Subtitles: Allow transitions between ATSC and SCTE captions.

In changeset d526385 (refs #9829),
code was added such that the first time ATSC caption data is seen, any
future SCTE caption data is explicitly ignored.  This was to deal with
broadcasts containing duplicate caption data.

This policy causes problems when the recording switches back and forth
between the two sources of caption data, such as when the broadcaster
splices in commercials.

The updated solution continues to favor ATSC data, but allows a switch
back to SCTE after SCTE data is seen 10 times in a row (a somewhat
arbitrarily chosen number) without intervening ATSC data.

Refs #12054.

(cherry picked from commit 68bd03f)

mythtv/libs/libmythtv/avformatdecoder.cpp

@@ -410,7 +410,7 @@ AvFormatDecoder::AvFormatDecoder(MythPlayer *parent,
       video_codec_id(kCodec_NONE),
       maxkeyframedist(-1),
       // Closed Caption & Teletext decoders
-      ignore_scte(false),
+      ignore_scte(0),
       invert_scte_field(0),
       last_scte_field(0),
       ccd608(new CC608Decoder(parent->GetCC608Reader())),
@@ -3676,17 +3676,31 @@ bool AvFormatDecoder::ProcessVideoFrame(AVStream *stream, AVFrame *mpa_pic)
 {
     AVCodecContext *context = stream->codec;
 
+    // We need to mediate between ATSC and SCTE data when both are present.  If
+    // both are present, we generally want to prefer ATSC.  However, there may
+    // be large sections of the recording where ATSC is used and other sections
+    // where SCTE is used.  In that case, we want to allow a natural transition
+    // from ATSC back to SCTE.  We do this by allowing 10 consecutive SCTE
+    // frames, without an intervening ATSC frame, to cause a switch back to
+    // considering SCTE frames.  The number 10 is somewhat arbitrarily chosen.
+
     uint cc_len = (uint) max(mpa_pic->scte_cc_len,0);
     uint8_t *cc_buf = mpa_pic->scte_cc_buf;
     bool scte = true;
 
+    // If we saw SCTE, then decrement a nonzero ignore_scte count.
+    if (cc_len > 0 && ignore_scte)
+        --ignore_scte;
+
     // If both ATSC and SCTE caption data are available, prefer ATSC
     if ((mpa_pic->atsc_cc_len > 0) || ignore_scte)
     {
-        ignore_scte = true;
         cc_len = (uint) max(mpa_pic->atsc_cc_len, 0);
         cc_buf = mpa_pic->atsc_cc_buf;
         scte = false;
+        // If we explicitly saw ATSC, then reset ignore_scte count.
+        if (cc_len > 0)
+            ignore_scte = 10;
     }
 
     // Decode CEA-608 and CEA-708 captions

mythtv/libs/libmythtv/avformatdecoder.h

@@ -328,7 +328,7 @@ class AvFormatDecoder : public DecoderBase
     int maxkeyframedist;
 
     // Caption/Subtitle/Teletext decoders
-    bool             ignore_scte;
+    uint             ignore_scte;
     uint             invert_scte_field;
     uint             last_scte_field;
     CC608Decoder     *ccd608;