Add temporally overlapping subtitle support.

* New subtitle sync algorithm added to sync work-object ("simultaneous").
  Classic algorithm preserved but disabled.
* Render work-object now supports queueing a /list/ of subtitles.
* FIFOs have been extended to support pushing/popping buffer-lists as single elements.
* Added SUBSYNC_VERBOSE_TIMING flag to debug timing issues related to subtitle display.

Observable behaviors changed in the new subtitle sync algorithm:
* Temporally overlapping subtitles are no longer trimmed to be non-overlapping.
* Subtitles less than two seconds long are no longer artificially extended. Sorry, Indochine fans.
* Subtitles that stop before they start will never be displayed. The old algorithm will display such subtitles if they begin in the future (relative to the current video frame being processed).



git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@3804 b64f7644-9d1e-0410-96f1-a4d463321fa5

5 files changed, 366 insertions, 34 deletions

diff --git a/libhb/decssasub.c b/libhb/decssasub.c
index 8d0a9995d..b0ed845d5 100644
--- a/libhb/decssasub.c
+++ b/libhb/decssasub.c
@@ -50,6 +50,8 @@ typedef enum {
               sec   * 1000L +\
               centi * 10L ) )
 
+#define SSA_VERBOSE_PACKETS 0
+
 static StyleSet ssa_parse_style_override( uint8_t *pos, StyleSet prevStyles )
 {
     StyleSet nextStyles = prevStyles;
@@ -592,6 +594,10 @@ static int decssaWork( hb_work_object_t * w, hb_buffer_t ** buf_in,
     hb_buffer_t * in = *buf_in;
     hb_buffer_t * out_list = NULL;
     
+#if SSA_VERBOSE_PACKETS
+    printf("\nPACKET(%"PRId64",%"PRId64"): %.*s\n", in->start/90, in->stop/90, in->size, in->data);
+#endif
+    
     if ( in->size > 0 ) {
         out_list = ssa_decode_packet(w, in);
     } else {
diff --git a/libhb/fifo.c b/libhb/fifo.c
index 755a6d164..cd348c863 100644
--- a/libhb/fifo.c
+++ b/libhb/fifo.c
@@ -190,6 +190,7 @@ void hb_buffer_realloc( hb_buffer_t * b, int size )
     }
 }
 
+// Frees the specified buffer list.
 void hb_buffer_close( hb_buffer_t ** _b )
 {
     hb_buffer_t * b = *_b;
@@ -294,6 +295,8 @@ float hb_fifo_percent_full( hb_fifo_t * f )
     return ret;
 }
 
+// Pulls the first packet out of this FIFO, blocking until such a packet is available.
+// Returns NULL if this FIFO has been closed or flushed.
 hb_buffer_t * hb_fifo_get_wait( hb_fifo_t * f )
 {
     hb_buffer_t * b;
@@ -323,6 +326,7 @@ hb_buffer_t * hb_fifo_get_wait( hb_fifo_t * f )
     return b;
 }
 
+// Pulls a packet out of this FIFO, or returns NULL if no packet is available.
 hb_buffer_t * hb_fifo_get( hb_fifo_t * f )
 {
     hb_buffer_t * b;
@@ -368,6 +372,8 @@ hb_buffer_t * hb_fifo_see_wait( hb_fifo_t * f )
     return b;
 }
 
+// Returns the first packet in the specified FIFO.
+// If the FIFO is empty, returns NULL.
 hb_buffer_t * hb_fifo_see( hb_fifo_t * f )
 {
     hb_buffer_t * b;
@@ -400,6 +406,8 @@ hb_buffer_t * hb_fifo_see2( hb_fifo_t * f )
     return b;
 }
 
+// Waits until the specified FIFO is no longer full or until FIFO_TIMEOUT milliseconds have elapsed.
+// Returns whether the FIFO is non-full upon return.
 int hb_fifo_full_wait( hb_fifo_t * f )
 {
     int result;
@@ -415,6 +423,8 @@ int hb_fifo_full_wait( hb_fifo_t * f )
     return result;
 }
 
