Supervision frame mask not handling negative end time

[ArthLeu]

lhotse/lhotse/cut/set.py

Lines 2908 to 2922 in c5f26af

	for ali in supervision.alignment[use_alignment_if_exists]:
	st = round(ali.start / frame_shift) if ali.start > 0 else 0
	et = (
	round(ali.end / frame_shift)
	if ali.end < cut.duration
	else num_frames
	)
	mask[st:et] = 1.0
	else:
	st = round(supervision.start / frame_shift) if supervision.start > 0 else 0
	et = (
	round(supervision.end / frame_shift)
	if supervision.end < cut.duration
	else num_frames
	)

CutSet.cut_into_windows() produces negative start times in returned Supervision and Alignment items. But when computing the mask using supervision_frame_masks(), the end time et (assumed obtained from start + duration) does not handle this case where alignment ends before cut start, e.g. et <= 0.

Please verify if this is the case and resolve if needed.

[pzelasko]

I see. I think your PR makes sense to help resolve this. You may also want to use keep_excessive_supervisions=False for such cases.

[ArthLeu]

I tried it. But it would remove an entire Supervision altogether, regardless of whether a portion of its Alignment items are within the Cut time range.

[desh2608]

I made some changes in alignment start times recently (#1193). Not sure if it is related, but may be worth checking if it resolved your issue?

[ArthLeu]

Want to understand this. With #1193, the expectation is to unify all the Supervision and Alignment timestamps to absolutes (w.r.t. Recording start), and no more relatives (w.r.t. Cut start)?

[desh2608]

@ArthLeu Alignments were always supposed to have absolute start times (w.r.t. the recording), but there was a bug in the implementation of some functions where they were being modified.