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From: andrea@hp-sdd.HP.COM (Andrea K. Frankel)
Newsgroups: net.graphics
Subject: Re: Y positive up or down?
Message-ID: <553@hp-sdd.HP.COM>
Date: Tue, 28-Oct-86 01:10:32 EST
Article-I.D.: hp-sdd.553
Posted: Tue Oct 28 01:10:32 1986
Date-Received: Tue, 28-Oct-86 06:55:59 EST
References: <469@vaxb.calgary.UUCP>
Reply-To: andrea@hp-sdd.UUCP (Andrea K. Frankel)
Distribution: net
Organization: Hewlett-Packard, San Diego Division
Lines: 30
Keywords: cartesian coordinates, pixel addressing

We debated this question in the context of graphics standards
(GKS, CGM, CGI), and came to the conclusion that it was best to
permit the client (i.e., the program or person using the system)
to SET the coordinate space, both to establish scaling and to
establish orientation.  In CGM and CGI this is done with the
"VDC EXTENT" function (VDC = Virtual Device Coordinates, the
space thus specified).

The default coordinate system for the graphics standards is always
first quadrant (i.e., (0,0) in lower left).

There are some nontrivial ramifications for changing; for example,
some circular arcs are specified in terms of a center point and
two rays, and the arc is drawn counter-clockwise from the first
to the second.  Now, what defines counter-clockwise?  The resolution
was "from the positive x-axis of VDC space to the positive y-axis
of VDC space".  Draw yourself some pictures, and you might be surprised
at what happens to arc specifications if the specifier assumes
counter-clockwise means "as I look at the screen" and the VDC space
is fourth-quadrant!


Andrea Frankel, Hewlett-Packard (San Diego Division) (619) 592-4664
 "every time that wheel goes round, bound to cover just a little more ground"
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