+// Pushes the specified buffer onto the specified FIFO,
+// blocking until the FIFO has space available.
 void hb_fifo_push_wait( hb_fifo_t * f, hb_buffer_t * b )
 {
     if( !b )
@@ -451,6 +461,7 @@ void hb_fifo_push_wait( hb_fifo_t * f, hb_buffer_t * b )
     hb_unlock( f->lock );
 }
 
+// Appends the specified packet list to the end of the specified FIFO.
 void hb_fifo_push( hb_fifo_t * f, hb_buffer_t * b )
 {
     if( !b )
@@ -482,6 +493,7 @@ void hb_fifo_push( hb_fifo_t * f, hb_buffer_t * b )
     hb_unlock( f->lock );
 }
 
+// Prepends the specified packet list to the start of the specified FIFO.
 void hb_fifo_push_head( hb_fifo_t * f, hb_buffer_t * b )
 {
     hb_buffer_t * tmp;
@@ -519,6 +531,29 @@ void hb_fifo_push_head( hb_fifo_t * f, hb_buffer_t * b )
     hb_unlock( f->lock );
 }
 
+// Pushes a list of packets onto the specified FIFO as a single element.
+void hb_fifo_push_list_element( hb_fifo_t *fifo, hb_buffer_t *buffer_list )
+{
+    hb_buffer_t *container = hb_buffer_init( 0 );
+    // XXX: Using an arbitrary hb_buffer_t pointer (other than 'next')
+    //      to carry the list inside a single "container" buffer
+    container->next_subpicture = buffer_list;
+    
+    hb_fifo_push( fifo, container );
+}
+
+// Removes a list of packets from the specified FIFO that were stored as a single element.
+hb_buffer_t *hb_fifo_get_list_element( hb_fifo_t *fifo )
+{
+    hb_buffer_t *container = hb_fifo_get( fifo );
+    // XXX: Using an arbitrary hb_buffer_t pointer (other than 'next')
+    //      to carry the list inside a single "container" buffer
+    hb_buffer_t *buffer_list = container->next_subpicture;
+    hb_buffer_close( &container );
+    
+    return buffer_list;
+}
+
 void hb_fifo_close( hb_fifo_t ** _f )
 {
     hb_fifo_t   * f = *_f;
diff --git a/libhb/internal.h b/libhb/internal.h
index 335c9a854..7896bee16 100644
--- a/libhb/internal.h
+++ b/libhb/internal.h
@@ -126,6 +126,8 @@ void          hb_fifo_push( hb_fifo_t *, hb_buffer_t * );
 void          hb_fifo_push_wait( hb_fifo_t *, hb_buffer_t * );
 int           hb_fifo_full_wait( hb_fifo_t * f );
 void          hb_fifo_push_head( hb_fifo_t *, hb_buffer_t * );
+void          hb_fifo_push_list_element( hb_fifo_t *fifo, hb_buffer_t *buffer_list );
+hb_buffer_t * hb_fifo_get_list_element( hb_fifo_t *fifo );
 void          hb_fifo_close( hb_fifo_t ** );
 void          hb_fifo_flush( hb_fifo_t * f );
 
diff --git a/libhb/render.c b/libhb/render.c
index 8c7e9a2d8..c16b9403d 100644
--- a/libhb/render.c
+++ b/libhb/render.c
@@ -67,6 +67,8 @@ static uint8_t *getV(uint8_t *data, int width, int height, int x, int y)
     return(&data[(y>>1) * w2 + (x>>1) + width*height + w2*h2]);
 }
 
+// Draws the specified PICTURESUB subtitle on the specified video packet.
+// Disposes the subtitle afterwards.
 static void ApplySub( hb_job_t * job, hb_buffer_t * buf,
                       hb_buffer_t ** _sub )
 {
@@ -397,11 +399,11 @@ int renderWork( hb_work_object_t * w, hb_buffer_t ** buf_in,
     /* Push subtitles onto queue just in case we need to delay a frame */
     if( in->sub )
     {
-        hb_fifo_push( pv->subtitle_queue, in->sub );
+        hb_fifo_push_list_element( pv->subtitle_queue, in->sub );
     }
     else
     {
-        hb_fifo_push( pv->subtitle_queue,  hb_buffer_init(0) );
+        hb_fifo_push_list_element( pv->subtitle_queue, NULL );
     }
 
