From: Carl Hetherington <cth@carlh.net>
Date: Sat, 21 Jan 2023 22:41:23 +0000 (+0100)
Subject: Align text subtitles from the bottom of the screen.
X-Git-Tag: v2.16.45~18^2~2
X-Git-Url: https://main.carlh.net/gitweb/?a=commitdiff_plain;h=0d87795d5305db9d75894ba9857662b1c0cfeeb3;p=dcpomatic.git

Align text subtitles from the bottom of the screen.
---

diff --git a/src/lib/text_decoder.cc b/src/lib/text_decoder.cc
index 2541bbc9b..58f631e59 100644
--- a/src/lib/text_decoder.cc
+++ b/src/lib/text_decoder.cc
@@ -153,13 +153,20 @@ TextDecoder::emit_plain_start (ContentTime from, sub::Subtitle const & sub_subti
 				switch (line.vertical_position.reference.get_value_or(sub::BOTTOM_OF_SCREEN)) {
 				case sub::BOTTOM_OF_SCREEN:
 				case sub::TOP_OF_SUBTITLE:
-					/* This 0.9 is an arbitrary value to lift the bottom sub off the bottom
+					/* This 0.1 is an arbitrary value to lift the bottom sub off the bottom
 					   of the screen a bit to a pleasing degree.
 					   */
-					v_position = 0.9 -
+					v_position = 0.1 +
 						(1 + bottom_line.get() - line.vertical_position.line.get()) * multiplier;
 
-					v_align = dcp::VAlign::TOP;
+					/* Align our subtitles to the bottom of the screen, because if we are making a SMPTE
+					 * DCP and the projection system uses the wrong standard to interpret vertical position,
+					 * a bottom-aligned subtitle will be less wrong than a top-aligned one.  This is because
+					 * in the top-aligned case the difference will be the distance between bbox top an
+					 * baseline, but in the bottom-aligned case the difference will be between bbox bottom
+					 * and baseline (which is shorter).
+					 */
+					v_align = dcp::VAlign::BOTTOM;
 					break;
 				case sub::TOP_OF_SCREEN:
 					/* This 0.1 is another fudge factor to bring the top line away from the top of the screen a little */