Mathematical XYZ ? 2004-02-26 - By Hans Kylberg
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> So there has to be a Mathematical XYZ Rule of where
> xyz ? is up and Down.
> xyz ? is left to right.
> xyz ? is front back.
Then you first has to know what up/down/left/right/front/back is
in universe, not just to you.
What is up for you is down for for someone in South Africa and
perhaps back for me here in Sweden, sitting at my computer.
When describing something in a 3d computer model I think there
is no general agreement of xyz directions. As we usually can
view the model only in 2D (a paper or a computer screen) the
most usual orientation is with x to the right and y "up" if
the screen is verical, or "back" if it is horisontal like a
paper on a table. But now we usually regard the screen to
be horisontal like the paper, and that makes y to be back.
This leaves z to be up.
Looking at tS, there is a raster plane to have something fixed
to relate to. When in top view, you can regard it as showing
map longitudes and latitudes, and we are looking down onto it.
Then the x axes is east-west and y axes north-south. But to
me the x values should be higher to the east (right), and y
values higher to the north (up). That is the way most other
geometry is shown, CAD programs for exanmple.
So tS is backwards here, and that has made me so confused that
I never get the directions right. The z axes is towards me,
in top view, so that is ok.
But there is another thing about the z axis for an object i tS.
When using "Look At" the z-axis used as "forward". This implies
that it was originally meant for starting rockets, as a newly
created object like a cube has its y-axes pointing upwards from
the tS raster plane. For me the y-axes should have been used
for "Look At". But as tS has y the "wrong" way, there would have
been confusions about the direction of the nose of the looking
object, it is not usual with eyes in the back of the head, they
are mostly in the nose direction.
- Hans
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