     /* If there's a chapter mark remember it in case we delay or drop its frame */
@@ -451,11 +453,15 @@ int renderWork( hb_work_object_t * w, hb_buffer_t ** buf_in,
             }
             else if( result == FILTER_DELAY )
             {
+                // Process the current frame later
+                
                 buf_tmp_in = NULL;
                 break;
             }
             else if( result == FILTER_DROP )
             {
+                // Drop the current frame
+                
                 /* We need to compensate for the time lost by dropping this frame.
                    Spread its duration out in quarters, because usually dropped frames
                    maintain a 1-out-of-5 pattern and this spreads it out amongst the remaining ones.
@@ -471,15 +477,28 @@ int renderWork( hb_work_object_t * w, hb_buffer_t ** buf_in,
                 pv->total_lost_time += temp_duration;
                 pv->dropped_frames++;
 
-                /* Pop the frame's subtitle and dispose of it. */
-                hb_buffer_t * subpicture_list = hb_fifo_get( pv->subtitle_queue );
-                hb_buffer_t * subpicture;
-                hb_buffer_t * subpicture_next;
-                for ( subpicture = subpicture_list; subpicture; subpicture = subpicture_next )
+                /* Pop the frame's subtitle list and dispose of it. */
+                hb_buffer_t * sub_list = hb_fifo_get_list_element( pv->subtitle_queue );
+                hb_buffer_t * sub;
+                hb_buffer_t * sub_next;
+                for ( sub = sub_list; sub; sub = sub_next )
                 {
-                    subpicture_next = subpicture->next_subpicture;
-                    hb_buffer_close( &subpicture );
+                    sub_next = sub->next;
+                    // XXX: Prevent hb_buffer_close from killing the whole list
+                    //      before we finish iterating over it
+                    sub->next = NULL;
+                    
+                    hb_buffer_t * subpicture_list = sub;
+                    hb_buffer_t * subpicture;
+                    hb_buffer_t * subpicture_next;
+                    for ( subpicture = subpicture_list; subpicture; subpicture = subpicture_next )
+                    {
+                        subpicture_next = subpicture->next_subpicture;
+                        
+                        hb_buffer_close( &subpicture );
+                    }
                 }
+                
                 buf_tmp_in = NULL;
                 break;
             }
@@ -503,16 +522,28 @@ int renderWork( hb_work_object_t * w, hb_buffer_t ** buf_in,
         pv->last_stop[0] = pv->last_start[0] + (buf_tmp_in->stop - buf_tmp_in->start);
     }
 
-    /* Apply subtitles */
+    /* Apply subtitles and dispose them */
     if( buf_tmp_in )
     {
-        hb_buffer_t * subpicture_list = hb_fifo_get( pv->subtitle_queue );
-        hb_buffer_t * subpicture;
-        hb_buffer_t * subpicture_next;
-        for ( subpicture = subpicture_list; subpicture; subpicture = subpicture_next )
+        hb_buffer_t * sub_list = hb_fifo_get_list_element( pv->subtitle_queue );
+        hb_buffer_t * sub;
+        hb_buffer_t * sub_next;
+        for ( sub = sub_list; sub; sub = sub_next )
         {
-            subpicture_next = subpicture->next_subpicture;
-            ApplySub( job, buf_tmp_in, &subpicture );
+            sub_next = sub->next;
+            // XXX: Prevent hb_buffer_close inside ApplySub from killing the whole list
+            //      before we finish iterating over it
+            sub->next = NULL;
+            
+            hb_buffer_t * subpicture_list = sub;
+            hb_buffer_t * subpicture;
+            hb_buffer_t * subpicture_next;
+            for ( subpicture = subpicture_list; subpicture; subpicture = subpicture_next )
+            {
+                subpicture_next = subpicture->next_subpicture;
+                
+                ApplySub( job, buf_tmp_in, &subpicture );
+            }
         }
     }
 
diff --git a/libhb/sync.c b/libhb/sync.c
index ad26fb656..9a532a94b 100644
--- a/libhb/sync.c
+++ b/libhb/sync.c
@@ -68,6 +68,9 @@ typedef struct
     uint64_t   st_counts[4];
     uint64_t   st_dates[4];
     uint64_t   st_first;
+    
+    /* Subtitles */
+    hb_buffer_t * sub_list;   /* list of subtitles to be passed thru or rendered */
 } hb_sync_video_t;
 
 struct hb_work_private_s
@@ -549,11 +552,204 @@ int syncVideoWork( hb_work_object_t * w, hb_buffer_t ** buf_in,
     }
 
