From:	"Arthur" <arfa AT clara DOT net>
To:	<djgpp AT delorie DOT com>
Subject:	RE: Dirty Rectangles
Date:	Sun, 11 Oct 1998 12:24:44 +0100
Message-ID:	<000101bdf509$becc1b00$f24e08c3@arthur>
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Reply-To:	djgpp AT delorie DOT com

> In the allegro demo game, one of the displaying method is dirty
> rectangles.
> I have read the code quite a few times, but since I am a beginner I still
> don't understand it completely, can someone explain it to me how it works.

No offence to Shawn Hargreaves (who wrote the game), but the code has pretty
poor style. For one it has no comments; and secondly it has no structure.

Dirty rectangles involves working out which bits of the previous frame are
going to change, and recording them in a list. Then you draw the stuff on to
the screen. After that screen has been displayed, you copy the data which is
in the list back to the screen. This has the advantage of not having to
completly redraw the screen from scratch, and hence saves time in most
cases. If you have a lot of elements in your list (ie, have a lot of moving
stuff on screen), however, you will find that the frame rate will drop below
the equivalent double-buffering method (without vsync).

You will notice that the framerate is much higher on dirty rectangles mode,
than any other mode. This is because there is no vsync. You will also notice
that shearing does not become apparant. This is because most of the screen
remains exactly the same colour (black). As soon as you add a moving
background to the game, you will find that shearing does occur - you just
didn't notice it before. Using dirty rectangles does not mean that you can
avoid using a vsync in all cases.

James Arthur
jaa AT arfa DOT clara DOT net
ICQ#15054819

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