Re: [JPP-Devel] [jts-devel] Re: Question on use cases of JTS trianguation API

[Michaël Michaud]

Hi Martin, Today, I compared the new JTS triangulation api with the triangulation plugin I wrote some years ago. In my tests, I just compared speed for triangulation of a random set of points (no constraint, no real data). Measures include initialization, triangulation, and feature(s) creation. Both libraries worked well. I did not check result correctness. Processing time was faster with my code (and ratio between two consecutive figures are always better). Of course, triangulation speed is only one thing and may not be the most important. Just want to show that there is still room for improvement.   In OpenJUMP, memory usage was better with JTS api than with my plugin (1560Mb vs 2033Mb). I suppose this is partly due to the number of features created (about 1M features in my case, only one feature in your case). On the other hand, as I told you  before, playing with a MultiPolygon composed of 1M triangles is not easy. In OJ, with a MultiPolygon of 1M triangles, the user interface do not respond anymore and I could not explode the MultiPolygon (if I had to implement a plugin, I would probably make it possible to explode it before it is displayed). I did not look at your implementation, but I tried to find its complexity in an empiric way. I found that your algo has about a n*sqrt(n) complexity, but this deduction may be due to measurement errors : MM : n*log(n)*2.4E-6 MD : n*sqrt(n)*0.1E-6 Even if it is not yet the fastest, I have no doubt JTS triangulation api will soon be a reference in the java world. Thank you to share your excellent work. Michaël Figures (times in sec for sets from 50 000 to 1 000 000 points)   Michaël Michaud a écrit : Hi Martin, You may already know the benchmark done by Erwan's team with some java implementations (http://conference.osgeo.org/index.php/foss4g/2008/paper/view/282/177) It eventually shows the triangulator I have written a few years ago (available on http://geo.michaelm.free.fr/OpenJUMP/resources/) is very fast (I have to add it is not 100% robust as it sometimes fails for large datasets  - more than 100k points) They also wrote a more recent paper about their new implementation for orbisgis : http://hal.archives-ouvertes.fr/docs/00/32/95/03/PDF/CDT-paper.pdf Now about your question : INPUT - should be able to input ponctual (sites) , lineal (breaklines), polygonal geometries (points and breaklines to be extracted) - collection of coordinates INPUT/OUTPUT - in my plugin, I decided that when the input was a single polygon, the output should be a triangulation containing only triangles inside the polygon (not all triangles inside the convex hull) (may not concern the low level api) OUTPUT - in my Triangulation class, I decided to return a simple Coordinate[] array containing n*3 objects (elements 0, 1 and 2 composing the first triangle...)  I'm not that sure, but I think this structure just contained references to input coordinates (memory-friendly) - i would prefer a collection of Linestrings or a collection of Polygons than a MultiLinestring or MultiPolygon, because I'm not sure a mutigeometry of one million elements is easy to deal with (spatial indexation inefficient for example). Moreover, individual geometries can give the oppurtunity to keep the tin structure with links between elements represented as attributes, or to compute height, slope, orientation on every individual triangle and keep them as attributes. I understand why a Tin structure is interesting (no duplicate), but not a multi-geometry. My 2 cents Michaël Martin Davis a écrit : To those that are interested in the upcoming JTS triangulation API, a question: What type of input and output structures would you find useful? Currently I'm developing the following: INPUT: - Geometry (from which the site/vertex coordinates are extracted) - Collection of Coordinates OUTPUT: - MultiLineString containing triangulation edges - GeometryCollection of Polygons containing triangles You can also work directly with the internal datastructures of the triangulation (Vertices, QuadEdges, etc), but this requires a higher level of understanding. Is there any other option I haven't though of?   _______________________________________________ jts-devel mailing list jts-de...@lists.jump-project.org http://lists.refractions.net/mailman/listinfo/jts-devel

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