h264decoder: Update latency dynamically

The actual number of reorder frames is unknown
unless frame reordering is disabled
(e.g., POC type 2 or constrained-* profiles).
Also derived maximum DPB size or max_num_reorder_frames in VUI
is not the upper bound of output delay.

Fixes: https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/2702
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5326>

subprojects/gst-plugins-bad/gst-libs/gst/codecs/gsth264decoder.c

@@ -153,6 +153,12 @@ struct _GstH264DecoderPrivate
 
   /* Return value from output_picture() */
   GstFlowReturn last_flow;
+
+  /* Latency report params */
+  guint32 max_reorder_count;
+  guint32 last_reorder_frame_number;
+  gint fps_n;
+  gint fps_d;
 };
 
 typedef struct
@@ -405,6 +411,17 @@ gst_h264_decoder_finalize (GObject * object)
   G_OBJECT_CLASS (parent_class)->finalize (object);
 }
 
+static void
+gst_h264_decoder_reset_latency_infos (GstH264Decoder * self)
+{
+  GstH264DecoderPrivate *priv = self->priv;
+
+  priv->max_reorder_count = 0;
+  priv->last_reorder_frame_number = 0;
+  priv->fps_n = 25;
+  priv->fps_d = 1;
+}
+
 static void
 gst_h264_decoder_reset (GstH264Decoder * self)
 {
@@ -420,6 +437,8 @@ gst_h264_decoder_reset (GstH264Decoder * self)
   priv->height = 0;
   priv->nal_length_size = 4;
   priv->last_flow = GST_FLOW_OK;
+
+  gst_h264_decoder_reset_latency_infos (self);
 }
 
 static gboolean
@@ -1290,6 +1309,9 @@ gst_h264_decoder_parse_slice (GstH264Decoder * self, GstH264NalUnit * nalu)
         gst_h264_picture_unref (picture);
         return ret;
       }
+
+      priv->last_reorder_frame_number++;
+      picture->reorder_frame_number = priv->last_reorder_frame_number;
     }
 
     /* This allows accessing the frame from the picture. */
@@ -1797,6 +1819,28 @@ gst_h264_decoder_do_output_picture (GstH264Decoder * self,
   }
 #endif
 
+  if (priv->last_reorder_frame_number > picture->reorder_frame_number) {
+    guint64 diff = priv->last_reorder_frame_number -
+        picture->reorder_frame_number;
+    guint64 total_delay = diff + priv->preferred_output_delay;
+    if (diff > priv->max_reorder_count && total_delay < G_MAXUINT32) {
+      GstClockTime latency;
+
+      priv->max_reorder_count = (guint32) diff;
+      latency = gst_util_uint64_scale_int (GST_SECOND * total_delay,
+          priv->fps_d, priv->fps_n);
+
+      if (latency != G_MAXUINT64) {
+        GST_DEBUG_OBJECT (self, "Updating latency to %" GST_TIME_FORMAT
+            ", reorder count: %" G_GUINT64_FORMAT ", output-delay: %u",
+            GST_TIME_ARGS (latency), diff, priv->preferred_output_delay);
+
+        gst_video_decoder_set_latency (GST_VIDEO_DECODER (self),
+            latency, latency);
+      }
+    }
+  }
+
   frame = gst_video_decoder_get_frame (GST_VIDEO_DECODER (self),
       GST_CODEC_PICTURE_FRAME_NUMBER (picture));
 
@@ -2309,16 +2353,14 @@ gst_h264_decoder_set_latency (GstH264Decoder * self, const GstH264SPS * sps,
 
   bump_level = get_bump_level (self);
   if (bump_level != GST_H264_DPB_BUMP_NORMAL_LATENCY) {
-    if (sps->pic_order_cnt_type == 2) {
+    GST_DEBUG_OBJECT (self, "Actual latency will be updated later");
-      /* POC type 2 has does not allow frame reordering */
     frames_delay = 0;
-    } else {
-      guint32 max_reorder_frames =
-          gst_h264_dpb_get_max_num_reorder_frames (priv->dpb);
-      frames_delay = MIN (max_dpb_size, max_reorder_frames);
-    }
   }
 
+  priv->max_reorder_count = frames_delay;
+  priv->fps_n = fps_n;
+  priv->fps_d = fps_d;
+
   /* Consider output delay wanted by subclass */
   frames_delay += priv->preferred_output_delay;
 
@@ -2433,6 +2475,8 @@ gst_h264_decoder_process_sps (GstH264Decoder * self, GstH264SPS * sps)
     if (ret != GST_FLOW_OK)
       return ret;
 
+    gst_h264_decoder_reset_latency_infos (self);
+
     g_assert (klass->new_sequence);
 
     if (klass->get_preferred_output_delay) {

subprojects/gst-plugins-bad/gst-libs/gst/codecs/gsth264picture.h

@@ -153,6 +153,9 @@ struct _GstH264Picture
 
   GstH264DecRefPicMarking dec_ref_pic_marking;
 
+  /* Set by decoder to trace the number of delayed output pictures */
+  guint32 reorder_frame_number;
+
   /* For interlaced decoding */
   gboolean second_field;
   GstH264Picture * other_field;