Re: [Update REQ 5461]: ellipsoids

[doolin at cs.utk.edu]

In message <199506232053.AA04411 at cameron.geom.umn.edu>, "Stuart Levy" writes:
>Date: Fri, 23 Jun 1995 15:53:18 -0500
>From: slevy
>Message-Id: <199506232053.AA17673 at plucker.geom.umn.edu>
>To: software@geom
>Subject: Re:  [ REQ 5461]: ellipsoids
>
>> 1) Can I easily construct an ellipsoid with a major, intermediate, and minor
>> axis?  This is for illustrating 3 dimensional, 2nd order tensor quantities.
>
>Yes.  Probably the easiest way to do this is to apply a (non-uniform scaling)
>transformation to a sphere.  You can express transformations as 4x4 matrices:
>
>INST transform {
>	.5 0 0 0
>	0 .8 0 0
>	0 0 1.3 0
>	0 0 0 1
>} geom { < sphere.mesh }
> 
>Matrices are the transpose of what you'd probably expect, so a translation
>component (the sphere's center) appears in the bottom row, rather than
>the rightmost column.
>
>For spheres which are smoother though slower to draw you could replace the
>{ < sphere.mesh } 
>with
>{ SPHERE 1 0 0 0 }   # A unit sphere centered on the origin
>
>
>assign: slevy
>state: closed
>
>
>[5461     closed   slevy at geom.umn.edu 06/26/95 David Doolin <doolin at cs.utk.edu
>>]
>[51 lines of previous history; see software -v 5461.]

Thanks, 
I got part of the transform yesterday afternoon.  I understand 3x3 systems,
I will be off to the library for some exercises on 4x4.

Great software, good documentation in the makefiles.  I am recompiling
small pieces of it (pssnap).  I hope to cite TGC in my MS thesis,
there will be a refereed article come out of it.  I learned of TGC
from an article that a colleague of my advisor is supposed to review.
He doesn't get the math (topology and geometry), my advisor sort
of gets it, and I am learning it now.  I shouldn't say what
the paper is, but TGC is cited.  I will say that I am also
working in fractured rock masses!

Thanks again,
Dave D