[apetrov]

I don’t mean to discount any other work. I have already disclaimed that I don’t work in academia and rely on second-hand feedback from my classmates (in Europe)—for example, the Fortran implementation of Yoshida’s method from N years ago that nobody could modify, or the pressure for publication. Building (or learning) a new rendering engine would be a losing strategy in an academic career, as it is a much more difficult path to getting published. There are far fewer postdoc positions than PhD positions, and rendering skills won’t help in this competition.

Regarding the work of Ralf Kaehler: I have seen his renderings and looked through his articles, but to the best of my knowledge, no source code is publicly available. I don’t consider it fair to count it as something actively used in the field, beyond his lab and affiliated projects.

Disclaimer: that doesn't mean that there are no others, but their availability to researchers is limited to be widely spread.