NetNews Usenet Archive 1992 #27/NN_1992_27.iso/spool/rec/games/programm/4696

Internet Message Format | 1992-11-15 | 1.8 KB

Path: sparky!uunet!gumby!wupost!cs.utexas.edu!ut-emx!ccwf.cc.utexas.edu
From: [e-mail] (Baylor)
Newsgroups: rec.games.programmer
Subject: Re: Xmode Scrolling
Message-ID: <[e-mail]>
Date: 15 Nov 92 23:09:14 GMT
Sender: [e-mail]
Organization: The University of Texas at Austin, Austin TX
Lines: 28

>  What I am wondering is how games do scrolling (smooth) with a static
>piece any other place on the screen, such as on the left.
>
    Honestly, they do it completely different then the way you think.
    They don't scroll anything onto the screen. Virtual screens
are almost always to short for anything except demos and title screens
(at which point it looks very nice).
    They define an array of screen tiles for background (not
jill, alligators or such) and then do a putpic-type blit for each
visible screen, that is, they constantly redraw it!
    Ok, get yourself a scratch piece of video memory. Put down the
background (can be done with movsw), each tile hopefully like 16*16
or 32*32. Then put down things that go over the background, keeping in
mind their priority (if two things collide on the screen, like the
birds and alligators, which overlaps which? Depends on the order
they're put down, so keep that in mind). Then lastly draw the things that
masks everything else, like the words DEMO in keen or other words
on jill (or anything you can walk behind that masks you, like columns
and hidden tunnels).
    That's also how you can have teh score on top or on the side and
scroll in a window, like in duke pukem.
    Someday look at the intro to Ultima 6. Turn your machine down to
4mhz and use the pause button alot - you'll see that it doesn't scroll
smoothly and that you can catch it before it finishes a redraw, something
that doesn't happen by scrolling with the start address.
                            - baylor