Handling of whitespace in subtitles

[forthrin]

A line containing only spaces will terminate parsing of a single SRT subtitle unit, eg. if "_" is a space character ($20),

00:00:00,000 --> 00:00:01,000
Hello
_
World

... will render as ...

00:00:00,000 --> 00:00:01,000
Hello

... whereas an empty line with no whitespace ...

00:00:00,000 --> 00:00:01,000
Hello

World

... will render as ...

00:00:00,000 --> 00:00:01,000
Hello
World

1. Is this according to de facto parsing conventions?
2. What would go wrong if a space-only line was just skipped, like an empty line?
3. Which code line makes this decision? Couldn't find an immediate candidate:

https://git.ffmpeg.org/gitweb/ffmpeg.git/blob/:/libavformat/srtdec.c

[na-na-hi]

1. Is this according to de facto parsing conventions?

SRT subtitle format doesn't support multiple line breaks. A "blank line" means the start of a new subtitle. In fact, SubRip treats these two examples identically (stops after reading the blank/space-only line), because it strips whitespace first and then check for emptiness, so a line with only whitespaces is considered blank.

These examples don't have a defined behavior, and different players will render them differently. See also #14271.

[forthrin]

Thanks! Pointing out ...

1. The referenced linked does not define what constitutes a "blank line".
2. nbsp (which is universally considered white-space), does -not- constitute a "blank line" (by ffmpeg, at least). It can thus be used to create empty lines in the ultimate on-screen subtitles.