Re: [GD-General] quaternion conversion

[Chris H. <c.h...@ke...>]

Hi Colin,

Yes, I think so. And that explains the strange results. The flipping was
more or less done out of 'I did not know what to do' :)
The problem is that our rendering system and scenegraph is left-handed while
a sensor connected to my PC is outputting quaternions in right-handed
space.....
I'll try the hack with the matrices.

Thanks, Chris

----- Original Message ----- 
From: "Colin Fahey" <cp...@ea...>
To: <gam...@li...>
Sent: Wednesday, July 02, 2003 8:00 PM
Subject: Re: [GD-General] quaternion conversion


>
> 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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