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From: gwyn@brl-smoke.ARPA (Doug Gwyn )
Newsgroups: net.graphics
Subject: Re: Y positive up or down?
Message-ID: <5139@brl-smoke.ARPA>
Date: Mon, 3-Nov-86 12:01:05 EST
Article-I.D.: brl-smok.5139
Posted: Mon Nov  3 12:01:05 1986
Date-Received: Tue, 4-Nov-86 05:26:09 EST
References: <478@vaxb.calgary.UUCP>
Reply-To: gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 29

In article <478@vaxb.calgary.UUCP> bonham@calgary.UUCP writes:
>However, there must be *some* reason Sun, and many other window
>system developers, chose the upper left with Y increasing downward.
>Perhaps it is because their systems were intended primarily for ease
>in programming text  displays.  Most typesetting / page description
>languages use the inverted, Y-positive-down coordinate system.

So far as adopting a convention goes, it really doesn't matter,
so you might as well provide programmers etc. with one that
follows mathematical/engineering conventions (1st-quadrant).
Whether this is appropriate for an end-user depends a lot on
what the user's background is.

HOWEVER, the order that bits are stored in a file DOES matter;
there are observable consequences, such as the order in which
an image is "painted" on the display.  It should be obvious
why text-oriented systems store the bits near the top of the
image first, and consequently tend to adopt 4th-quadrant address
conventions.  I also think that it's aesthetically more pleasing
for graphic images to roll onto the display from top to bottom,
but not everyone agrees with this.

Image storage order and coordinate conventions have only a loose
connection and can be chosen independently if desired.

Henry stated that the Blit was 4th-quadrant; I believe the Blit
was originally 1st-quadrant, changed to 4th-quadrant for the DMD
(which is NOT a Blit!).  There is evidence for this in some of
the older DMD software.