Creating a perfect circle in hi order .gdf files


I had a question regarding the output of .gdf files. An example I think would explain my problem better that anything else so here goes.

  1. I create a simple surface which includes a circular cross section.

  2. I then export this to a WAMIT style hi order .gdf file and on opening this up I get a geometry where the circular cross section has now morphed into a square with rounded edges. This discrepancy is seen when using WAMIT as the volume of the structure is larger than expected from the wanted circular cross section.

  3. How in this case can I represent the surface as a proper circle in the .gdf file?

Thanks in advance and let me know if I should clarify anything.

Best regards

Hi emrys- what is the size of the object, and what is the Rhino file’s absolute tolerance (DocumentProperties > Units page)?



Hi Pascal

Absolute tolerance is 0.001 (model units: Mils)

Object is a cylinder with diameter 0.2m and length 1.44m but I’ve seen the same thing with a model that is 25m in diameter and 200m long (not sure what the tolerance was in that case though).

My guess then is that the rational arcs/circles/cylinders are not coming across as rational - that is the corner control points are weighted at 1 rather than .7071 as they are in Rhino. Is there any way to check that? A workaround, if the target application does not support rational curves, may be to rebuild the cylinders or the input curves (circles) that made them. A rebuild of a circle to degree 5 and 24 points comes remarkably close to a true circle.


Great thanks Pascal, I’ll use the work around for now as it does the job