Which way is up?

[Mike McDonnell <mike@etl.arpa>]

-*- Mode: Text -*-

This is about the coordinate system used on Symbolics computers
and why I don't like it.  It is also a request for solutions to
my problem.  

The main problem is that I am doing graphics programming on a
Symbolics and the coordinate system of all windows starts in the
upper left corner [let's forget about dynamic windows for the
sake of discussion] with Y being positive downward; call this
Y-down. The graphics texts I am using [Foley and Van Dam, Rogers,
Newman and Sproull] all assume coordinates that start in the
lower left corner with Y being positive upward; call this Y-up.
Besides violating my intuition, developed in years of schooling,
that coordinate systems should be Y-up by default, this makes it
more difficult to use the standard texts on graphics and
particularly causes me trouble when I have to figure out some
trigonometric thing such as vector operations.  Looking around in
the lispm, I can't find any global way of changing the
coordinates to what I want.

So I have a couple of questions:

1.  Why is this "antique" form of coordinate system so persistent
in computers when major texts no longer use it and we were all
trained to think in Y-up coordinates?  [I notice that even the
"new" graphics system X Windows uses Y-down coordinates.]

2.  What is the best way of transforming window coordinates to
Y-up while making as few changes to the Symbolics software as
possible?

Regarding question 2, I am familiar with what Lynn Quam did in
Image-Calc (tm) as of about two years ago in which he wrapped
each graphics method in a whopper that transforms coordinates.
This works but seems crude to me, sort of like defining my own
graphics methods such as my-draw-line, my-draw-circle, etc.  I
was thinking of something like a homogeneous coordinate
transformation, or at least conversion from Y-down to Y-up, that
can be done at the screen level so that drawing methods will not
have to be altered but rather the coordinate system itself is
altered.  Comments?