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From: ekalenda@cup.portal.com (Edward John Kalenda)
Newsgroups: rec.games.programmer,comp.os.msdos.programmer
Subject: Re: A big map with scrolling.  How???
Message-ID: <33812@cup.portal.com>
Date: 12 Sep 90 06:04:08 GMT
References: <1990Sep10.183857.15821@watserv1.waterloo.edu>
Organization: The Portal System (TM)
Lines: 33

> 	I have some questions regarding a very large graphic or bitmap
> image.  I am programming a game which will have a large map surface
> displayed on the screen.  This map will be larger than the displayable
> screen size, so it will need to scroll.  Much like a spreadsheet.
> 
> ... verbage removed
> 
> 	As to scrolling, I think that the only acceptable method may
> be direct screen memory writes.  Unless someone can direct me
> otherwise.

Having just finished an animated educational game for a client, I'll say
a few things about this. The MCGA mode has only one page, cannot have it's
starting address changed, and is best dealt with by direct memory writes
(you don't have to worry about synching to the retrace).

Panning (it's not scrolling) will have to be a matter of copying the portion
of the screen which will remain visible to it's new location and filling
in the area which was "off screen". If you do "smooth panning" by moving
things a small number of pixels (<5 or so), it shouldn't look to bad.

Your biggest headache, after you get the code to scribble on the screen,
will be managing the pixel data which is off screen. A single screenful
is all you will be able to store per memory allocation since 64K is the
limit. Taking a peice from this block and a peice from that block will
need some very careful program design. THINK it through BEFORE you start
writing.

Sorry, but I can't give you any examples of how I wrote it since coding
for money generally means the finished product is their's, not mine.

Ed
ekalenda@cup.portal.com