codecs: gstmpeg2decoder: add support for render delay

Some decoding APIs support delayed output for performance reasons.
One example would be to request decoding for multiple frames and
then query for the oldest frame in the output queue.
This also increases throughput for transcoding and improves seek
performance when supported by the underlying backend.

Introduce support in the mpeg2 base class, so that backends that
support render delays can actually implement it.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1013>

subprojects/gst-plugins-bad/gst-libs/gst/codecs/gstmpeg2decoder.c

@@ -29,6 +29,7 @@
 #include <config.h>
 #endif
 
+#include <gst/base/base.h>
 #include "gstmpeg2decoder.h"
 
 GST_DEBUG_CATEGORY (gst_mpeg2_decoder_debug);
@@ -258,6 +259,12 @@ struct _GstMpeg2DecoderPrivate
   GstMpeg2Picture *current_picture;
   GstVideoCodecFrame *current_frame;
   GstMpeg2Picture *first_field;
+
+  guint preferred_output_delay;
+  /* for delayed output */
+  GstQueueArray *output_queue;
+  /* used for low-latency vs. high throughput mode decision */
+  gboolean is_live;
 };
 
 #define UPDATE_FLOW_RETURN(ret,new_ret) G_STMT_START { \
@@ -265,6 +272,14 @@ struct _GstMpeg2DecoderPrivate
     *(ret) = new_ret; \
 } G_STMT_END
 
+typedef struct
+{
+  GstVideoCodecFrame *frame;
+  GstMpeg2Picture *picture;
+  GstMpeg2Decoder *self;
+} GstMpeg2DecoderOutputFrame;
+
+
 #define parent_class gst_mpeg2_decoder_parent_class
 G_DEFINE_ABSTRACT_TYPE_WITH_CODE (GstMpeg2Decoder, gst_mpeg2_decoder,
     GST_TYPE_VIDEO_DECODER,
@@ -283,6 +298,11 @@ static GstFlowReturn gst_mpeg2_decoder_handle_frame (GstVideoDecoder * decoder,
     GstVideoCodecFrame * frame);
 static void gst_mpeg2_decoder_do_output_picture (GstMpeg2Decoder * self,
     GstMpeg2Picture * picture, GstFlowReturn * ret);
+static void gst_mpeg2_decoder_clear_output_frame (GstMpeg2DecoderOutputFrame *
+    output_frame);
+static void gst_mpeg2_decoder_drain_output_queue (GstMpeg2Decoder *
+    self, guint num, GstFlowReturn * ret);
+
 
 static void
 gst_mpeg2_decoder_class_init (GstMpeg2DecoderClass * klass)
@@ -326,6 +346,11 @@ gst_mpeg2_decoder_start (GstVideoDecoder * decoder)
   priv->profile = -1;
   priv->progressive = TRUE;
 
+  priv->output_queue =
+      gst_queue_array_new_for_struct (sizeof (GstMpeg2DecoderOutputFrame), 1);
+  gst_queue_array_set_clear_func (priv->output_queue,
+      (GDestroyNotify) gst_mpeg2_decoder_clear_output_frame);
+
   return TRUE;
 }
 
@@ -337,6 +362,7 @@ gst_mpeg2_decoder_stop (GstVideoDecoder * decoder)
 
   g_clear_pointer (&self->input_state, gst_video_codec_state_unref);
   g_clear_pointer (&priv->dpb, gst_mpeg2_dpb_free);
+  gst_queue_array_free (priv->output_queue);
 
   return TRUE;
 }
@@ -347,6 +373,7 @@ gst_mpeg2_decoder_set_format (GstVideoDecoder * decoder,
 {
   GstMpeg2Decoder *self = GST_MPEG2_DECODER (decoder);
   GstMpeg2DecoderPrivate *priv = self->priv;
+  GstQuery *query;
 
   GST_DEBUG_OBJECT (decoder, "Set format");
 
@@ -358,6 +385,11 @@ gst_mpeg2_decoder_set_format (GstVideoDecoder * decoder,
   priv->width = GST_VIDEO_INFO_WIDTH (&state->info);
   priv->height = GST_VIDEO_INFO_HEIGHT (&state->info);
 
+  query = gst_query_new_latency ();
+  if (gst_pad_peer_query (GST_VIDEO_DECODER_SINK_PAD (self), query))
+    gst_query_parse_latency (query, &priv->is_live, NULL, NULL);
+  gst_query_unref (query);
+
   return TRUE;
 }
 
@@ -373,6 +405,8 @@ gst_mpeg2_decoder_drain (GstVideoDecoder * decoder)
     gst_mpeg2_decoder_do_output_picture (self, picture, &ret);
   }
 
+  gst_mpeg2_decoder_drain_output_queue (self, 0, &ret);
+  gst_queue_array_clear (priv->output_queue);
   gst_mpeg2_dpb_clear (priv->dpb);
 
   return ret;
@@ -391,6 +425,7 @@ gst_mpeg2_decoder_flush (GstVideoDecoder * decoder)
   GstMpeg2DecoderPrivate *priv = self->priv;
 
   gst_mpeg2_dpb_clear (priv->dpb);
+  gst_queue_array_clear (priv->output_queue);
   priv->state &= GST_MPEG2_DECODER_STATE_VALID_SEQ_HEADERS;
   priv->pic_hdr = PIC_HDR_INIT;
   priv->pic_ext = PIC_HDR_EXT_INIT;
@@ -702,10 +737,16 @@ gst_mpeg2_decoder_handle_picture (GstMpeg2Decoder * decoder,
   /* 6.1.1.6: Conversely if no sequence_xxx_extension() occurs between
      the first sequence_header() and the first picture_header() then
      sequence_xxx_extension() shall not occur in the bitstream. */
-  if (priv->seq_changed && klass->new_sequence) {
+  if (priv->seq_changed) {
     GstFlowReturn ret;
 
