In https://github.com/robotology/blender-robotics-utils/blob/master/script/urdfToBlender.py we have a URDF --> Blender rig converter that is working fine on some URDF model, but creating problems on some other models. However, debugging the problem is tricky as we do not have a clear mathematical definition and/or understanding of how the Blender Armature system ( https://docs.blender.org/manual/en/latest/animation/armatures/structure.html#chains-of-bones ) works. In this issue we will document this with mathematical notation, so it will simplify future mantainance of the software.

Unless noted otherwise, the notation used for expressing homegenous transforms is the one defined in https://research.tue.nl/en/publications/multibody-dynamics-notation-version-2, and as LaTeX formulas will be used in the issue, please install an appropriate plugin such as https://chrome.google.com/webstore/detail/mathjax-plugin-for-github/ioemnmodlmafdkllaclgeombjnmnbima?hl=en for Chrome or https://addons.mozilla.org/en-US/firefox/addon/green-pi/ for Firefox to visualize them in the browser.

cc @Nicogene