From: brlcad <brlcad@ma...>  20100608 19:43:25
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Harun Thanks for the details on your project! That's a pretty interesting comparison, but I believe you're just "getting lucky" with the small version giving a computed volume close to the actual volume. The default working units are millimeters and your smaller intersecting volume looks to be less than 1mm^3. I believe the default 0.25 lower limit is an absolute grid spacing limit (in mm) so it's just hitting that limit quickly (and if not that limit, it should run into absolute computation limits before too long). Given you want to control the timing, you should compare a 1m^3 with a 100m^3 model, and then specify a volume tolerance with the V option in addition to specifying the g grid sample tolerance. The gqa manual page has more details on those two options. The manual page also covers how to import a density table that can be used for calculating center of gravity, moments, etc. See the section on volume and weight calculations. It looks like the manual page is not yet updated with the flags for centroid and moments of inertia but they are just Ac and Am respectively. Cheers! Sean On Jun 07, 2010, at 04:20 PM, Harun Bayraktar <hhbayraktar@...> wrote: Hi, I have gathered some performance data for the calculation of the intersecting volume between an elliptical crosssection cylinder (tgc) and a hexahedron (arb8) using gqa in MGED. I created a 100x scaled version of the same model and again used gqa to calculate the volume. I used the following command to get the volume and time (time "gqa ..."): gqa Av g XX int.c where XX is the grid spacing I used except for the default case. I see that the default 0.25 lower limit for the grid size is not optimal for either the original or the 100x scaled version of the model. Also, I can get to within 0.3% error in 0.05s for the small model but for the 100x model requires more than 0.15s to get to the same error level. Here is the data in a table (hoping the formatting will stay as it is) Characteristic Size 1 100 V Actual 0.7853982 785398.2 Grid Size (% of charc. Size) Volume Time (microsec) % error Volume Time (microsec) % error Def. Grid Size (025 abs lower limit) 0.731724 23535 6.8% 787744 368938 0.3% 10.0% 0.76912 38972 2.1% 834063 25095 6.2% 5.0% 0.78306 51085 0.3% 811749 41145 34% 2.5% 0.78539 85981 0.0% 799036 67201 1.7% 1.0% 0.785166 187265 0.0% 790717 162074 0.7% 0.5% 0.785536 404860 0.0% 788228 298249 0.4% I also need to calculate the center of gravity for the combination but I can't seem to find out how to assign the density and then get the center of gravity/moments of inertia information. Any help with the volume calculation accuracy and speed as well as the density assignment and center of gravity calculations is much appreciated. Thanks, Harun  ThinkGeek and WIRED's GeekDad team up for the Ultimate GeekDad Father's Day Giveaway. ONE MASSIVE PRIZE to the lucky parental unit. See the prize list and enter to win: http://p.sf.net/sfu/thinkgeekpromo _______________________________________________ BRLCAD Users mailing list brlcadusers@... https://lists.sourceforge.net/lists/listinfo/brlcadusers 