--- On Tue, 21/4/09, J.P. Delport <jpdelp...@csir.co.za> wrote:
> From: J.P. Delport <jpdelp...@csir.co.za>
> Subject: [osg-users] Quat * Vec3 proposals - request for comments/help
> To: "osg users" <osg-users@lists.openscenegraph.org>
> Date: Tuesday, 21 April, 2009, 1:36 PM
> Hi all,
>
> it's been known for a while [1] that there are
> inconsistencies with the way OSG handles Quat * Vec3. In
> short: Quat * Vec3 is written in code as a post-multiply,
> but the result of the operation is as if a pre-multiply was
> performed. The attached test app also shows the problem
> (more on it later).
>
> [1]
> http://thread.gmane.org/gmane.comp.graphics.openscenegraph.user/21003
> and
> http://thread.gmane.org/gmane.comp.graphics.openscenegraph.user/33099
>
> What doesn't work?
> There are many examples that can be constructed of where a
> mathematical expression usings quats and vectors would not
> provide the expected results. See also [1]. The easiest one
> I could come up with is this:
>
> ((q1 * q2) * v) != (q1 * (q2 * v))
>
> Why are there not more complaints?
Well I curse this all the time, but not for the same reason. For me it's q1 *
q2 that's annoying as I see it as the opposite of what I expect from the
mathematics of applying a quaternion to a vector but then therein lies the
problem.
Using operator* for matrix multiplications is fine as that mirrors precisely
what you are doing with the matrices mathematically. Using operator* for
quaternion multiplication is a loose analogy since you are actually doing
q*v*q' mathematically when applying q to v. If you have two rotations, q1
followed by q2 then you would get q2*q1*v*q1'*q2' mathematically and so I
expect the interpretation of operator* to allow me to write q2*q1*v in code.
Of course that's just my interpretation and what I'm effectively saying within
that interpretation is that it's quat * quat that is wrong but there's no way
in the world you can swap that now! :)
Alternatively you could drop osg::Vec3::operator*( osg::Quat ) altogether and
have osg::Vec3::apply( osg::Quat ) or osg::Vec3::rotate( osg::Quat ). I've used
math libraries that do it this way (and where vec::operator*( quat ) is just
that i.e. the mathematical multiplication rather than the full blown rotation.
I guess it depends on whether you value the brevity of operator* over the
underlying mathematics. If operator* is still going to be used for applying a
quaternion to a vector then I support the idea of making the multiplication
orders consistent between quat/quat and vec/quat as per your proposal.
Making it consistent with OSG's row-major matrix multiplication is probably
easier for coders who are not concerned with the details of the maths (and as I
noted above it's not a workable solution to swap it now anyway).
Whatever happens I second the comment of Tanguy Fautre who asked for it to be
documented clearly somewhere. :)
Anyway I hope you don't mind me chipping in. It certainly is interesting that
there hasn't been more discussion. Maybe people just do what works and leave
it. ;)
Paul Fotheringham.
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