[Edu-sig] On the front page

[Kirby Urner]

>One specific place I know I saw the term used in the Alice 
>dissertation is in connection with their use of 'up, 'right', 
>'forward' instead of X,Y,Z axis references. Which again is 
>fine to me if you're building a VR system.

I'm not well-versed in Alice, but it's true that in computer
graphics in general, you have your world coordinates, and 
then the local coordinates of the various objects.  

Words like 'up','right','left' are relative to an 
observer/object/camera/viewpoint, not "the world in general", 
which is why we have those confusions about "your left 
is my right", and "exit, stage right" means "from the 
_director's_ point of view" (yes?).  

It's not altogether pointless to want to have these 
"relative-to-the-observer" commands in the picture, given 
they map to semantics we encounter a lot.  Given we live 
on a sphere, "up" is likewise relative, and is more 
accurately "out", "down" more accurately "in" (out/in 
from/towards the planet's center).

Indeed, left versus right is a confusing area that needs 
clarification.  A "flatland" text book will invariably 
show the "positive X axis" pointing off to the right, 
always neglecting the viewpoint of someone "behind the
page" (looking out), who would necessarily see the positives 
going to the left (so why not equal time? why all the 
asymmetry?).

A lot of kids start stumbling in math as soon as you 
introduce "negative numbers", because suddenly the symmetry 
seems broken:  numbers on the one side of zero behave 
differently from those on the other side, such that 
root(5) produces a point on the positive line, but 
root(-5) does not produce a mirror image point on the 
other side.  Why should "left" versus "right" make such
a big difference, they wonder, especially in light of
the fact that "your left is my right and vice versa".

Kirby