avviddec, video.c, h265parse: Workaround for broken field-based interlaced encoders

Some encoders (e.g. Makito) have H265 field-based interlacing, but then
also specify an 1:2 pixel aspect ratio. That makes it kind-of work with
decoders that don't properly support field-based decoding, but makes us
end up with the wrong aspect ratio if we implement everything properly.
As a workaround, detect 1:2 pixel aspect ratio for field-based
interlacing, and check if making that 1:1 would make the new display
aspect ratio common. In that case, we override it with 1:1.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2577>

subprojects/gst-libav/ext/libav/gstavviddec.c

@@ -1101,6 +1101,8 @@ gst_ffmpegviddec_update_par (GstFFMpegVidDec * ffmpegdec,
   gboolean decoder_par_set = FALSE;
   gint demuxer_num = 1, demuxer_denom = 1;
   gint decoder_num = 1, decoder_denom = 1;
+  GstVideoCodecFrame *out_frame;
+  GstFFMpegVidDecVideoFrame *out_dframe;
 
   if (in_info->par_n && in_info->par_d) {
     demuxer_num = in_info->par_n;
@@ -1127,14 +1129,38 @@ gst_ffmpegviddec_update_par (GstFFMpegVidDec * ffmpegdec,
   if (decoder_par_set && !demuxer_par_set)
     goto use_decoder_par;
 
+  /* Special case for some encoders which provide an 1:2 pixel aspect ratio
+   * for HEVC interlaced content, possibly to work around decoders that don't
+   * support field-based interlacing. Add some defensive checks to check for
+   * a "common" aspect ratio. */
+  out_dframe = ffmpegdec->picture->opaque;
+  out_frame = out_dframe->frame;
+
+  if (demuxer_num == 1 && demuxer_denom == 1 &&
+      decoder_num == 1 && decoder_denom == 2 &&
+      GST_BUFFER_FLAG_IS_SET (out_frame->input_buffer,
+          GST_VIDEO_BUFFER_FLAG_ONEFIELD) &&
+      gst_video_is_common_aspect_ratio (ffmpegdec->pic_width,
+          ffmpegdec->pic_height, 1, 2) &&
+      !gst_video_is_common_aspect_ratio (ffmpegdec->pic_width,
+          ffmpegdec->pic_height, 1, 1)) {
+    GST_WARNING_OBJECT (ffmpegdec,
+        "PAR 1/2 makes the aspect ratio of "
+        "a %d x %d frame uncommon. Switching to 1/1",
+        ffmpegdec->pic_width, ffmpegdec->pic_height);
+    goto use_demuxer_par;
+  }
+
   /* Both the demuxer and the decoder provide a PAR. If one of
    * the two PARs is 1:1 and the other one is not, use the one
    * that is not 1:1. */
-  if (demuxer_num == demuxer_denom && decoder_num != decoder_denom)
+  if (demuxer_num == demuxer_denom && decoder_num != decoder_denom) {
     goto use_decoder_par;
+  }
 
-  if (decoder_num == decoder_denom && demuxer_num != demuxer_denom)
+  if (decoder_num == decoder_denom && demuxer_num != demuxer_denom) {
     goto use_demuxer_par;
+  }
 
   /* Both PARs are non-1:1, so use the PAR provided by the demuxer */
   goto use_demuxer_par;

subprojects/gst-plugins-bad/gst/videoparsers/gsth265parse.c

@@ -2086,6 +2086,7 @@ gst_h265_parse_update_src_caps (GstH265Parse * h265parse, GstCaps * caps)
   gboolean modified = FALSE;
   GstBuffer *buf = NULL;
   GstStructure *s = NULL;
+  gint width, height;
 
   if (G_UNLIKELY (!gst_pad_has_current_caps (GST_BASE_PARSE_SRC_PAD
               (h265parse))))
@@ -2242,7 +2243,6 @@ gst_h265_parse_update_src_caps (GstH265Parse * h265parse, GstCaps * caps)
     if (G_UNLIKELY (modified || h265parse->update_caps)) {
       gint fps_num = h265parse->fps_num;
       gint fps_den = h265parse->fps_den;
-      gint width, height;
       GstClockTime latency = 0;
 
