| Re: Rendering and Code like Points2polys [message #21826] |
Tue, 26 September 2000 22:10  |
Mark Hadfield
Messages: 783
Registered: May 1995
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Senior Member |
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"Larry Busse" <ljb@ljbdev.com> wrote in message
news:39CB7E28.596301A1@ljbdev.com...
> I've been given a list of xyz points on the surface of an object and I'd
> like to be able to use the IDLgrPolygon object to view them. This
> requires trigulation....generating lists of vertices (the original
> points) and a list of faces (list of indices that define each polygon,
> or triangle, on the surfaces.) I found a WindowsNT program by Parasoft
> called Points2Polys that will do this but it would certainly be more
> convenient if I could do it directly within IDL.
I am very much a novice in the area of triangulation, computational geometry
etc, but I'll contribute my $0.02 and see who contradicts me.
Do you mean that the xyz points *define* the surface of the object and you
want to visualise the object? In that case what you want is a 3-dimensional
triangulation of your points. (If that is not what you want then the rest of
this post is off the topic.)
The IDL triangulation procedure, TRIANGULATE,
won't do it because it only does planar or spherical triangulations (the
latter referring to locations on the surface of a sphere).
MESH_OBJ generates 3-D triangulated data sets, but I don't think it will do
what you want. In one of its modes of operation it triangulates irregular
data, but this is just a planar triangulation of the x & y components of the
data. Its other modes of operation generate various 3D objects of simplified
geometry (extrusions, solids of revolution)
SHADE_VOLUME generates 3-D meshes, but it fits iso-surfaces to 3D gridded
datasets, which is not what you have.
So AFAIK IDL will not do what you want. Needless to say, it would be
possible to write a routine for 3-D triangulation and if you do I'd love to
see it!
I hate to say this but there is a very good spatial & geometric analysis
toolbox for Matlab:
http://puddle.mit.edu/~glenn/kirill/saga.html
It has a multi-dimensional triangulation routine. You might want to take a
look at its FAQ for an introduction to spatial & geometric analysis
concepts.
---
Mark Hadfield
m.hadfield@niwa.cri.nz http://katipo.niwa.cri.nz/~hadfield/
National Institute for Water and Atmospheric Research
PO Box 14-901, Wellington, New Zealand
Hi! I'm a .signature virus! copy me into your .signature file to help me
spread!
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| Re: Rendering and Code like Points2polys [message #21828 is a reply to message #21826] |
Tue, 26 September 2000 00:00  |
Sylvain Carette
Messages: 19
Registered: May 2000
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Junior Member |
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<!doctype html public "-//w3c//dtd html 4.0 transitional//en">
<html>
Hi Larry,
<p>This can answer you need as it continue my thread about "spherical gridding"
<br>Here I got finally something - it seem we try to do about the same;
building a "tin" surface over a sphere.
<br>Special thanks to Ben who point me to some good cues in private email
about the "orb" object and the CV_COORD procedure. I ran also almost by
accident on the "mesh_obj.pro" in the lib directory which use triangulate
procedure in conjunction whith cv_coord to build spherical surface among
other types.
<p>Now in the proccess of building my own "sphere_surf.pro" which I just
have tested the basic inner working.
<br>I will be more or less based on mesh_obj interface:
<br>sphere_surf, type, vertex_list,[ /full], [/degrees], [renderStyle=value],
[shading=value], [color=value], [etc... ]
<br>in which: type 0= spherically triangulated irregular network, 1= spherically
gridded mesh
<br>vertex_list is the input xyz vertices for type =1 and a 2 dim array
for type 1
<br>/full is to build the mesh or tin over a full sphere
<br>/degrees is to input the x, y as degrees
<br>for render_style, shading, color, etc, these are the parameters of
the inner IDLgrPolygon used to construct the result - and here I'm not
sure if I should implement as a procedure, a function or derivate a new
object - suggestion welcome.
<p>Anyhow, for the moment I badly only need just the tin over the incomplete
sphere so I'll keep the rest for later.