     /*
-     * Track the video sequence number localy so that we can sync the audio
+     * Track the video sequence number locally so that we can sync the audio
      * to it using the sequence number as well as the PTS.
      */
     sync->video_sequence = cur->sequence;
+    
+    /* Process subtitles that apply to this video frame */
+    
+    // NOTE: There is no logic in either subtitle-sync algorithm that waits for the
+    //       subtitle-decoder if it is lagging behind the video-decoder.
+    //       
+    //       Therefore there is the implicit assumption that the subtitle-decoder 
+    //       is always faster than the video-decoder. This assumption is definitely 
+    //       incorrect in some cases where the SSA subtitle decoder is used.
+    //       Enable the SUBSYNC_VERBOSE_TIMING flag below to debug.
+    
+#define SUBSYNC_ALGORITHM_SIMULTANEOUS 1
+#define SUBSYNC_ALGORITHM_CLASSIC 0
+
+/*
+ * Enables logging of three kinds of events:
+ *   SUB***: Subtitle received by sync object
+ *   SUB+++: Subtitle now shown
+ *   SUB---: Subtitle now hidden and disposed
+ * 
+ * Lead times on SUB*** events should be positive.
+ *   Negative lead times lead to lag times on SUB+++ or the complete drop of a subtitle.
+ * Lag times on SUB+++ and SUB--- should be small positive values in the 0-40ms range.
+ */
+#define SUBSYNC_VERBOSE_TIMING 0
+    
+#if SUBSYNC_ALGORITHM_SIMULTANEOUS
+    #define sub_list sync->sub_list
+    /*
+     * 1. Find all subtitles that need to be burned into the current video frame
+     *    and attach them to the frame.
+     * 2. Find all subtitles that need to be passed thru and do so immediately.
+     */
+    for( i = 0; i < hb_list_count( job->list_subtitle ); i++)
+    {
+        subtitle = hb_list_item( job->list_subtitle, i );
+        
+        // If this subtitle track's packets are to be passed thru, do so immediately
+        if( subtitle->config.dest == PASSTHRUSUB )
+        {
+            while ( ( sub = hb_fifo_get( subtitle->fifo_raw ) ) != NULL )
+            {
+                if ( subtitle->source == VOBSUB )
+                {
+                    hb_fifo_push( subtitle->fifo_sync, sub );
+                }
+                else
+                {
+                    hb_fifo_push( subtitle->fifo_out, sub );
+                }
+            }
+        }
+        // If this subtitle track's packets are to be rendered, identify the
+        // packets that need to be rendered on the current video frame
+        else if( subtitle->config.dest == RENDERSUB )
+        {
+            // Migrate subtitles from 'subtitle->fifo_raw' to 'sub_list' immediately.
+            // Note that the size of 'sub_list' is unbounded.
+            while ( ( sub = hb_fifo_see( subtitle->fifo_raw ) ) != NULL )
+            {
+                sub = hb_fifo_get( subtitle->fifo_raw );  // pop
+                
+                #if SUBSYNC_VERBOSE_TIMING
+                    printf( "\nSUB*** (%"PRId64"/%"PRId64":%"PRId64") @ %"PRId64"/%"PRId64":%"PRId64" (lead by %"PRId64"ms)\n",
+                        sub->start/90, sub->start/90/1000/60, sub->start/90/1000%60,
+                        cur->start/90, cur->start/90/1000/60, cur->start/90/1000%60,
+                        (sub->start - cur->start)/90);
+                    if (pv->common->video_pts_slip)
+                    {
+                        printf( "  VIDEO-LAG: %"PRId64"\n", pv->common->video_pts_slip );
+                    }
+                #endif
+                
+                // Prepend to sub_list
+                hb_buffer_t *sub_list_next = sub_list;
+                sub_list = sub;
+                sub_list->next = sub_list_next;
+            }
+            
+            hb_buffer_t *last_sub = NULL;
+            for ( sub = sub_list; sub != NULL; )
+            {
+                // NOTE: Strictly speaking this sequence check is probably unnecessary.
+                //       It is a holdover behavior inherited from the classic subsync algorithm.
+                if ( sub->sequence > cur->sequence )
+                {
+                    // Subtitle sequence in the future
+                    
+                    // (Keep the subtitle in the stream)
+                    last_sub = sub;
+                    sub = sub->next;
+                    continue;
+                }
+                
+                if ( cur->start < sub->start )
+                {
+                    // Subtitle starts in the future
+                    
