- _subtitle = sub;
- _subtitle_content_time = piece->content->start ();
- _subtitle_offset = sc->subtitle_offset ();
- _subtitle_scale = sc->subtitle_scale ();
+ dcpomatic::Rect<double> in_rect = _in_subtitle.rect;
+ libdcp::Size scaled_size;
+
+ in_rect.y += sc->subtitle_offset ();
+
+ /* We will scale the subtitle up to fit _video_container_size, and also by the additional subtitle_scale */
+ scaled_size.width = in_rect.width * _video_container_size.width * sc->subtitle_scale ();
+ scaled_size.height = in_rect.height * _video_container_size.height * sc->subtitle_scale ();
+
+ /* Then we need a corrective translation, consisting of two parts:
+ *
+ * 1. that which is the result of the scaling of the subtitle by _video_container_size; this will be
+ * rect.x * _video_container_size.width and rect.y * _video_container_size.height.
+ *
+ * 2. that to shift the origin of the scale by subtitle_scale to the centre of the subtitle; this will be
+ * (width_before_subtitle_scale * (1 - subtitle_scale) / 2) and
+ * (height_before_subtitle_scale * (1 - subtitle_scale) / 2).
+ *
+ * Combining these two translations gives these expressions.
+ */
+
+ _out_subtitle.position.x = rint (_video_container_size.width * (in_rect.x + (in_rect.width * (1 - sc->subtitle_scale ()) / 2)));
+ _out_subtitle.position.y = rint (_video_container_size.height * (in_rect.y + (in_rect.height * (1 - sc->subtitle_scale ()) / 2)));
+
+ _out_subtitle.image = _in_subtitle.image->scale (libdcp::Size (scaled_size.width, scaled_size.height), Scaler::from_id ("bicubic"), true);
+ _out_subtitle.from = _in_subtitle.from + piece->content->start ();
+ _out_subtitle.to = _in_subtitle.to + piece->content->start ();