Today I have made a commit that improves the IK algorithm of the blender importer. It is not yet perfect but at least the models I checked started to look more similar to those in blender than before.
I replaced the previously used ‘triangulation algorithm’ with more commonly used CCD (Cyclic Coordinate Descent).
NOTE: The computations will take more time than before but I think the result will be better.
The importer does not yet support some of the constraint’s options: pole target, rotation and position weights, influence, using tail and stretching. I hope to add some of them even this week (at least ‘use tail’ and ‘stretch’).
There still will be issues with this algorithm because of its nature: it can have many valid solutions. But I hope to improve it with time.
Great thanks to @wezrule for providing me with the initial algorithm