+                    // (Keep the subtitle in the stream)
+                    last_sub = sub;
+                    sub = sub->next;
+                    continue;
+                }
+                else
+                {
+                    // Subtitle starts in the past...
+                    
+                    if ( cur->start < sub->stop )
+                    {
+                        // Subtitle starts in the past and finishes in the future
+                        
+                        // Attach a copy of the subtitle packet to the current video packet
+                        // to be burned in by the 'render' work-object.
+                        // (Can't just alias it because we don't know when the 'render'
+                        //  work-object will dispose of it.)
+                        hb_buffer_t * old_sublist_head = cur->sub;
+                        cur->sub = copy_subtitle( sub );
+                        cur->sub->next = old_sublist_head;
+                        
+                        #if SUBSYNC_VERBOSE_TIMING
+                            if (!(sub->new_chap & 0x01))
+                            {
+                                printf( "\nSUB+++ (%"PRId64"/%"PRId64":%"PRId64") @ %"PRId64"/%"PRId64":%"PRId64" (lag by %"PRId64"ms)\n",
+                                    sub->start/90, sub->start/90/1000/60, sub->start/90/1000%60,
+                                    cur->start/90, cur->start/90/1000/60, cur->start/90/1000%60,
+                                    (cur->start - sub->start)/90 );
+                                if (pv->common->video_pts_slip)
+                                {
+                                    printf( "  VIDEO-LAG: %"PRId64"\n", pv->common->video_pts_slip );
+                                }
+                                
+                                sub->new_chap |= 0x01;
+                            }
+                        #endif
+                        
+                        // (Keep the subtitle in the stream)
+                        last_sub = sub;
+                        sub = sub->next;
+                        continue;
+                    }
+                    else
+                    {
+                        // Subtitle starts in the past and has already finished
+                        
+                        #if SUBSYNC_VERBOSE_TIMING
+                            printf( "\nSUB--- (%"PRId64"/%"PRId64":%"PRId64") @ %"PRId64"/%"PRId64":%"PRId64" (lag by %"PRId64"ms)\n",
+                                sub->start/90, sub->start/90/1000/60, sub->start/90/1000%60,
+                                cur->start/90, cur->start/90/1000/60, cur->start/90/1000%60,
+                                (cur->start - sub->stop)/90 );
+                            if (pv->common->video_pts_slip)
+                            {
+                                printf( "  VIDEO-LAG: %"PRId64"\n", pv->common->video_pts_slip );
+                            }
+                        #endif
+                        
+                        // Remove it from the stream...
+                        if (last_sub != NULL)
+                        {
+                            last_sub->next = sub->next;
+                        }
+                        if (sub_list == sub)
+                        {
+                            sub_list = sub->next;
+                        }
+                        
+                        // ...and trash it
+                        hb_buffer_t *next_sub = sub->next;
+                        // XXX: Prevent hb_buffer_close from killing the whole list
+                        //      before we finish iterating over it
+                        sub->next = NULL;
+                        
+                        hb_buffer_t * subpicture_list = sub;
+                        hb_buffer_t * subpicture;
+                        hb_buffer_t * subpicture_next;
+                        for ( subpicture = subpicture_list; subpicture; subpicture = subpicture_next )
+                        {
+                            subpicture_next = subpicture->next_subpicture;
+                            
+                            hb_buffer_close( &subpicture );
+                        }
+                        
+                        // (last_sub remains the same)
+                        sub = next_sub;
+                        continue;
+                    }
+                }
+            }
+        }
+    } // end subtitles
+    #undef sub_list
 
