URDF to SDF conversion ignores links with mass smaller than 1e-6

[traversaro]

Spin-off of #199 for visibility. Quoting from #199 (comment) :

Original comment by Kaelin (Bitbucket: Kaelin).

I happen to have run into a variant of this issue today and spent a while debugging it. In addition to the case of having not declared an inertia node in the XML at all, this also occurs if you have a mass value which is too small. A mass value of 0.00001 (1e-5) works fine, but if you decrease it to 0.000001 (1e-6), the link is silently omitted. This is pretty dangerous, since it is easy to gloss over an additional 0 when scanning for issues. I saw that having no mass was disallowed, so my natural inclination was to make it some very small number, and happened to hit too many zeroes.

An error in this case, even if massless links aren’t implemented, would be much appreciated.

[traversaro]

The issue is due to the following line:

• https://github.com/ignitionrobotics/sdformat/blob/sdformat12_12.4.0/src/parser_urdf.cc#L2644

it may not be immediatly clear why the cutoff value is 1e-6, but this come from the fact that the second argument of ignition::math::equal is not specified and by default it is 1e-6:

• https://github.com/ignitionrobotics/ign-math/blob/ignition-math6_6.10.0/include/ignition/math/Helpers.hh#L557

[azeey]

Fixed by #1238

[aaronchongth]

Fixed by #1238

More specifically, #1238 does not allow link mass with value of <= 0, without any epsilon values, https://github.com/gazebosim/sdformat/pull/1238/files#diff-2bc5ca23bcfc66fe173f513399bec8065b1cb4e607d741566517d7368e6bce0fR2670