demux_mkv: add demuxer-mkv-crop-compat option

I don't expect the Matroska specification to be adjusted for this to
work soon, so in the meantime, follow the spec as written and add an
option to support most files in the wild.

I would prefer this to never be an option, but it looks like it's
unavoidable.

Recently, FFmpeg added cropping support for Matroska, so we don’t want
to be the odd one out by doing it differently...

See for more details:
ietf-wg-cellar/matroska-specification#947
https://gitlab.com/mbunkus/mkvtoolnix/-/issues/2389
mpv-player#13446

Fixes: mpv-player#15800

Author: kasper93

DOCS/interface-changes/demuxer-mkv-crop-compat.txt

@@ -0,0 +1 @@
+add `demuxer-mkv-crop-compat` option

DOCS/man/options.rst

@@ -4032,6 +4032,29 @@ Demuxer
     file and can make a reliable estimate even without an index present (such
     as partial files).
 
+``--demuxer-mkv-crop-compat=<yes|no>``
+    Enable compatibility mode for files that do not fully comply with the
+    Matroska specification. (default: no)
+
+    Most files containing cropping metadata require this mode to display correctly.
+
+    If this option is enabled, crop metadata will be applied before calculating
+    the video's aspect ratio, ensuring it is cropped accordingly. If this option
+    is disabled, the image will be cropped first and then stretched to match
+    DisplayWidth and DisplayHeight.
+
+    According to the Matroska specification, the Pixel Aspect Ratio (PAR) should
+    be calculated after cropping. However, the majority of files do not adhere
+    to this rule, as it would cause incompatibility with crop-unaware players.
+    Additionally, MKVToolNix does not automatically adjust DisplayWidth and
+    DisplayHeight when cropping metadata is applied, leading to most of files
+    created with it also failing to conform to the specification.
+
+    See for more details:
+    https://github.com/ietf-wg-cellar/matroska-specification/pull/947
+    https://gitlab.com/mbunkus/mkvtoolnix/-/issues/2389
+    https://github.com/mpv-player/mpv/pull/13446
+
 ``--demuxer-rawaudio-channels=<value>``
     Number of channels (or channel layout) if ``--demuxer=rawaudio`` is used
     (default: stereo).

demux/demux_mkv.c

@@ -236,6 +236,7 @@ struct demux_mkv_opts {
     double subtitle_preroll_secs_index;
     int probe_duration;
     bool probe_start_time;
+    bool crop_compat;
 };
 
 const struct m_sub_options demux_mkv_conf = {
@@ -249,6 +250,7 @@ const struct m_sub_options demux_mkv_conf = {
         {"probe-video-duration", OPT_CHOICE(probe_duration,
             {"no", 0}, {"yes", 1}, {"full", 2})},
         {"probe-start-time", OPT_BOOL(probe_start_time)},
+        {"crop-compat", OPT_BOOL(crop_compat)},
         {0}
     },
     .size = sizeof(struct demux_mkv_opts),
@@ -1683,9 +1685,24 @@ static int demux_mkv_open_video(demuxer_t *demuxer, mkv_track_t *track)
         sh_v->crop = crop;
     }
 
-    int dw = track->v_dwidth_set ? track->v_dwidth : mp_rect_w(crop);
-    int dh = track->v_dheight_set ? track->v_dheight : mp_rect_h(crop);
-    struct mp_image_params p = {.w = mp_rect_w(crop), .h = mp_rect_h(crop)};
+    mkv_demuxer_t *mkv_d = demuxer->priv;
+
+    // The Matroska specification mandates that the PAR is calculated after
+    // cropping. However, most files on the internet do not follow this, as it
+    // would make them incompatible with crop-unaware players. Additionally,
+    // MKVToolNix does not automatically adjust DisplayWidth/DisplayHeight when
+    // cropping metadata is added, so most files created with it also do not
+    // follow the specification.
+    // See the following links for more information:
+    // <https://github.com/ietf-wg-cellar/matroska-specification/pull/947>
+    // <https://gitlab.com/mbunkus/mkvtoolnix/-/issues/2389>
+    // <https://github.com/mpv-player/mpv/pull/13446>
+    int crop_compat_w = mkv_d->opts->crop_compat ? track->v_width : mp_rect_w(crop);
+    int crop_compat_h = mkv_d->opts->crop_compat ? track->v_height : mp_rect_h(crop);
+
+    int dw = track->v_dwidth_set ? track->v_dwidth : crop_compat_w;
+    int dh = track->v_dheight_set ? track->v_dheight : crop_compat_h;
+    struct mp_image_params p = {.w = crop_compat_w, .h = crop_compat_h};
     mp_image_params_set_dsize(&p, dw, dh);
     sh_v->par_w = p.p_w;
     sh_v->par_h = p.p_h;