Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!watmath!clyde!rutgers!caip!think!husc6!cfa!mink
From: mink@cfa.harvard.edu (Doug Mink)
Newsgroups: net.graphics
Subject: Re: Y positive up or down?
Message-ID: <528@husc6.HARVARD.EDU>
Date: Sun, 26-Oct-86 15:39:57 EST
Article-I.D.: husc6.528
Posted: Sun Oct 26 15:39:57 1986
Date-Received: Mon, 27-Oct-86 01:46:06 EST
References: <469@vaxb.calgary.UUCP>
Sender: news@husc6.HARVARD.EDU
Distribution: net
Organization: Harvard University, Cambridge Mass
Lines: 24
Keywords: cartesian coordinates, pixel addressing

In article <469@vaxb.calgary.UUCP>, bonham@calgary.UUCP writes:
> and text-oriented displays in 2-D.  The subject of picking a coordinate
> system origin has come up.  Several designers propose putting pixel
> <0,0> at the top left of the screen, with Y increasing downward;
> others propose putting it at the bottom left, with Y increasing upward.
> 
> Both have advantages and disadvantages.  We think it a hard enough
> and interesting enough philosophical question to put it to the net:
> Should positive values represent upward or downward Y increments?

As an applications programmer, I find it much easier to conceptualize a display
in standard, bottom-left origin coordinates because that is what the equations
before the programs assume.  A display holds far more bytes of graphics than of
text, and all of those bytes have to be processed by the display driver.  If
your system is going to be used for interactive graphics a significant amount
of the time, any speed you gain will be important.

When I wrote the Tektronix emulator part of xterm, the X Windo System's
terminal emulator, I had to handle the translation from bottom-left (Tek) to
Top-left (X).  It's not difficult; it just adds one more operation to every
point plotted.
				Doug Mink
				cfa.harvard.edu
				{ihnp4|seismo}!harvard!cfa!mink