Path: utzoo!utgpu!watmath!clyde!att!ucbvax!hplabs!hpda!hpwala!boba
From: boba@hpwala.wal.hp.com (Bob Alexander)
Newsgroups: comp.binaries.ibm.pc.d
Subject: Re: Making balls move with TC 2.0 graphics, a tough problem HELP
Message-ID: <781@hpwala.wal.hp.com>
Date: 11 Jan 89 13:35:38 GMT
References: <4343@freja.diku.dk>
Reply-To: boba@hpwala.wal.hp.com (Bob Alexander)
Organization: Hewlett Packard, Waltham, Mass
Lines: 28
UUCP-Path: hpwala!boba

The EGA, VGA and Hercules graphics boards support 2 graphics pages.  You
can swap between these using the setactivepage and setvisualpage functions.
Graphics functions always write to the active page.  The user sees only
the visual page.

So here's what to do: set the active page to 0 and the visual page to 1.
Draw the ball on the active page.  Switch the visual page to 0 and active
to 1.  Now you can see the ball on the screen.  Draw the ball's new position
on the active page and swap again.  Now, you have to erase the ball from
its old position (since you're swapping pages, you have to remember where
the ball is *on each page* to erase it.)  I recommend that you use putimage
with COPY_PUT.  At initialization time, draw a ball on the screen, then
save it with a getimage.  Also, save an area of blank screen.  Erase the
ball by doing a putimage of the blank screen area.

I'm currently writing a game, too, and I've found that this animation
technique works very well.  You won't see any flicker.  Alas, it's still
too slow for my taste, but it will be fast enough for a breakout game.

Have fun,


  Bob Alexander      | *Controversial remark deleted* 
  boba@hpwala.hp.com |
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