[FFmpeg-devel] [PATCH] avcodec/mpeg12dec: extract only one type of CC substream

[Marth64]

In MPEG-2 user data, there can be different types of Closed Captions
formats embedded (A53, SCTE-20, or DVD). The current behavior of the
CC extraction code in the MPEG-2 decoder is to not be aware of
multiple formats if multiple exist, therefore allowing one format
to overwrite the other during the extraction process since the CC
extraction shares one output buffer for the normalized bytes.

This causes sources that have two CC formats to produce flawed output.
There exist real-world samples which contain both A53 and SCTE-20 captions
in the same MPEG-2 stream, and that manifest this problem. Example of symptom:
THANK YOU (expected) --> THTHANANK K YOYOUU (actual)

The solution is to pick only the first CC substream observed with valid bytes,
and ignore the other types. Additionally, provide an option for users
to manually "force" a type in the event that this matters for a particular source.

In tandem, I am working on an improvement to src_movie to allow passing decoder
options (as src_movie via lavfi is the "de facto" way to extract CCs right now).
This way, users can realize the newly added option.

Signed-off-by: Marth64 <marth64 at proxyid.net>
---
 libavcodec/mpeg12dec.c | 49 +++++++++++++++++++++++++++++++++++++++---
 1 file changed, 46 insertions(+), 3 deletions(-)

diff --git a/libavcodec/mpeg12dec.c b/libavcodec/mpeg12dec.c
index 3a2f17e508..a42e1c661f 100644
--- a/libavcodec/mpeg12dec.c
+++ b/libavcodec/mpeg12dec.c
@@ -62,6 +62,13 @@
 
 #define A53_MAX_CC_COUNT 2000
 
+enum Mpeg2ClosedCaptionsFormat {
+    CC_FORMAT_AUTO,
+    CC_FORMAT_A53_PART4,
+    CC_FORMAT_SCTE20,
+    CC_FORMAT_DVD
+};
+
 typedef struct Mpeg1Context {
     MpegEncContext mpeg_enc_ctx;
     int mpeg_enc_ctx_allocated; /* true if decoding context allocated */
@@ -70,6 +77,7 @@ typedef struct Mpeg1Context {
     AVStereo3D stereo3d;
     int has_stereo3d;
     AVBufferRef *a53_buf_ref;
+    enum Mpeg2ClosedCaptionsFormat cc_format;
     uint8_t afd;
     int has_afd;
     int slice_count;
@@ -1908,7 +1916,8 @@ static int mpeg_decode_a53_cc(AVCodecContext *avctx,
 {
     Mpeg1Context *s1 = avctx->priv_data;
 
-    if (buf_size >= 6 &&
+    if ((!s1->cc_format || s1->cc_format == CC_FORMAT_A53_PART4) &&
+        buf_size >= 6 &&
         p[0] == 'G' && p[1] == 'A' && p[2] == '9' && p[3] == '4' &&
         p[4] == 3 && (p[5] & 0x40)) {
         /* extract A53 Part 4 CC data */
@@ -1927,9 +1936,15 @@ static int mpeg_decode_a53_cc(AVCodecContext *avctx,
                 memcpy(s1->a53_buf_ref->data + old_size, p + 7, cc_count * UINT64_C(3));
 
             avctx->properties |= FF_CODEC_PROPERTY_CLOSED_CAPTIONS;
+
+            if (!s1->cc_format) {
+                s1->cc_format = CC_FORMAT_A53_PART4;
+                av_log(avctx, AV_LOG_DEBUG, "CC: first seen substream is A53 format\n");
+            }
         }
         return 1;
-    } else if (buf_size >= 2 &&
+    } else if ((!s1->cc_format || s1->cc_format == CC_FORMAT_SCTE20) &&
+               buf_size >= 2 &&
                p[0] == 0x03 && (p[1]&0x7f) == 0x01) {
         /* extract SCTE-20 CC data */
         GetBitContext gb;
@@ -1974,9 +1989,15 @@ static int mpeg_decode_a53_cc(AVCodecContext *avctx,
                 }
             }
             avctx->properties |= FF_CODEC_PROPERTY_CLOSED_CAPTIONS;
+
+            if (!s1->cc_format) {
+                s1->cc_format = CC_FORMAT_SCTE20;
+                av_log(avctx, AV_LOG_DEBUG, "CC: first seen substream is SCTE-20 format\n");
+            }
         }
         return 1;
-    } else if (buf_size >= 11 &&
+    } else if ((!s1->cc_format || s1->cc_format == CC_FORMAT_DVD) &&
+               buf_size >= 11 &&
                p[0] == 'C' && p[1] == 'C' && p[2] == 0x01 && p[3] == 0xf8) {
         /* extract DVD CC data
          *
@@ -2034,6 +2055,11 @@ static int mpeg_decode_a53_cc(AVCodecContext *avctx,
                 }
             }
             avctx->properties |= FF_CODEC_PROPERTY_CLOSED_CAPTIONS;
+
+            if (!s1->cc_format) {
+                s1->cc_format = CC_FORMAT_DVD;
+                av_log(avctx, AV_LOG_DEBUG, "CC: first seen substream is DVD format\n");
+            }
         }
         return 1;
     }
@@ -2598,11 +2624,28 @@ const FFCodec ff_mpeg1video_decoder = {
                            },
 };
 
+#define M2V_OFFSET(x) offsetof(Mpeg1Context, x)
+#define M2V_PARAM     AV_OPT_FLAG_VIDEO_PARAM | AV_OPT_FLAG_DECODING_PARAM
+
+static const AVOption mpeg2video_options[] = {
+    { "cc_format", "extract a specific Closed Captions format (0=auto)", M2V_OFFSET(cc_format), AV_OPT_TYPE_INT, { .i64 = 0 }, CC_FORMAT_AUTO, CC_FORMAT_DVD, M2V_PARAM },
+    { NULL }
+};
+
+static const AVClass mpeg2video_class = {
+    .class_name = "MPEG-2 video",
+    .item_name  = av_default_item_name,
+    .option     = mpeg2video_options,
+    .version    = LIBAVUTIL_VERSION_INT,
+    .category   = AV_CLASS_CATEGORY_DECODER,
+};
+
 const FFCodec ff_mpeg2video_decoder = {
     .p.name         = "mpeg2video",
     CODEC_LONG_NAME("MPEG-2 video"),
     .p.type         = AVMEDIA_TYPE_VIDEO,
     .p.id           = AV_CODEC_ID_MPEG2VIDEO,
+    .p.priv_class   = &mpeg2video_class,
     .priv_data_size = sizeof(Mpeg1Context),
     .init           = mpeg_decode_init,
     .close          = mpeg_decode_end,
-- 
2.34.1