Dear Karim,
The post here is already a few years old, but it concerns exactly the problem that is bothering me right now. By the way, I have desperately searched several forums for an answer, unfortunately in vain.
Would it be possible for you to post an example showing how polytempo works in LilyPond? Or how the text file generated by the om->lily function would have to be modified in order to display the overlay of different tempos in LilyPond?
I have been trying to understand how this could work for some time. Unfortunately, when I export an OM poly object with multiple voices, each with its own tempo, using om->lily, and then open it in LilyPond (or Frescobaldi), the overlaying of the different tempos does not work (in addition to displaying the more unusual microtonal accidentals). I thought until now that it must be enough to move the Metronome_mark_engraver from the Score context to the Staff context in LilyPond, but that doesn’t seem to be enough. This way, the metronome markings can be assigned to the individual staffs, but the horizontal display of the rhythms remains unaffected.
So I’m not concerned here with the representation of polymetrics - that’s easily achieved by moving the Timing_translator and the Default_bar_line_engraver from the Score context to the Staff context (as is the case with the files exported from OM). It’s about overlaying different tempos. Ultimately, this is about displaying more complicated OM poly objects in LilyPond.
I am extremely grateful for any tips and hints!
Best regards,
Adrian