Fix scaling of bitmap subs when the corresponding video is cropped (#2670).

Previously we would scale the bitmap size/position to a proportion
of the original video frame, then scale it back up again to the DCP
container.  This didn't take into account some cropped cases where
the picture would end up the same shape but the subtitles would be
stretched.

diff --git a/src/lib/ffmpeg_decoder.cc b/src/lib/ffmpeg_decoder.cc
index 6e9d671a2fa87792e2ea5b58d6b2d9b2e136822c..dcad184fa81ea17ea3ddc5e4a0cdf7b9052984db 100644 (file)
--- a/src/lib/ffmpeg_decoder.cc
+++ b/src/lib/ffmpeg_decoder.cc
@@ -781,11 +781,23 @@ FFmpegDecoder::process_bitmap_subtitle (AVSubtitleRect const * rect)
        if (target_height == 0 && video_codec_context()) {
                target_height = video_codec_context()->height;
        }
-       DCPOMATIC_ASSERT (target_width);
-       DCPOMATIC_ASSERT (target_height);
+
+       int x_offset = 0;
+       int y_offset = 0;
+       if (_ffmpeg_content->video && _ffmpeg_content->video->use()) {
+               auto const crop = _ffmpeg_content->video->actual_crop();
+               target_width -= crop.left + crop.right;
+               target_height -= crop.top + crop.bottom;
+               x_offset = -crop.left;
+               y_offset = -crop.top;
+       }
+
+       DCPOMATIC_ASSERT(target_width > 0);
+       DCPOMATIC_ASSERT(target_height > 0);
+
        dcpomatic::Rect<double> const scaled_rect (
-               static_cast<double>(rect->x) / target_width,
-               static_cast<double>(rect->y) / target_height,
+               static_cast<double>(rect->x + x_offset) / target_width,
+               static_cast<double>(rect->y + y_offset) / target_height,
                static_cast<double>(rect->w) / target_width,
                static_cast<double>(rect->h) / target_height
                );