| Re: Surface mesh [message #59780 is a reply to message #59743] |
Thu, 10 April 2008 23:17   |
twhaw
Messages: 5
Registered: February 2008
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Junior Member |
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Dear Rick,
Thanks for your very insight reply. It helps a lot.
I have managed to construct the surface mesh now, though I still have
problem with the polygon's normals near the transition point. Is there
any way to make sure that polygon's vertices are always in the counter-
clockwise manner?
Cheers.
On Apr 11, 12:53 am, rtow...@gmail.com wrote:
> On Apr 9, 4:26 am, twhaw <wooihaw....@gmail.com> wrote:
>
>> Good day.
>
>> I have created a 3D model consisting of points sampled on the outline
>> (surface) of a 3D object. The 3D object is initially translated so
>> that the origin is at its center of mass. The points are then sampled
>> in the spherical coordinates and converted back to the rectangular
>> coordinated and stored as a (3, n) array. The outline of the model is
>> sampled at a fixed interval longitudinally (every10 deg).
>
> <snip>
>
>> Now, I would like to create the surface mesh of the 3D model from the
>> sample points. I have used MESH_OBJ (type 0) but the resulting mesh
>> does not correspond to the surface of the 3D model. I have also
>> triangulated the points first to get the polygons and passed them to
>> IDLgrPolygon, but could not get the desired surface mesh.
>
> This problem is similar to an earlier thread titled "how to compute
> the merged volume of two 3d objects" although you absolutely need to
> find the equilibrium surface since your object is concave. Unlike the
> poster in that thread, your task is much easier. Your vertices are
> ordered by the slice and you know that the verts in one slice will
> "connect" to the verts in the next. Easier still since the number of
> samples per slice is fixed. It gets even easier if your verts from
> each slice are sampled in the same order. Given this, you should be
> able to generate a polygon mesh comprised of quad strips that create
> bands connecting one slice to the next.
>
> Since the planes pass thru the center of mass of your object (and I
> would assume the axis of rotation for your cutting planes is fixed)
> each slice should share 2 common vertices. Given your example object,
> the only trick I can see is that these common verts will be transition
> points where you will have to reverse the order of the polygon winding
> since the "top" and "bottom" row of verts will flip-flop. This
> wouldn't affect the wire-frame view, but if you don't change the
> winding the normals will be pointed inward which would affect lighting/
> shading.
>
> You'll have a couple of for loops. The outer loops nSlices-1 (17)
> times. The inner will loop k times. In the inner you'll fill in your
> polygon array, starting at the common point on slice n, and connecting
> it's counterclockwise neighbor, then the neighbor's compliment on
> slice n+1, then the commons's compliment on n+1. Repeat for each
> point on that slice, keeping in mind that when you get to the
> transition point again you'll need to change the order that you
> connect the verts. The last slice is then meshed to the first after
> your nested for loops in the same manner. For more complicated
> objects this gets harder but I think this should work for your example
> object.
>
> You'll want to read the IDL docs on "About Polygon and Polyline
> Objects" and "Polygon Optimization" for more info on creating polygon
> arrays.
>
> Have fun.
>
> -Rick
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