<br>To complete the sphere (there is still a gap in this form as I expected),
I suppose it will be a matter of using the "B" optionnal variable in triangulate
which return a list of the indices of the boundary point in ccw order and
to triangulate those again and add the resulting connectivity to the preceeding
before feeding IDLgrPolygon.
<br>I also tested in double precision; it is OK. (although, trying to view
in xobjview generate zclipping plane error which I dont mind at all as
long the object is ok).
<p>So here the sample code which build the tin:
<br> -------------------------------8<-------------------- --
<br><tt>; Create array to hold vertices</tt>
<br><tt>vertexlist = dblarr(3, 200)</tt>
<br><tt>; Create some random longitude points:</tt>
<br><tt>vertexlist[0, *] = RANDOMU(seed, 200) * 360. - 180.</tt>
<br><tt>; Create some random latitude points:</tt>
<br><tt>vertexlist[1, *] = RANDOMU(seed, 200) * 180. - 90.</tt>
<br><tt>; Set z to uniform value</tt>
<br><tt>vertexlist[2, *] = 500</tt><tt></tt>
<p><tt>; Triangulate a spherical surface</tt>
<br><tt>Triangulate, vertexlist[0, *], vertexlist[1, *], tri, /degrees</tt>
<br><tt>polylist = [Replicate(3L, 1L, (N_Elements(tri) / 3L)), Temporary(tri)]</tt>
<br><tt>polylist = polylist[*]</tt><tt></tt>
<p><tt>vertex = CV_COORD(From_Sphere=vertexlist, /To_Rect, /degrees)</tt><tt></tt>
<p><tt>; Create model.</tt>
<br><tt>oGroup = OBJ_NEW('IDLgrModel')</tt><tt></tt>
<p><tt>; Create the surface.</tt>
<br><tt>oTin = OBJ_NEW('IDLgrPolygon', vertex, polygon=PolyList, STYLE=2,
$</tt>
<br><tt>
SHADING=1, COLOR=[0,20,70])</tt>
<br><tt>oGroup->Add, oTin</tt><tt></tt>
<p><tt>oLightGroup = OBJ_NEW('IDLgrModel')</tt>
<br><tt>; Create some lights.</tt>
<br><tt>oLight = OBJ_NEW('IDLgrLight', LOCATION=[2,2,2], TYPE=1)</tt>
<br><tt>oLightGroup->Add, oLight</tt>
<br><tt>oLight = OBJ_NEW('IDLgrLight', TYPE=0, INTENSITY=0.5)</tt>
<br><tt>oLightGroup->Add, oLight</tt><tt></tt>
<p><tt>xobjview, oGroup, stationary=oLightGroup</tt><tt></tt>
<p><tt>end</tt>
<br> -----------------------------------------8<---------- -----
<br>Just run as is. If you have other ideas about how the final object
should work, I'll be glad to hear about it.
<br>My purpose is to have a way to construct and project and merge all
kind of dem from lat/long gridded to xyz irregular surveyed points onto
the WGS84 geoid at any kind of resolution from crude to exact. Plus the
ability to map satellite imagery over the resulting tile or "patch" and
calculating exact texture coordinate on each part. Later, I'll like also
to implement "constrained" delanoy triangulation to include VMAP and higher
resolution vectors to constrain the tin along rivers, roads, etc
<p>Hope it help
<p>Sylvain Carette
<br>VRML designer-composer
<p>Larry Busse wrote:
<blockquote TYPE=CITE>Hello,
<p>I've been given a list of xyz points on the surface of an object and
I'd
<br>like to be able to use the IDLgrPolygon object to view them.
This
<br>requires trigulation....generating lists of vertices (the original
<br>points) and a list of faces (list of indices that define each polygon,
<br>or triangle, on the surfaces.) I found a WindowsNT program by
Parasoft
<br>called Points2Polys that will do this but it would certainly be more
<br>convenient if I could do it directly within IDL.
<p>Has anyone written such code thqat they would be willing to share? or
<br>could provide pointers??
