Fix overlapping burnt-in subtitles in some cases (#959).

Firstly, when finding subtitles that exist during a period, only
return those which overlap more than half the period.  This means
that, in a fight over a frame, the longest-running subtitle in that
frame will win.

Secondly, make SubtitleDecoder::get pick the wanted subtitles from
the cache simply by comparing their periods to those that were
requested.  I think this is nicer than what was there before
(basically reevaulating 'what subtitle(s) for this period') and
also makes the first part of this commit effective.

1 files changed, 11 insertions, 1 deletions

diff --git a/src/lib/text_subtitle_decoder.cc b/src/lib/text_subtitle_decoder.cc
index bec2ab9b7..05bde829c 100644
--- a/src/lib/text_subtitle_decoder.cc
+++ b/src/lib/text_subtitle_decoder.cc
@@ -85,10 +85,20 @@ TextSubtitleDecoder::text_subtitles_during (ContentTimePeriod p, bool starting)
 
 	list<ContentTimePeriod> d;
 
+	/* Only take `during' (not starting) subs if they overlap more than half the requested period;
+	   here's the threshold for being significant.
+	*/
+	ContentTime const significant (p.duration().get() / 2);
+
 	for (vector<sub::Subtitle>::const_iterator i = _subtitles.begin(); i != _subtitles.end(); ++i) {
 		ContentTimePeriod t = content_time_period (*i);
-		if ((starting && p.contains (t.from)) || (!starting && p.overlap (t))) {
+		if (starting && p.contains(t.from)) {
 			d.push_back (t);
+		} else if (!starting) {
+			optional<ContentTimePeriod> const o = p.overlap (t);
+			if (o && o->duration() > significant) {
+				d.push_back (t);
+			}
 		}
 	}