[pstricks] pst-3dplot

[camus.philippe at free.fr]

Selon Christoph Bersch <usenet at bersch.net>:

> camus.philippe at free.fr schrieb:
> >
> > \pspicture(-5,-5)(5,5)
> > \psset{Alpha=150,Beta=40}
> > \pstThreeDCoor[xMin=0,xMax=4,yMin=0,yMax=4,zMin=0,zMax=4]
> > \def\al{60}\def\be{130} %just because I can't use \Alpha and \Beta !
> > \FPeval{a}{cos(\be)*sin(\al)}
> > \FPeval{b}{cos(\be)*cos(\al)}
> > \FPeval{c}{sin(\be)}
> >
> > \pstThreeDLine(\a,\b,\c)(0,0,0)
> >
> > \endpspicture
> >
> > I expect that the 3DLine is reduced to a point (the origin).
>
> I don't known why you expect this. \a \b and \c each contain a number
> (which are all != 0 for your angles), so you plot a line from the point
> specified by theses coordinates to the origin.
>
> Christoph

Because the transformation frome 3D to 2D is made through the matrix :
     (-cos(al)         sin(al)          0      )
   M=(                                         )
     (-sin(al)sin(be)  -cos(al)sin(be)  cos(be))
and (if my computations are OK),
     (\a)  (0)
   M* (\b)= ( ).
     (\c)  (0)

(this is what I mean by "kernel" of the matrix.
More generally, every projection is made following a direction, and I'm looking
for this direction...

Philippe