Re: [GD-General] quaternion conversion

[Colin F. <cp...@ea...>]

By "flipping the sign of the x-coordinate" do you mean you
are doing the following:

    q = ( x, y, z, w )

  -->  ( n; theta ) = ( nx, ny, nz; theta )

  --> ( n'; theta ) = ( -nx, ny, nz; theta ) ?

Flipping the x-component normal vector (the "axis" part of
axis-angle) is not the same as flipping the x-coordinate of
points transformed by that quarternion.

One temporary hack experiment is to convert the quaternion
to a matrix, invert the sign of elements in the row that
yields the x-coordinate ( i.e., x = r31 * x + r32 * y + r33 * z
==> Invert to:  (-r31), (-r32), (-r33) ).

Basically, I think you have to postpone switching to a
left-handed coordinate system until you are finished doing
your calculations.  You cannot, for example, benefit from
taking the modified matrix described in the previous
paragraph and converting it back to a right-handed
quaternion; it's meaning would be perverse and useless.


OFF-TOPIC:

D3D had more than five years to switch to a right-handed
coordinate system.  Isn't there an incentive to do so, to
coincide with conventional right-handed cross-products,
quaternions, and all the mathematical results based on
such things?  Doesn't anyone complain?  If as many
textbooks on D3D point out "It is easy to switch to a
right-handed coordinate system by changing the transformation
matrix", why not simply change the default to a right-handed
system?


--- Colin


----- Original Message -----
From: "Chris Haarmeijer" <c.h...@ke...>
To: "gdgeneral" <gam...@li...>
Sent: Wednesday, July 02, 2003 2:16 AM
Subject: [GD-General] quaternion conversion


Hi,

I've got an incoming quaternion from a sensor. This sensor has a coordinate system that is right-handed. Our rendering system is
based on a left-handed system. I've tried converting the quaternion to Axis-Angle and then flipping the sign of the
x-coordinate, but that produces strange results.

Anyone knows an answer or hint to this problem?

Chris

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