[metapost] surfaces

[Renaud AUBIN]

The subject is interesting... The following link seems pertinent:
http://www.inria.fr/rrrt/rr-5667.html

For testing:
http://www.loria.fr/equipes/isa/qi/server/
http://www.loria.fr/equipes/isa/qi/server/in_examples.php

Cheers,

Renaud

lfinsto1 at gwdg.de a écrit :

>>hi metapost friends, i have a big question (and problem to me), how i can
>>draw t he intersection of surfaces in metapost code, for example i want
>>draw
>>the intersection between a parabolid and a cylinder. I reed somethig about
>>featpost and mp 3D but i dont get good results. Well a hope that you can
>>help me.
>>    
>>
>
>I'm interested in intersections and quadric surfaces, too.  I've worked on
>these subjects a bit, but I've only gotten up to the intersection of an
>ellipsoid and a plane
>(http://www.gnu.org/software/3dldf/ellpsoid.html#Intersection_Ellipsoid_Plane).
> I would like to continue working on this, but unfortunately, it's not
>possible at the moment.  I know there's an algebraic solution for finding
>the intersection figure of two quadric surfaces, but the "explanation"
>involved references to earlier results, which I hadn't read.  If I
>remember correctly, it's in an appendix of this book at around page 360:
>
>Brauner, Heinrich, _Lehrbuch der konstruktiven Geometrie_.
>Wien [u.a.] : Springer, 1986
>ISBN:  3-211-81833-2 - ISBN 0-387-81833-2
>
>There are various approaches to using MetaPost for 3D drawing.  One is to
>write a macro package in the MetaPost language.  I've chosen to write a
>package in C++ that writes MetaPost code (GNU 3DLDF --- 
>http://www.gnu.org/software/3dldf/LDF.html).  Another possibility might be
>to have MetaPost write code to be passed to a rendering package using
>OpenGL, Renderman, or some other standard.  Denis Roegel published an
>article about his work on this not too long ago.  According to my
>understanding of the matter, in renderers, the problem isn't really
>solved, it just "comes out in the wash".  That is, objects in the
>resulting image appear to intersect, but the points of intersection are
>not calculated.  Nor is the equation of the resulting figure calculated,
>if it happens to be a figure that has an equation.
>
>Yet another approach would be to define the quadric surfaces using NURBs
>surfaces.  Then the problem of finding the intersections is the same as
>finding those of any other two NURBs surfaces.  I presume this is done by
>iterating through all the combinations of the individual sections of the
>two surfaces, except where some kind of optimization is possible.  This
>seems like a brute-force approach to me, but it has the advantage of being
>able to handle different shapes.  The main reason I've worked on finding
>the intersections of geometric shapes using their equations is because I
>find the problem interesting in itself.
>
>I discovered that just finding points on an ellipse doesn't necessarily
>lead to accurately determining the center, axes, etc., even if you can
>find as many points as you want (using the Braikenridge-Maclaurin
>construction).  By the same token, it's not always possible to determine
>its equation (after transforming it so that it lies in standard position).
> I strongly suspect the same is true of the quadric surfaces.  The conic
>sections and quadric surfaces may be "easy" for good mathematicians, but
>I've had a hard time finding the information I wanted for purposes of
>programming.  I've found even less information on this subject in books on
>computer graphics.
>
>I hate to say this, but if you want fast results rather than a programming
>challenge, you might want to think about using some other package where
>this is already implemented.
>
>Laurence Finston
>
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