From: Alan W. Irwin <irwin@be...>  20091215 23:12:17

Hi Arjen: I think it is time to take our offlist conversation about a replacement for the code in plP_plfclp back to the list. For the others here, Arjen fixed all the fill issues shown by example 25, but neither Arjen nor I were positive that the code in plP_plfclp would work for an arbitrary polygon (even for the "simple" case where the polygon is guaranteed not to selfintersect). Even if that code can be proved to be correct by some code guru, it is a huge bunch of code which makes the code difficult to understand for mere mortals. Furthermore, I was not satisfied with the plgradient software fallback defined area approach so I wanted to try a different approach there by specifying the fill of the intersection of two arbitrary polygons which is a generalization of the fill of the intersection of a polygon + rectangle that is solved by plP_plfclp. For both the general case and the special case where one of the polygons is a rectangle, I felt the most reliable algorithm would be a recursive one where the boundary of polygon 1 is traversed looking for the first two intersections that have not been found before, and splitting polygon 2 into two parts with the boundary between the two parts specified by the two intersections and the polygon 1 boundary (if any) between those, and dealing recursively with the intersection of polygon 1 with those two parts. I have implemented a partial version of this algorithm as of revision 10720. The only way to test this new algorithm (rather than using the traditional plP_plfclp code) is to specify DUSE_FILL_INTERSECTION_POLYGON=ON. The valgrind applications gives the new code a clean bill of health (no memory management issues, no memory leaks) for example 25. Also, the code is quite straightforward so mere mortals can understand it. However, there are some limitations to the current implementation which generate a lot of PLplot warnings and bad looking example 25 plots. * Intersections between polygon boundaries that occur at vertices are not handled correctly yet. I still have to implement code to decide if a vertex intersection corresponds to an actual crossing of the two polygon boundaries or just a "touch" between them with no crossing. * Once the algorithm reduces polygon 2 to pieces that do not cross the polygon 1 boundary, then the tests have to be refined to deal properly with the case where polgyon 1 is inscribed inside the polygon 2 piece or vice versa. * The code to deal with selfintersecting polygons has not been implemented yet. However, example 25 does not test that case yet so you probably won't notice this deficiency. I am confident thes three issues should be straightforward to deal with, but until at least the first two of them are solved do not expect much from the DUSE_FILL_INTERSECTION_POLYGON=ON experience. What I am working on at the moment is another issue where I found pointinpolygon was giving an incorrect result. (The documentation of that routine specifically states it returns false if the test point is on the boundary, but I have a test case generated by the new code where the test point was actually a vertex of the polygon, and pointinpolygon incorrectly returned true.) More later. Alan __________________________ Alan W. Irwin Astronomical research affiliation with Department of Physics and Astronomy, University of Victoria (astrowww.phys.uvic.ca). Programming affiliations with the FreeEOS equationofstate implementation for stellar interiors (freeeos.sf.net); PLplot scientific plotting software package (plplot.org); the libLASi project (unifont.org/lasi); the Loads of Linux Links project (loll.sf.net); and the Linux Brochure Project (lbproject.sf.net). __________________________ Linuxpowered Science __________________________ 