screen.
>2) draw new sprites on hidden screen.
>3) wail for vertical retrace.
>4) swap screens.


Yes this is what I am doing.

>
>HTH
>
>James Arthur
>jaa AT arfa DOT clara DOT net
>
>
>

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