[vlc-devel] [PATCH] Support H.264 SEI recovery points

[Steinar H. Gunderson]

Hi,

Below is a patch to support H.264 SEI recovery points; ie., points that are
not keyframes, but that mark “if you decode the next N frames, you will have
a [perfectly or approximately] valid picture no matter what your starting
point was”. In particular, this is needed to decode streams encoded with
Periodic Intra Refresh (e.g. --sout-x264-intra-refresh true), at least if you
don't see the beginning of the stream, e.g. tuning into a multicast stream.
This may also help with some kinds of streams from AVCHD cameras that use
similar techniques.

One could argue that this functionality should live inside libavcodec
instead, but given that VLC does its own H.264 depacketization, this seems to
be the best place. I've tested it with streaming over UDP, and it seems to
work fine.

/* Steinar */

diff --git a/modules/packetizer/h264.c b/modules/packetizer/h264.c
index 35e1010..a6e929e 100644
--- a/modules/packetizer/h264.c
+++ b/modules/packetizer/h264.c
@@ -101,6 +101,7 @@ struct decoder_sys_t
     bool   b_pps;
     block_t *pp_sps[SPS_MAX];
     block_t *pp_pps[PPS_MAX];
+    int    i_recovery_frames;  /* -1 = no recovery */
 
     /* avcC data */
     int i_avcC_length_size;
@@ -217,6 +218,7 @@ static int Open( vlc_object_t *p_this )
         p_sys->pp_sps[i] = NULL;
     for( i = 0; i < PPS_MAX; i++ )
         p_sys->pp_pps[i] = NULL;
+    p_sys->i_recovery_frames = -1;
 
     p_sys->slice.i_nal_type = -1;
     p_sys->slice.i_nal_ref_idc = -1;
@@ -714,7 +716,18 @@ static block_t *OutputPicture( decoder_t *p_dec )
     decoder_sys_t *p_sys = p_dec->p_sys;
     block_t *p_pic;
 
-    if( !p_sys->b_header && p_sys->slice.i_frame_type != BLOCK_FLAG_TYPE_I)
+    if ( !p_sys->b_header && p_sys->i_recovery_frames != -1 )
+    {
+        if( p_sys->i_recovery_frames == 0 )
+        {
+            msg_Dbg( p_dec, "Recovery from SEI recovery point complete" );
+            p_sys->b_header = true;
+        }
+        --p_sys->i_recovery_frames;
+    }
+
+    if( !p_sys->b_header && p_sys->i_recovery_frames == 0 &&
+         p_sys->slice.i_frame_type != BLOCK_FLAG_TYPE_I)
         return NULL;
 
     const bool b_sps_pps_i = p_sys->slice.i_frame_type == BLOCK_FLAG_TYPE_I &&
@@ -760,6 +773,8 @@ static block_t *OutputPicture( decoder_t *p_dec )
     p_pic->i_length = 0;    /* FIXME */
     p_pic->i_flags |= p_sys->slice.i_frame_type;
     p_pic->i_flags &= ~BLOCK_FLAG_PRIVATE_AUD;
+    if( !p_sys->b_header )
+        p_pic->i_flags |= BLOCK_FLAG_PREROLL;
 
     p_sys->slice.i_frame_type = 0;
     p_sys->p_frame = NULL;
@@ -1176,6 +1191,27 @@ static void ParseSei( decoder_t *p_dec, block_t *p_frag )
                 cc_Extract( &p_sys->cc_next, true, &p_t35[3], i_t35 - 3 );
             }
         }
+
+        /* Look for SEI recovery point */
+        if( i_type == 6 )
+        {
+            bs_t s;
+            const int      i_rec = i_size;
+            const uint8_t *p_rec = &pb_dec[i_used];
+    
+            bs_init( &s, p_rec, i_rec );
+            int i_recovery_frames = bs_read_ue( &s );
+            //bool b_exact_match = bs_read( &s, 1 );
+            //bool b_broken_link = bs_read( &s, 1 );
+            //int i_changing_slice_group = bs_read( &s, 2 );
+            if( !p_sys->b_header )
+            {
+                msg_Dbg( p_dec, "Seen SEI recovery point, %d recovery frames", i_recovery_frames );
+                if ( p_sys->i_recovery_frames == -1 || i_recovery_frames < p_sys->i_recovery_frames )
+                    p_sys->i_recovery_frames = i_recovery_frames;
+            }
+        }
+
         i_used += i_size;
     }
 

-- 
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