+    if (klass->get_preferred_output_delay)
+      priv->preferred_output_delay =
+          klass->get_preferred_output_delay (decoder, priv->is_live);
+
     priv->seq_changed = FALSE;
+
+    if (klass->new_sequence) {
       ret = klass->new_sequence (decoder, &priv->seq_hdr,
           _seq_ext_is_valid (&priv->seq_ext) ? &priv->seq_ext : NULL,
           _seq_display_ext_is_valid (&priv->seq_display_ext) ?
@@ -718,6 +759,7 @@ gst_mpeg2_decoder_handle_picture (GstMpeg2Decoder * decoder,
         return ret;
       }
     }
+  }
 
   priv->state &= (GST_MPEG2_DECODER_STATE_GOT_SEQ_HDR |
       GST_MPEG2_DECODER_STATE_GOT_SEQ_EXT);
@@ -1044,9 +1086,9 @@ static void
 gst_mpeg2_decoder_do_output_picture (GstMpeg2Decoder * decoder,
     GstMpeg2Picture * to_output, GstFlowReturn * ret)
 {
-  GstMpeg2DecoderClass *klass = GST_MPEG2_DECODER_GET_CLASS (decoder);
   GstVideoCodecFrame *frame = NULL;
-  GstFlowReturn flow_ret = GST_FLOW_OK;
+  GstMpeg2DecoderPrivate *priv = decoder->priv;
+  GstMpeg2DecoderOutputFrame output_frame;
 
   g_assert (ret != NULL);
 
@@ -1065,15 +1107,12 @@ gst_mpeg2_decoder_do_output_picture (GstMpeg2Decoder * decoder,
     return;
   }
 
-  g_assert (klass->output_picture);
+  output_frame.frame = frame;
-  GST_LOG_OBJECT (decoder,
+  output_frame.picture = to_output;
-      "Output picture %p (frame_num %d, poc %d, pts: %" GST_TIME_FORMAT
+  output_frame.self = decoder;
-      "), from DPB",
+  gst_queue_array_push_tail_struct (priv->output_queue, &output_frame);
-      to_output, to_output->system_frame_number, to_output->pic_order_cnt,
+  gst_mpeg2_decoder_drain_output_queue (decoder, priv->preferred_output_delay,
-      GST_TIME_ARGS (frame->pts));
+      ret);
-  flow_ret = klass->output_picture (decoder, frame, to_output);
-
-  UPDATE_FLOW_RETURN (ret, flow_ret);
 }
 
 static GstFlowReturn
@@ -1113,6 +1152,21 @@ gst_mpeg2_decoder_output_current_picture (GstMpeg2Decoder * decoder)
   return ret;
 }
 
+static void
+gst_mpeg2_decoder_clear_output_frame (GstMpeg2DecoderOutputFrame * output_frame)
+{
+  if (!output_frame)
+    return;
+
+  if (output_frame->frame) {
+    gst_video_decoder_release_frame (GST_VIDEO_DECODER (output_frame->self),
+        output_frame->frame);
+    output_frame->frame = NULL;
+  }
+
+  gst_mpeg2_picture_clear (&output_frame->picture);
+}
+
 static GstFlowReturn
 gst_mpeg2_decoder_handle_frame (GstVideoDecoder * decoder,
     GstVideoCodecFrame * frame)
@@ -1200,3 +1254,31 @@ failed:
     return ret;
   }
 }
+
+static void
+gst_mpeg2_decoder_drain_output_queue (GstMpeg2Decoder * self, guint num,
+    GstFlowReturn * ret)
+{
+  GstMpeg2DecoderPrivate *priv = self->priv;
+  GstMpeg2DecoderClass *klass = GST_MPEG2_DECODER_GET_CLASS (self);
+  GstFlowReturn flow_ret;
+
+  g_assert (klass->output_picture);
+
+  while (gst_queue_array_get_length (priv->output_queue) > num) {
+    GstMpeg2DecoderOutputFrame *output_frame = (GstMpeg2DecoderOutputFrame *)
+        gst_queue_array_pop_head_struct (priv->output_queue);
+    GST_LOG_OBJECT (self,
+        "Output picture %p (frame_num %d, poc %d, pts: %" GST_TIME_FORMAT
+        "), from DPB",
+        output_frame->picture, output_frame->picture->system_frame_number,
+        output_frame->picture->pic_order_cnt,
+        GST_TIME_ARGS (output_frame->frame->pts));
+
+    flow_ret =
+        klass->output_picture (self, output_frame->frame,
+        output_frame->picture);
+
+    UPDATE_FLOW_RETURN (ret, flow_ret);
+  }
+}

subprojects/gst-plugins-bad/gst-libs/gst/codecs/gstmpeg2decoder.h

@@ -178,6 +178,21 @@ struct _GstMpeg2DecoderClass
                                      GstVideoCodecFrame * frame,
                                      GstMpeg2Picture * picture);
 
+  /**
+   * GstMpeg2DecoderClass::get_preferred_output_delay:
+   * @decoder: a #GstMpeg2Decoder
+   * @is_live: whether upstream is live or not
+   *
+   * Optional. Called by baseclass to query whether delaying output is
+   * preferred by subclass or not.
+   *
+   * Returns: the number of perferred delayed output frames
+   *
+   * Since: 1.20
+   */
+  guint (*get_preferred_output_delay) (GstMpeg2Decoder * decoder,
+                                       gboolean is_live);
+
   /*< private >*/
   gpointer padding[GST_PADDING_LARGE];
 };