codecs: h264dec: Calculate the latency by its bump mode.

The current latency calculation just uses the num_reorder_frames,
which is not very precise. We should consider the bump mode of the
DPB, the faster it bumps, the lower latency we will have.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1046>

diff --git a/subprojects/gst-plugins-bad/gst-libs/gst/codecs/gsth264decoder.c b/subprojects/gst-plugins-bad/gst-libs/gst/codecs/gsth264decoder.c
index aee2eee..7c71987 100644 (file)
--- a/subprojects/gst-plugins-bad/gst-libs/gst/codecs/gsth264decoder.c
+++ b/subprojects/gst-plugins-bad/gst-libs/gst/codecs/gsth264decoder.c
@@ -1036,8 +1036,8 @@ gst_h264_decoder_init_current_picture (GstH264Decoder * self)
   /* If the slice header indicates we will have to perform reference marking
    * process after this picture is decoded, store required data for that
    * purpose */
-  if (priv->current_slice.header.dec_ref_pic_marking.
-      adaptive_ref_pic_marking_mode_flag) {
+  if (priv->current_slice.header.
+      dec_ref_pic_marking.adaptive_ref_pic_marking_mode_flag) {
     priv->current_picture->dec_ref_pic_marking =
         priv->current_slice.header.dec_ref_pic_marking;
   }
@@ -2331,7 +2331,8 @@ gst_h264_decoder_set_latency (GstH264Decoder * self, const GstH264SPS * sps,
   GstClockTime min, max;
   GstStructure *structure;
   gint fps_d = 1, fps_n = 0;
-  guint32 num_reorder_frames;
+  GstH264DpbBumpMode bump_level;
+  guint32 frames_delay;
 
   caps = gst_pad_get_current_caps (GST_VIDEO_DECODER_SRC_PAD (self));
   if (!caps)
@@ -2352,18 +2353,30 @@ gst_h264_decoder_set_latency (GstH264Decoder * self, const GstH264SPS * sps,
     fps_d = 1;
   }
 
-  num_reorder_frames = priv->is_live ? 0 : 1;
-  if (sps->vui_parameters_present_flag
-      && sps->vui_parameters.bitstream_restriction_flag)
-    num_reorder_frames = sps->vui_parameters.num_reorder_frames;
-  if (num_reorder_frames > max_dpb_size)
-    num_reorder_frames = priv->is_live ? 0 : 1;
+  bump_level = get_bump_level (self);
+  frames_delay = 0;
+  switch (bump_level) {
+    case GST_H264_DPB_BUMP_NORMAL_LATENCY:
+      /* We always wait the DPB full before bumping. */
+      frames_delay = max_dpb_size;
+      break;
+    case GST_H264_DPB_BUMP_LOW_LATENCY:
+      /* We bump the IDR if the second frame is not a minus POC. */
+      frames_delay = 1;
+      break;
+    case GST_H264_DPB_BUMP_VERY_LOW_LATENCY:
+      /* We bump the IDR immediately. */
+      frames_delay = 0;
+      break;
+    default:
+      g_assert_not_reached ();
+      break;
+  }
 
   /* Consider output delay wanted by subclass */
-  num_reorder_frames += priv->preferred_output_delay;
+  frames_delay += priv->preferred_output_delay;
 
-  min = gst_util_uint64_scale_int (num_reorder_frames * GST_SECOND, fps_d,
-      fps_n);
+  min = gst_util_uint64_scale_int (frames_delay * GST_SECOND, fps_d, fps_n);
   max = gst_util_uint64_scale_int ((max_dpb_size + priv->preferred_output_delay)
       * GST_SECOND, fps_d, fps_n);