+#elif SUBSYNC_ALGORITHM_CLASSIC
+    // NOTE: This algorithm does not correctly support the simultaneous display of temporally overlapping subtitles.
+    
     /*
      * Look for a subtitle for this frame.
      *
@@ -651,13 +847,19 @@ int syncVideoWork( hb_work_object_t * w, hb_buffer_t ** buf_in,
                 duration = sub->stop - sub->start;
                 sub_stop = sub_start + duration;
 
-                /* If two subtitles overlap, make the first one stop
+                /* If two DVD subtitles overlap, make the first one stop
                    when the second one starts */
-                sub2 = hb_fifo_see2( subtitle->fifo_raw );
-                if( sub2 && sub->stop > sub2->start )
+                // TODO: Consider removing this entirely. Currently retained
+                //       to preserve old DVD subtitle behavior.
+                if ( subtitle->source == VOBSUB )
                 {
-                    sub->stop = sub2->start;
+                    sub2 = hb_fifo_see2( subtitle->fifo_raw );
+                    if( sub2 && sub->stop > sub2->start )
+                    {
+                        sub->stop = sub2->start;
+                    }
                 }
+
                 
                 // hb_log("0x%x: video seq: %"PRId64" subtitle sequence: %"PRId64,
                 //       sub, cur->sequence, sub->sequence);
@@ -674,8 +876,22 @@ int syncVideoWork( hb_work_object_t * w, hb_buffer_t ** buf_in,
                     break;
                 }
                 
+                #if SUBSYNC_VERBOSE_TIMING
+                    if (!(sub->new_chap & 0x02))
+                    {
+                        printf( "\nSUB*** (%"PRId64"/%"PRId64":%"PRId64") @ %"PRId64"/%"PRId64":%"PRId64" (lead by %"PRId64"ms)\n",
+                            sub->start/90, sub->start/90/1000/60, sub->start/90/1000%60,
+                            cur->start/90, cur->start/90/1000/60, cur->start/90/1000%60,
+                            (sub->start - cur->start)/90);
+                        
+                        sub->new_chap |= 0x02;
+                    }
+                #endif
+                
                 if( sub_stop > start ) 
                 {
+                    // CONDITION: cur->start < sub->stop
+                    
                     /*
                      * The stop time is in the future, so fall through
                      * and we'll deal with it in the next block of
@@ -687,6 +903,8 @@ int syncVideoWork( hb_work_object_t * w, hb_buffer_t ** buf_in,
                      */
                     if( sub_stop > sub_start)
                     {
+                        // CONDITION: {cur->start, sub->start} < sub->stop
+                        
                         /*
                          * Normal subtitle which ends after it starts, 
                          * check to see that the current video is between 
@@ -695,6 +913,8 @@ int syncVideoWork( hb_work_object_t * w, hb_buffer_t ** buf_in,
                         if( start > sub_start &&
                             start < sub_stop )
                         {
+                            // CONDITION: sub->start < cur->start < sub->stop
+                            
                             /*
                             * We should be playing this, so leave the
                             * subtitle in place.
@@ -704,6 +924,8 @@ int syncVideoWork( hb_work_object_t * w, hb_buffer_t ** buf_in,
                         }
                         else
                         {
+                            // CONDITION: cur->start < sub->start < sub->stop
+                            
                             /*
                              * Defer until the play point is within 
                              * the subtitle
@@ -713,12 +935,16 @@ int syncVideoWork( hb_work_object_t * w, hb_buffer_t ** buf_in,
                     }
                     else
                     {
+                        // CONDITION: cur->start < sub->stop < sub->start
+                        
                         /*
                          * The end of the subtitle is less than the start, 
                          * this is a sign of a PTS discontinuity.
                          */
                         if( sub_start > start )
                         {
+                            // CONDITION: cur->start < sub->stop < sub->start
+                            
                             /*
                              * we haven't reached the start time yet, or
                              * we have jumped backwards after having
@@ -726,6 +952,8 @@ int syncVideoWork( hb_work_object_t * w, hb_buffer_t ** buf_in,
                              */
                             if( start < sub_stop )
                             {
+                                // CONDITION: cur->start < sub->stop < sub->start
+                                
                                 /*
                                  * We have jumped backwards and so should
                                  * continue displaying this subtitle.
@@ -735,6 +963,8 @@ int syncVideoWork( hb_work_object_t * w, hb_buffer_t ** buf_in,
                             }
                             else
                             {
+                                // CONDITION: Mathematically impossible to get here
+                                
                                 /*
                                  * Defer until the play point is 
                                  * within the subtitle
@@ -742,6 +972,8 @@ int syncVideoWork( hb_work_object_t * w, hb_buffer_t ** buf_in,
                                 sub = NULL;
                             }
                         } else {
+                            // CONDITION: Mathematically impossible to get here
+                            
                             /*
                             * Play this subtitle as the start is 
                             * greater than our video point.
@@ -754,6 +986,14 @@ int syncVideoWork( hb_work_object_t * w, hb_buffer_t ** buf_in,
                 }
                 else
                 {
+                    // CONDITION: sub->stop < cur->start
+                    
+                    #if SUBSYNC_VERBOSE_TIMING
+                        printf( "\nSUB--- (%"PRId64"/%"PRId64":%"PRId64") @ %"PRId64"/%"PRId64":%"PRId64" (lag by %"PRId64"ms)\n",
+                            sub->start/90, sub->start/90/1000/60, sub->start/90/1000%60,
+                            cur->start/90, cur->start/90/1000/60, cur->start/90/1000%60,
+                            (cur->start - sub->stop)/90 );
+                    #endif
                     
