input: Add --auto-adjust-pts-delay, this allows to stream/receive with an extremely low latency.

Simple demo:
vlc --sout="#duplicate{dst=display,dst='transcode{vcodec=mp4v}:std{access=http,dst=0.0.0.0:8080,mux=ts}'}" --ignore-config  --use-stream-immediate movie.avi &
vlc --use-stream-immediate http://127.0.0.1:8080
and
vlc --use-stream-immediate http://127.0.0.1:8080 --auto-adjust-pts-delay

You'll have to wait a bit until the pts delay is auto adjusted. Generally it takes 2-4 seconds, because access set a very high default pts delay value. One amelioration would be to lower the pts_delay when this option is set to allow a quicker convergence.

The general algorithm requires some tuning, but results are here.

Note, this only works if there is a video track. A similar function could be developped for the audio tracks.

src/input/decoder.c

@@ -726,6 +726,96 @@ static void VoutFlushPicture( vout_thread_t *p_vout )
     }
     vlc_mutex_unlock( &p_vout->picture_lock );
 }
+
+
+static void optimize_video_pts( decoder_t *p_dec )
+{
+    picture_t * oldest_pict = NULL;
+    picture_t * youngest_pict = NULL;
+    int i;
+
+    input_thread_t * p_input = p_dec->p_owner->p_input;
+    vout_thread_t * p_vout = p_dec->p_owner->p_vout;
+    input_thread_private_t * p_priv = p_input->p;
+
+    if( !p_priv->pts_adjust.auto_adjust ) return;
+
+    for( i = 0; i < I_RENDERPICTURES; i++ )
+    {
+        picture_t * pic = PP_RENDERPICTURE[i];
+        if( pic->i_status != READY_PICTURE )
+            continue;
+
+        if( !oldest_pict || pic->date < oldest_pict->date )
+            oldest_pict = pic;
+        if( !youngest_pict || pic->date > youngest_pict->date )
+            youngest_pict = pic;
+    }
+
+    if( !youngest_pict || !oldest_pict )
+        return;
+
+    /* Try to find if we can reduce the pts
+     * This first draft is way to simple, and we can't say if the
+     * algo will converge. It's also full of constants.
+     * But this simple algo allows to reduce the latency
+     * to the minimum. */
+    mtime_t buffer_size = youngest_pict->date - oldest_pict->date;
+    int64_t pts_slide = 0;
+    if( buffer_size < 10000 )
+    {
+        if( p_priv->pts_adjust.i_num_faulty > 10 )
+        {
+            pts_slide = __MAX(p_input->i_pts_delay *3 / 2, 10000);
+            p_priv->pts_adjust.i_num_faulty = 0;
+        }
+        if( p_priv->pts_adjust.to_high )
+        {
+            p_priv->pts_adjust.to_high = !p_priv->pts_adjust.to_high;
+            p_priv->pts_adjust.i_num_faulty = 0;
+        }
+        p_priv->pts_adjust.i_num_faulty++;
+    }
+    else if( buffer_size > 100000 )
+    {
+        if( p_priv->pts_adjust.i_num_faulty > 25 )
+        {
+            pts_slide = -buffer_size/2;
+            p_priv->pts_adjust.i_num_faulty = 0;
+        }
+        if( p_priv->pts_adjust.to_high )
+        {
+            p_priv->pts_adjust.to_high = !p_priv->pts_adjust.to_high;
+            p_priv->pts_adjust.i_num_faulty = 0;
+        }
+        p_priv->pts_adjust.i_num_faulty++;
+    }
+    if( pts_slide )
+    {
+        mtime_t origi_delay = p_input->i_pts_delay;
+
+        p_input->i_pts_delay += pts_slide;
+
+        /* Don't play with the pts delay for more than -2<->3sec */
+        if( p_input->i_pts_delay < -2000000 )
+            p_input->i_pts_delay = -2000000;
+        else if( p_input->i_pts_delay > 3000000 )
+            p_input->i_pts_delay = 3000000;
+        pts_slide = p_input->i_pts_delay - origi_delay;
+
+        msg_Dbg( p_input, "Sliding the pts by %dms pts delay at %dms picture buffer was %dms",
+            (int)pts_slide/1000, (int)p_input->i_pts_delay/1000, (int)buffer_size/1000);
+
+        vlc_mutex_lock( &p_vout->picture_lock );
+        /* Slide all the picture */
+        for( i = 0; i < I_RENDERPICTURES; i++ )
+            PP_RENDERPICTURE[i]->date += pts_slide;
+        /* FIXME: slide aout/spu */
+        vlc_mutex_unlock( &p_vout->picture_lock );
+
+    }
+}
+
 static void DecoderDecodeVideo( decoder_t *p_dec, block_t *p_block )
 {
     input_thread_t *p_input = p_dec->p_owner->p_input;
@@ -757,6 +847,9 @@ static void DecoderDecodeVideo( decoder_t *p_dec, block_t *p_block )
 
         vout_DatePicture( p_dec->p_owner->p_vout, p_pic,
                           p_pic->date );
+
+        optimize_video_pts( p_dec );
+
         vout_DisplayPicture( p_dec->p_owner->p_vout, p_pic );
     }
 }

src/input/input_internal.h

@@ -112,6 +112,13 @@ struct input_thread_private_t
     int            i_slave;
     input_source_t **slave;
 
+    /* pts delay fixup */
+    struct {
+        int  i_num_faulty;
+        bool to_high;
+        bool auto_adjust;
+    } pts_adjust;
+
     /* Stats counters */
     struct {
         counter_t *p_read_packets;

src/input/var.c

@@ -165,6 +165,9 @@ void input_ControlVarInit ( input_thread_t *p_input )
     var_Change( p_input, "spu-delay", VLC_VAR_SETVALUE, &val, NULL );
     ADD_CALLBACK( "spu-delay", EsDelayCallback );
 
+    p_input->p->pts_adjust.auto_adjust = var_CreateGetBool(
+            p_input, "auto-adjust-pts-delay" );
+
     /* Video ES */
     var_Create( p_input, "video-es", VLC_VAR_INTEGER | VLC_VAR_HASCHOICE );
     text.psz_string = _("Video Track");

src/libvlc-module.c

@@ -1822,6 +1822,8 @@ vlc_module_begin();
 
     add_bool( "use-stream-immediate", false, NULL,
                USE_STREAM_IMMEDIATE, USE_STREAM_IMMEDIATE_LONGTEXT, false );
+    add_bool( "auto-adjust-pts-delay", false, NULL,
+              "auto-adjust-pts-delay", "auto-adjust-pts-delay", false );
 
 #if !defined(__APPLE__) && !defined(SYS_BEOS) && defined(LIBVLC_USE_PTHREAD)
     add_bool( "rt-priority", false, NULL, RT_PRIORITY_TEXT,