Align text subtitles from the bottom of the screen.

diff --git a/src/lib/text_decoder.cc b/src/lib/text_decoder.cc
index 2541bbc9bfbf25efe1148729f417d98223c00e24..58f631e59124855aae836432d4eac5f7ff67e03c 100644 (file)
--- 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 */