                     /*
                      * The subtitle is older than this picture, trash it
@@ -766,24 +1006,39 @@ int syncVideoWork( hb_work_object_t * w, hb_buffer_t ** buf_in,
             /* If we have a subtitle for this picture, copy it */
             if( sub )
             {
+                #if SUBSYNC_VERBOSE_TIMING
+                    if (!(sub->new_chap & 0x01))
+                    {
+                        printf( "\nSUB+++ (%"PRId64"/%"PRId64":%"PRId64") @ %"PRId64"/%"PRId64":%"PRId64" (lag by %"PRId64"ms)\n",
+                            sub->start/90, sub->start/90/1000/60, sub->start/90/1000%60,
+                            cur->start/90, cur->start/90/1000/60, cur->start/90/1000%60,
+                            (cur->start - sub->start)/90 );
+                        
+                        sub->new_chap |= 0x01;
+                    }
+                #endif
+                
                 if( sub->size > 0 )
                 {
                     if( subtitle->config.dest == RENDERSUB )
                     {
-                        // Only allow one subtitle to be showing at once; ignore others
-                        if ( cur->sub == NULL )
-                        {
-                            /*
-                             * Tack onto the video buffer for rendering
-                             */
-                            /* FIXME: we should avoid this memcpy */
-                            cur->sub = copy_subtitle( sub );
-                            cur->sub->start = sub_start;
-                            cur->sub->stop = sub_stop;
+                        /*
+                         * Tack onto the video buffer for rendering.
+                         * 
+                         * Note that there may be multiple subtitles
+                         * whose time intervals overlap which must display
+                         * on the same frame.
+                         */
+                        hb_buffer_t * old_sublist_head = cur->sub;
+                        
+                        /* FIXME: we should avoid this memcpy */
+                        cur->sub = copy_subtitle( sub );
+                        cur->sub->next = old_sublist_head;
+                        cur->sub->start = sub_start;
+                        cur->sub->stop = sub_stop;
                             
-                            // Leave the subtitle on the raw queue
-                            // (until it no longer needs to be displayed)
-                        }
+                        // Leave the subtitle on the raw queue
+                        // (until it no longer needs to be displayed)
                     } else {
                         /*
                          * Pass-Through, pop it off of the raw queue, 
@@ -811,6 +1066,9 @@ int syncVideoWork( hb_work_object_t * w, hb_buffer_t ** buf_in,
             }
         }
     } // end subtitles
+#else
+    #error "Must select a subtitle sync algorithm."
+#endif
 
     /*
      * Adjust the pts of the current frame so that it's contiguous