deflection unit

blindlb's picture
blindlb
Fri, 04/03/2009 - 04:58
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I'm a mechanical engineering student in Canada and I'm working on a program that use OpenCascade and STEP files. When i'm building a new mesh, i use the deflection, but I don't know if it's a percent of something or in the unit of the STEP file... What is the exact "unit" of deflection (I know it represent the maximum distance between edge of the mesh and the corresponding surface but in terms of what!)

Thanks a lot

Rob Bachrach's picture
Rob Bachrach
Fri, 04/03/2009 - 14:28

Once you load the STEP file, it is OpenCascade (OCC) geometry. OCC geometry is always in millimeters, irregardless of the units used in the STEP file. Your deflection is in millimeters as well.

blindlb's picture
blindlb
Tue, 04/07/2009 - 18:14

I'm sorry to ask but are you sure of that?

Because I did meshes on different models with a deflection of 1. After that, I did an interpolation to built a grid of points in each triangle. Project every points to find Nearest points on surface, and then, look for the distance between these points... maximum distance value : 4 to 9 mm depend on the model!!

Is it possible that the deflection is a percentage of the dimensions of the model or the face?

Thanks

Rob Bachrach's picture
Rob Bachrach
Tue, 04/07/2009 - 18:22

I'm not positive, but I am relatively sure. We set our deflection as a percentage of the bounding box of our geometry. Some, therefore, have quite a large deflection, but we get a pretty good and controlable tesselation. However, I am intrigued by your results. I suggest you limit your analysis to the distance between the face edges and the corresponding tesselation of those edges. See if the deflection remains smaller there.

blindlb's picture
blindlb
Tue, 04/07/2009 - 18:43

Hi!

In the case of a deflection of 0.1, I obtain much better results (maximum of around 0.2 mm for the 2 models tested with some complex surfaces). But the triangles are smaller so it's probably that, in the case of the only edges are checked, smaller triangles are more accurated than bigger one and that's really commonsense!

Thanks for your help! (There's somewhere in the OCC documentation where all this stuff is specify? On the units things?)