Fix subtitle vertical position (#2367).

Previously we would not account for the differences in what vertical
position means between Interop and SMPTE.  For interop, vertical
position is the distance from the reference point to the text
baseline, whereas for SMPTE it is the distance from the reference
point to the top/middle/bottom of the subtitle (depending on the
reference).

This caused differences between the preview and the DCP for some
cases (notably, using SRT/SSA and making Interop DCPs, or converting
Interop DCP subs to SMPTE, or vice versa).

1 files changed, 29 insertions, 0 deletions

diff --git a/src/lib/reel_writer.cc b/src/lib/reel_writer.cc
index f9e29a16f..94b12ec7d 100644
--- a/src/lib/reel_writer.cc
+++ b/src/lib/reel_writer.cc
@@ -114,6 +114,7 @@ ReelWriter::ReelWriter (
 	, _content_summary (film()->content_summary(period))
 	, _job (job)
 	, _text_only (text_only)
+	, _font_metrics(film()->frame_size().height)
 {
 	/* Create or find our picture asset in a subdirectory, named
 	   according to those film's parameters which affect the video
@@ -889,6 +890,33 @@ ReelWriter::empty_text_asset (TextType type, optional<DCPTextTrack> track, bool
 }
 
 
+float
+ReelWriter::convert_vertical_position(StringText const& subtitle, dcp::Standard to) const
+{
+	if (subtitle.valign_standard == to) {
+		return subtitle.v_position();
+	}
+
+	auto const baseline_to_bottom = _font_metrics.baseline_to_bottom(subtitle);
+	auto const height = _font_metrics.height(subtitle);
+
+	float correction = 0;
+	switch (subtitle.v_align()) {
+	case dcp::VAlign::TOP:
+		correction = height - baseline_to_bottom;
+		break;
+	case dcp::VAlign::CENTER:
+		correction = (height / 2) - baseline_to_bottom;
+		break;
+	case dcp::VAlign::BOTTOM:
+		correction = baseline_to_bottom;
+		break;
+	}
+
+	return subtitle.v_position() + ((subtitle.valign_standard == dcp::Standard::SMPTE) ? correction : -correction);
+}
+
+
 void
 ReelWriter::write (PlayerText subs, TextType type, optional<DCPTextTrack> track, DCPTimePeriod period, FontIdMap const& fonts)
 {
@@ -928,6 +956,7 @@ ReelWriter::write (PlayerText subs, TextType type, optional<DCPTextTrack> track,
 	for (auto i: subs.string) {
 		i.set_in  (dcp::Time(period.from.seconds() - _period.from.seconds(), tcr));
 		i.set_out (dcp::Time(period.to.seconds() - _period.from.seconds(), tcr));
+		i.set_v_position(convert_vertical_position(i, film()->interop() ? dcp::Standard::INTEROP : dcp::Standard::SMPTE));
 		auto sub = make_shared<dcp::SubtitleString>(i);
 		if (type == TextType::OPEN_SUBTITLE) {
 			sub->set_font(fonts.get(i.font));