Newsgroups:	comp.os.msdos.djgpp
From:	tob AT world DOT std DOT com
Subject:	Re: Graphics not drawn properly problem. (Allegro).
Message-ID:	<EC19rF.Mo5@world.std.com>
Sender:	tob AT world DOT std DOT com (Tom Breton)
Reply-To:	tob AT world DOT std DOT com
Organization:	BREnterprises
References:	<TwvHYEAiCFqzEw0k AT talula DOT demon DOT co DOT uk>
Date:	Thu, 19 Jun 1997 17:26:03 GMT
Lines:	47
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

Shawn Hargreaves <Shawn AT talula DOT demon DOT co DOT uk> writes:
> The other possibility is of course that it's a problem with my S3
> driver, but I don't think that's very likely (I've tried it on several
> different S3 boards, and noone else has reported any trouble with it so
> I think it's one of the most stable native drivers).

Here's another data point: I get a similar problem with the Trident
card&driver. Sometimes part of a horizontal line that should be drawn in
one place gets drawn somewhere else, making two messes: Missing partial
line, unwanted partial line.

From David Welch's description, I can't tell whether or not it's the
same thing. Maybe he's just seeing the extra line, where the missing
line isn't obvious.

Patterns:
It usually occurs when a bitmap significantly smaller than the whole
screen is being drawn. It's generally in a bitmap, but I couldn't swear
that's the only place.
It usually occurs in GUI dialogs, but maybe that's just because I've
spent disproportionate time in those.
It does not occur in the same place in successive invocations of the
same program, or neccessarily at all.
It occurs in different programs in much the same way, understanding that
"much the same way" means largely random.
It never takes an entire scan line. It certainly doesn't start at the
beginning of a line, and continues to the end of a bitmap's line.

It sounds very much like a problem with banks getting confused, but I
can't see why it should be so unpredictable then, and why it should
occur in different programs. I doubt anything I'm doing should corrupt
your bank array in any of them, let alone so many.

I keep thinking it might be some subtle logic error in setting up the
banks so a line is entirely contained in one bank, but if that were the
case it should happen the same way every time.

> Does this problem happen in any of my examples (eg. demo.exe in 640x480
> double-buffered mode), or only with your program?

FWIW, I don't remember seeing that happen in your examples, but I didn't
spend a lot of time with them. Basically "Does this work? It does! Cool.
OK, I've seen it, now I'll delete it."


        Tom

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