       caps = gst_caps_copy (sink_caps);
@@ -2345,9 +2345,22 @@ gst_h265_parse_update_src_caps (GstH265Parse * h265parse, GstCaps * caps)
     gst_h265_parse_get_par (h265parse, &par_n, &par_d);
     if (par_n != 0 && par_d != 0 &&
         (!s || !gst_structure_has_field (s, "pixel-aspect-ratio"))) {
-      GST_INFO_OBJECT (h265parse, "PAR %d/%d", par_n, par_d);
+      gint new_par_d = par_d;
+      /* Special case for some encoders which provide an 1:2 pixel aspect ratio
+       * for HEVC interlaced content, possibly to work around decoders that don't
+       * support field-based interlacing. Add some defensive checks to check for
+       * a "common" aspect ratio. */
+      if (par_n == 1 && par_d == 2
+          && gst_h265_parse_is_field_interlaced (h265parse)
+          && !gst_video_is_common_aspect_ratio (width, height, par_n, par_d)
+          && gst_video_is_common_aspect_ratio (width, height, 1, 1)) {
+        GST_WARNING_OBJECT (h265parse, "PAR 1/2 makes the aspect ratio of "
+            "a %d x %d frame uncommon. Switching to 1/1", width, height);
+        new_par_d = 1;
+      }
+      GST_INFO_OBJECT (h265parse, "PAR %d/%d", par_n, new_par_d);
       gst_caps_set_simple (caps, "pixel-aspect-ratio", GST_TYPE_FRACTION,
-          par_n, par_d, NULL);
+          par_n, new_par_d, NULL);
     }
 
     /* set profile and level in caps */

subprojects/gst-plugins-base/gst-libs/gst/video/video.c

@@ -183,6 +183,44 @@ gst_video_guess_framerate (GstClockTime duration, gint * dest_n, gint * dest_d)
   return (best_error != G_MAXUINT64);
 }
 
+/**
+ * gst_video_is_common_aspect_ratio:
+ * @width: Width of the video frame
+ * @height: Height of the video frame
+ * @par_n: Pixel aspect ratio numerator
+ * @par_d: Pixel aspect ratio denominator
+ *
+ * Given a frame's dimensions and pixel aspect ratio, this function will
+ * calculate the frame's aspect ratio and compare it against a set of
+ * common well-known "standard" aspect ratios.
+ *
+ * Returns: %TRUE if a known "standard" aspect ratio was
+ * recognised, and %FALSE otherwise.
+ *
+ * Since: 1.22
+ */
+gboolean
+gst_video_is_common_aspect_ratio (gint width, gint height, gint par_n,
+    gint par_d)
+{
+  gint dar_n, dar_d;
+
+  gst_util_fraction_multiply (width, height, par_n, par_d, &dar_n, &dar_d);
+
+  if (dar_n == 16 && dar_d == 9)
+    return TRUE;
+  if (dar_n == 4 && dar_d == 3)
+    return TRUE;
+  if (dar_n == 14 && dar_d == 9)
+    return TRUE;
+  if (dar_n == 8 && dar_d == 5)
+    return TRUE;
+  if (dar_n == 21 && dar_d == 11)
+    return TRUE;
+
+  return FALSE;
+}
+
 
 /**
  * gst_video_alignment_reset:

subprojects/gst-plugins-base/gst-libs/gst/video/video.h

@@ -149,6 +149,9 @@ GST_VIDEO_API
 gboolean       gst_video_guess_framerate (GstClockTime duration,
                                           gint * dest_n, gint * dest_d);
 
+GST_VIDEO_API
+gboolean       gst_video_is_common_aspect_ratio (gint width, gint height, gint par_n, gint par_d);
+
 /* convert/encode video sample from one format to another */
 
 typedef void (*GstVideoConvertSampleCallback) (GstSample * sample, GError *error, gpointer user_data);