Some arcs may have incorrect reference vector

[chriswu1997]

Some arcs may have incorrect reference vector

[ChrisWu1997]

Thanks for pointing this out! The parsing of arc did bother me a lot.

Your understanding of the arc definition is correct. I think the problem comes from parsing data from onshape. When extracting data from onshape, the way that I determine arc is to query the midpoint (see here).

I re-parse this example and printout the midpoint, which gives [-0.043971567347481526, -0.18829072000371472, 0.0]. This suggests it's on the large half of the circle, so it's regarded as a major arc in the final parsed data.

Therefore, I think either I incorrectly use the API to get the midpoint, or I get the wrong edge_id when parsing edge loop of a face. I need more time to review the code, but I'll let you know once I fixe this bug.

[JoeLambourne]

Thank you for looking. Calling evEdgeTangentLine looks like it should work. Maybe a bug in OnShape. Arcs are notoriously bad for bugs in CAD systems. Is there a way to get the start and end parameter value of the arc without using the arcLengthParameterization option. The bug might be in the OnShape arc length computation.

[ChrisWu1997]

After taking a deeper look, I think both the data parsing and arc interpretation are correct. The misunderstanding comes from the definition of sketch profile.

The sketch is composed of multiple profiles, each is a region enclosed by one or more edge loops. And we did not merge those profiles, even though they are extruded by the same extrusion operation (following the convention of onshape API). What you visualized above is just one profile JSa of the entity Sketch 5.

See below for illustration. If you search for the major arc JSF in the json file, you will find it appears in both profiles JSa and JSO.

[JoeLambourne]

@ChrisWu1997,

Thank you for the detailed explanation. This makes a lot of sense. Yes. he shape I'm expecting will be the union of all the profiles extruded by that operation. Let's close this as the solution is going to be to perform some post processing.