geom3d
======

projection from 3D on 2D after a 3D plot



Calling Sequence
~~~~~~~~~~~~~~~~


::

    [x,y]=geom3d(x1,y1,z1)




Arguments
~~~~~~~~~

:x1,y1,z1 real vectors of the same size (points in 3D).
: :x,y real vectors of the same size as `x1`, `y1` and `z1`.
:



Description
~~~~~~~~~~~

After having used a 3D plot function such as `plot3d`, `plot3d1` or
`param3d`, `geom3d` gives the mapping between a point in 3D space
`(x1(i),y1(i),z1(i))` and the corresponding point `(x(i),y(i))` in the
projected 2D plan. Then all the 2D graphics primitives working on
`(x,y)` can be used for superposition on the 3D plot.



Sample
~~~~~~



Examples
~~~~~~~~


::

    `deff`_("[z]=surface(x,y)","z=sin(x)*cos(y)")
    t=%pi*(-10:10)/10;
    // 3D plot of the surface
    `fplot3d`_(t,t,surface,35,45,"X@Y@Z")
    // now (t,t,sin(t).*cos(t)) is a curve on the surface
    // which can be drawn using geom3d and xpoly
    [x,y]=geom3d(%pi/2,0,surface(%pi/2,0))