[GRASS5] TINs as 3D vectors

Tue Apr 19 06:33:59 EDT 2005

Pieter Bos wrote:
> Hmm, indeed, it does not, which means it can't be distributed with
> GRASS. But should be ok to just use/distribute it as a add-on module
> separately, as this is a separate program (right?). Not an ideal
> solution indeed. Would have to email the author to find out. We're using
> it for research purposes, which is allowed.

Hopefully you convince him to release Triangle under some decent 
license. TIN is general task which should be included in standard GRASS 
package.

> Conversion from Raster to TIN is not directly done, however, Triangle
> handles a square grid of points just fine. I haven't tried r.to.vect,
> but it looks like it can convert raster data to a vectormap with a point
> for every pixel in the map, which is what is needed as input. It is
> possible to simplify the surface a bit by removing adjacent triangles in
> the same plane and replacing them by fewer triangles. This is currently
> being finished.

It would be nice to have it in one module (r.to.tin) to avoid large 
vector before simplification, it is simple to read points from raster.

> With very small modifications it's possible to triangulate contour lines
> as well.
> 
> Triangle works on quite large datasets, it should be able to handle a
> million points in less than 20 seconds on a pentium II, according to the
> Triangle website. However, GRASS will take quite a while building
> topology in this case and will create huge files. Haven't tried anything
> this big. We tried using some ascii raster data files and it worked
> fine, but these only have about 55000-70000 points with a data value.
> V.delaunay already has problems with this kind of amounts of data.
> 
> Data is stored in the same way as v.delaunay does it - GRASS format, an
> area for every triangle. We didn't want to limit the use of other
> modules with the data. It's far from ideal, however, real TIN support in
> GRASS would be much faster, but also cost a lot of time to implement.
> 
> The alpha-shapes algorithm can be used to create triangulations that do
> not form the convex hull of the set of points. It takes as input a
> delaunay triangulation and creates a subset of the triangulation that
> will look like the 'shape' of a set of points, using one parameter,
> which defines the detail of the shape. This parameter can be set
> automatically as well to include every point in at least one triangle,
> which gives quite ok results. It works very well if you're using data
> with a more or less regular sample density. This is completely written
> by us because no suitable other implementation was found. No problems
> with any license here. However, you would need some module to create a
> delaunay triangulation, v.delaunay works fine for small datasets...

Sounds good, do you have anything which can generate contours from TIN?

Radim

> Regards,
> 
> Pieter 
> 
> -----Original Message-----
> From: Radim Blazek [mailto:blazek at itc.it] 
> Sent: Tuesday, April 19, 2005 10:15 AM
> To: Pieter Bos
> Cc: GRASS Developers list
> Subject: Re: [GRASS5] TINs as 3D vectors
> 
> Triangle does not appear to have a clear license compatible with GPL:
> 
> "Please note that although Triangle is freely available, it is
> copyrighted by the author and may not be sold or included in commercial
> products without a license."
> 
> Am I wrong? If you want to include that to GRASS distribution, the
> library must have a license which gives at least the same rights as GPL.
> 
> 
> Do you have a module for conversion from raster to TIN?
> 
> How do you store the data, do you use your format? Did you try to use 
> GRASS format?
> 
> Radim
> 
> 
> Pieter Bos wrote:
> 
>>There is no possibility of creating 3D-vectors from v.delaunay
> 
> directly.
> 
>>We are currently implementing a system for vague regions in GRASS that
>>uses triangulations with 3D vectors, and have linked Triangle to
> 
> GRASS.
> 
>>Triangle is a much faster and memory efficient delaunay triangulation
>>with support for constraints. See
>>http://www-2.cs.cmu.edu/~quake/triangle.html for more information.
>>
>>As soon as everything is done we'll release a few useful GRASS
> 
> modules,
> 
>>including this better Delaunay triangulation and the 2D alpha-shape
>>algorithm, and perhaps some 2D visualization of TINs which might be
>>useful as a general module with some slight modifications. If you need
>>it now I can send you the module.
>>
>>Regards,
>>
>>Pieter Bos
>>
>>-----Original Message-----
>>From: grass5-admin at grass.itc.it [mailto:grass5-admin at grass.itc.it] On
>>Behalf Of Jachym Cepicky
>>Sent: Wednesday, April 13, 2005 2:10 PM
>>To: GRASS Developers list
>>Subject: [GRASS5] TINs as 3D vectors
>>
>>Hallo,
>>is there any possibilty, how to create 3D-vector of TIN (triangulated
>>irregular network) of the output from v.delauny? I have 3D points and
>>v.delauny makes only 2D-polygons from it.. 
>>
>>I wrote some script which solves this, but before I post it, I would
>>like to know, that I did not something useless.
>>
>>Thank you a lot
>>
>>Jachym
>>--
>>Jachym Cepicky
>>e-mail: jachym.cepicky at centrum.cz
>>URL: http://les-ejk.cz
>>GPG: http://www.fle.czu.cz/~jachym/gnupg_public_key/
>>
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> 
> 5