R: [Plib-users] ssg camera woes / tranformation/coordsystem confusion

[Paolo L. <p.l...@ci...>]

> -----Messaggio originale-----
> Da: pli...@li...
[mailto:pli...@li...]=20
> Per conto di Eduardo Alberto Hern=E1ndez Mu=F1oz
> Inviato: domenica 12 febbraio 2006 21.49
> A: pli...@li...
> Oggetto: Re: [Plib-users] ssg camera woes / tranformation/coordsystem
confusion
>=20
>=20
> > Hi. I have some pre-SSG/own code, I'm trying to migrate to SSG.=20
> > However, my camera code screws up; it's as if the axes are switched, =
or
> > worse.
> >=20
> > For example, my code has a direction/rotation matrix, that indicates
> > the users' rotation around the y axis (it could be a rotation angle
instead,=20
> > but I'm exploiting that it's a matrix).
> >=20
> > However, when I multiply this matrix onto the ssg camera, it rotates
around
> > the _Z_ axis instead!
> >   Similar problems happen with other parts of the transform (which =
I've=20
> > commented out to dissect the problem).
>=20
>=20
> This is a quote from a page at Steve Baker's tuxkart site at
> http://tuxkart.sourceforge.net/trackdesign.html :
>=20
> "I come from a flight-simulation background where we use the =
convention
that Z-is-up - so all my
> software works like that. However, the PLIB loaders know that some
modellers use Y-is-up and do
>  the axis swap as needed. Hence, in AC3D, Y-is-up - but in the game, =
your
models will be converted
> to Z-is-up. If you find this confusing, forget I mentioned it - =
everything
comes out OK automagically."
>=20
> So I would beleive the Z axis is the Y axis are swapped in plib, so =
just
swapping these values when you
> load models and in your coordinate handling code would do the trick.
>=20
> -Coz

To be more precise, the matrix used for such SSG to OpenGL coordinate
adaption can be found in ssg.cxx:

sgMat4 _ssgOpenGLAxisSwapMatrix =3D
{
  {  1.0f,  0.0f,  0.0f,  0.0f },
  {  0.0f,  0.0f, -1.0f,  0.0f },
  {  0.0f,  1.0f,  0.0f,  0.0f },
  {  0.0f,  0.0f,  0.0f,  1.0f }
} ;

This actually describes a 90=B0 rotation around the x-axis, in such a =
way that
SSG is "z+ is up, y+ points towards the observer", OpenGL is "y+ is up, =
z+
points inside the screen" (I think it's the same handness).

Greetings -

Paolo Leoncini