<p>David Fanning's article "Gridding XYZ Triples to form a Surface Plot"
is
<br>a step in the right direction but I was thinking the polygon approach
<br>would be better for closed surfaces and for datasets with a large number
<br>of points.
<p>Any thoughts on the topic would be appreciated.</blockquote>
</html>
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| Re: Rendering and Code like Points2polys [message #21911 is a reply to message #21826] |
Wed, 27 September 2000 00:00 |
Sylvain Carette
Messages: 19
Registered: May 2000
|
Junior Member |
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Mark Hadfield wrote:
> "Larry Busse" <ljb@ljbdev.com> wrote in message
> news:39CB7E28.596301A1@ljbdev.com...
>> I've been given a list of xyz points on the surface of an object and I'd
>> like to be able to use the IDLgrPolygon object to view them. This
>> requires trigulation....generating lists of vertices (the original
>> points) and a list of faces (list of indices that define each polygon,
>> or triangle, on the surfaces.) I found a WindowsNT program by Parasoft
>> called Points2Polys that will do this but it would certainly be more
>> convenient if I could do it directly within IDL.
>
> I am very much a novice in the area of triangulation, computational geometry
> etc, but I'll contribute my $0.02 and see who contradicts me.
>
> Do you mean that the xyz points *define* the surface of the object and you
> want to visualise the object? In that case what you want is a 3-dimensional
> triangulation of your points. (If that is not what you want then the rest of
> this post is off the topic.)
>
> The IDL triangulation procedure, TRIANGULATE,
> won't do it because it only does planar or spherical triangulations (the
> latter referring to locations on the surface of a sphere).
>
> MESH_OBJ generates 3-D triangulated data sets, but I don't think it will do
> what you want. In one of its modes of operation it triangulates irregular
> data, but this is just a planar triangulation of the x & y components of the
> data. Its other modes of operation generate various 3D objects of simplified
> geometry (extrusions, solids of revolution)
But if you look in MESH_OBJ, you'll see that for spherical gridding, it use
triangulate and trigrid to generate a planar grid. Then it convert all the
vertices with CV_COORD from spherical space (x, y, radius) to cartesian (x, y,
z). So to get only a triangulation (without gridding) on a sphere, I used
triangulate alone to get the connectivity and convert the vertices as in
MESH_OBJ (look my reply on this thread). There is still a gap that must be
addressed - begin to work on it.
I still suspect there is a way to use the "sphere=s" variable in triangulate
since it provide vertices and connectivity, but the ordering of the vertices is
differents (xxxx, yyyy, zzzz instead of xyz, xyz, xyz, xyz - why they have done
this?!?!) and also the function "reorganize" the vertices order so probably the
connectivity too... Anyhow, up to then, I could only get a kind of shrinkwrapped
shrinkwrap blob from it... :-( I'll have to investigate how to mess with the
connectivity to get the right result.
Now if you know of a function (in any source code) that will perform delaunay
triangulation from gridded data (to remove extraneous vertices keeping vertices
where it is needed -up to 80% polygon reduction can be acheived without any
noticable artefact), I'll be glad to hear about it
Sylvain Carette
VRML designer-composer
>
>
> SHADE_VOLUME generates 3-D meshes, but it fits iso-surfaces to 3D gridded
> datasets, which is not what you have.
>
> So AFAIK IDL will not do what you want. Needless to say, it would be
> possible to write a routine for 3-D triangulation and if you do I'd love to
> see it!
>
> I hate to say this but there is a very good spatial & geometric analysis
> toolbox for Matlab:
>
> http://puddle.mit.edu/~glenn/kirill/saga.html
>
> It has a multi-dimensional triangulation routine. You might want to take a
> look at its FAQ for an introduction to spatial & geometric analysis
> concepts.
>
> ---
> Mark Hadfield
> m.hadfield@niwa.cri.nz http://katipo.niwa.cri.nz/~hadfield/
> National Institute for Water and Atmospheric Research
> PO Box 14-901, Wellington, New Zealand
>
> Hi! I'm a .signature virus! copy me into your .signature file to help me
> spread!
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