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explanatory comment for MidiRegion::set_start_beats_from_start_frames()

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Paul Davis 2016-11-02 15:20:54 +00:00
parent 4c749120b6
commit e800d595dc
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14
libs/ardour/midi_region.cc
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@ -223,6 +223,20 @@ MidiRegion::post_set (const PropertyChange& pc)
void
MidiRegion::set_start_beats_from_start_frames ()
{
/* _start gives an offset *in samples* into the source file. we want to
compute (and cache) the offset into the source file using musical
time.
_pulse gives the position of the region in musical time, which has
the same relationship to _position that _start_beats does to _start.
so, convert pulses into quarter notes, then compute the offset from
the result (region position in quarter notes) from the quarter note
position computed using position and start. The use of quarter notes
here seems optional, but we happen to have API to compute that in
TempoMap.
*/
_start_beats = (pulse() * 4.0) - _session.tempo_map().quarter_note_at_frame (_position - _start);
}