Hi David
I am not an expert on the matter. First of all we need to clarify whether YafaRay is currently using LGPL 2 or
LGPL 2+ license which corresponds to the "any later version" clause. I have been looking into source files and I don't have a clear cut answer about it. This is an important distinction because if we using the
LGPL 2+ license then migrating to LGPL 3 might not be necessary, since LGPL 2+ and LGPL 3 are compatible licenses. We can keep the LGPL 2+ in our source code but release binaries under LGPL 3 when adding libraries released under Apache 2.0. This is basically what the Blender Foundation is doing with Blender (GPL 2+) & Cycles (Apache 2.0)= GPL3 releases.
About LGPL 3 you are right that, given
FSF policy on this matter, a LGPL3 library may be linked to a proprietary applications as long as the render API, if used, is LGPL 3 too. Doubts and misunderstanding comes from the fact that the Apache Foundation
considers GPL3 (and LGPL3 for that matter) incompatible with Apache 2.0 because of the the dynamic linking question. This means that, at least for some important people, dynamic linking to a LGPL 3 library is considered hereditary to the whole "combined work".
Besides, with LGPL 3 there are very clear requirements about notices that did not exist in LGPL 2.1:
"You must give prominent notice with each copy of the work that the Library is used in it and that the Library and its use are covered by this License. You must supply a copy of this License. If the work during execution displays copyright notices, you must include the copyright notice for the Library among them, as well as a reference directing the user to the copy of this License"
I welcome this addition in some cases (pcon planner), but are commercial host applications that already collaborate with us worried about giving us publicity in installation or execution time if we release under LGPL3?