Dan Connelly <djc### [at] flashnet> writes:

> I said "pseudovector".  A velocity vector, for example, is a real
> thing.  If I have a right-hand or left-hand coordinate system
> doesn't matter.  But if I take the cross product of two sides
> of a cube side in a fashion which yields a normal pointing
> outside the box in a LHS, when I switch to a RHS the same
> cross product will yield a normal pointing inside the box.

  Yes, sure.  The whole point is:

POV-Ray uses a right-handed coordinate system whatever your camera

, period.  I know this is going to make the whole crowd hurling mad and
becoming epileptic and throwing rotten tomatoes at me, but that's the way
it is.  The coordinate system has *nothing* to do with the way you display
it.  The cross-product of <x1, y1, z1> by <x2, y2, z2> is <y1*z2-z1*y2,
z1*x2-x1*z2, x1*y2-y1*x2>, whatever your camera is.  The confusing thing is
that the POV-Doc says that you can change the coordinate system handedness
by giving different combinations of up, right and direction vectors, which
is not true.  I repeat, POV-Ray uses a right-handed coordinate system
whatever your camera.  You can only change the way the camera looks at the
scene, in which you can choose to look at it as it is (that is, choose a
right-handed *camera*) of to flip it left-right (or upside-down, or
front-back, depending on the camera component you change into minus
itself).  This is a rendering-only characteristic and has nothing to do
with the real handedness of the coordinate system, which stays the real
right-handed system used by every mathematician in the world (or at least,
every mathematician I have heard of).

> In Physics, this is important when considering conserved
> quantities like parity.

  I won't deny it.  In fact, I keep saying it: we are in a right-handed
coordinate system, so the Physics theorems and laws are safe.  Now, you
just have to take care when you look at your scene...

  Okay, throw the tomatoes.  I won't read here till next Tuesday, so please
don't waste your ammunition till then, or till you're *really* sure I'm
wrong, in which case we will probably have an interesting discussion next
week >:-)

Roland.
-- 
Les francophones m'appellent Roland Mas,
English speakers call me Rowlannd' Mass,
Nihongode hanasu hitoha [Lolando Masu] to iimasu.
Choisissez ! Take your pick ! Erande kudasai !

Post a reply to this message