[FFmpeg-devel] [PATCH 2/4] avformat/ttmlenc: enable writing out additional header values

Tue Mar 30 11:23:44 EEST 2021

From: Jan Ekström <jan.ekstrom at 24i.com>

This way the encoder may pass on the following values to the muxer:
1) Additional root "tt" element attributes, such as the subtitle
   canvas reference size.
2) Anything before the body element of the document, such as regions
   in the head element, which can configure styles.

Signed-off-by: Jan Ekström <jan.ekstrom at 24i.com>
---
 libavcodec/ttmlenc.h  |  5 +++
 libavformat/ttmlenc.c | 78 ++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 78 insertions(+), 5 deletions(-)

diff --git a/libavcodec/ttmlenc.h b/libavcodec/ttmlenc.h
index c1dd5ec990..c3bb11478d 100644
--- a/libavcodec/ttmlenc.h
+++ b/libavcodec/ttmlenc.h
@@ -25,4 +25,9 @@
 #define TTMLENC_EXTRADATA_SIGNATURE "lavc-ttmlenc"
 #define TTMLENC_EXTRADATA_SIGNATURE_SIZE (sizeof(TTMLENC_EXTRADATA_SIGNATURE) - 1)
 
+static const char ttml_default_namespacing[] =
+"  xmlns=\"http://www.w3.org/ns/ttml\"\n"
+"  xmlns:ttm=\"http://www.w3.org/ns/ttml#metadata\"\n"
+"  xmlns:tts=\"http://www.w3.org/ns/ttml#styling\"\n";
+
 #endif /* AVCODEC_TTMLENC_H */
diff --git a/libavformat/ttmlenc.c b/libavformat/ttmlenc.c
index 940f8bbd4e..5d9ad6b756 100644
--- a/libavformat/ttmlenc.c
+++ b/libavformat/ttmlenc.c
@@ -37,18 +37,23 @@ enum TTMLPacketType {
     PACKET_TYPE_DOCUMENT,
 };
 
+struct TTMLHeaderParameters {
+    char *tt_element_params;
+    char *pre_body_elements;
+};
+
 typedef struct TTMLMuxContext {
     enum TTMLPacketType input_type;
     unsigned int document_written;
+    struct TTMLHeaderParameters header_params;
 } TTMLMuxContext;
 
 static const char ttml_header_text[] =
 "<?xml version=\"1.0\" encoding=\"utf-8\"?>\n"
 "<tt\n"
-"  xmlns=\"http://www.w3.org/ns/ttml\"\n"
-"  xmlns:ttm=\"http://www.w3.org/ns/ttml#metadata\"\n"
-"  xmlns:tts=\"http://www.w3.org/ns/ttml#styling\"\n"
+"%s"
 "  xml:lang=\"%s\">\n"
+"%s"
 "  <body>\n"
 "    <div>\n";
 
@@ -72,6 +77,47 @@ static void ttml_write_time(AVIOContext *pb, const char tag[],
                 tag, hour, min, sec, millisec);
 }
 
+static int ttml_set_header_values_from_extradata(
+    AVCodecParameters *par, struct TTMLHeaderParameters *header_params)
+{
+    size_t additional_data_size =
+        par->extradata_size - TTMLENC_EXTRADATA_SIGNATURE_SIZE;
+
+    if (!additional_data_size) {
+        header_params->tt_element_params =
+            av_strndup(ttml_default_namespacing,
+                       sizeof(ttml_default_namespacing) - 1);
+        header_params->pre_body_elements = av_strndup("", 1);
+
+        if (!header_params->tt_element_params ||
+            !header_params->pre_body_elements)
+            return AVERROR(ENOMEM);
+
+        return 0;
+    }
+
+    {
+        char *value =
+            (char *)par->extradata + TTMLENC_EXTRADATA_SIGNATURE_SIZE;
+        size_t value_size = av_strnlen(value, additional_data_size);
+
+        if (!(header_params->tt_element_params = av_strndup(value, value_size)))
+            return AVERROR(ENOMEM);
+
+        additional_data_size -= value_size + 1;
+        value += value_size + 1;
+        if (additional_data_size <= 0)
+            return AVERROR_INVALIDDATA;
+
+        value_size = av_strnlen(value, additional_data_size);
+
+        if (!(header_params->pre_body_elements = av_strndup(value, value_size)))
+            return AVERROR(ENOMEM);
+
+        return 0;
+    }
+}
+
 static int ttml_write_header(AVFormatContext *ctx)
 {
     TTMLMuxContext *ttml_ctx = ctx->priv_data;
@@ -103,8 +149,21 @@ static int ttml_write_header(AVFormatContext *ctx)
 
         avpriv_set_pts_info(st, 64, 1, 1000);
 
-        if (ttml_ctx->input_type == PACKET_TYPE_PARAGRAPH)
-            avio_printf(pb, ttml_header_text, printed_lang);
+        if (ttml_ctx->input_type == PACKET_TYPE_PARAGRAPH) {
+            int ret = ttml_set_header_values_from_extradata(
+                st->codecpar, &ttml_ctx->header_params);
+            if (ret < 0) {
+                av_log(ctx, AV_LOG_ERROR,
+                       "Failed to parse TTML header values from extradata: "
+                       "%s!\n", av_err2str(ret));
+                return ret;
+            }
+
+            avio_printf(pb, ttml_header_text,
+                        ttml_ctx->header_params.tt_element_params,
+                        printed_lang,
+                        ttml_ctx->header_params.pre_body_elements);
+        }
     }
 
     return 0;
@@ -159,6 +218,14 @@ static int ttml_write_trailer(AVFormatContext *ctx)
     return 0;
 }
 
+static void ttml_free(AVFormatContext *ctx)
+{
+    TTMLMuxContext *ttml_ctx = ctx->priv_data;
+
+    av_freep(&(ttml_ctx->header_params.tt_element_params));
+    av_freep(&(ttml_ctx->header_params.pre_body_elements));
+}
+
 AVOutputFormat ff_ttml_muxer = {
     .name              = "ttml",
     .long_name         = NULL_IF_CONFIG_SMALL("TTML subtitle"),
@@ -171,4 +238,5 @@ AVOutputFormat ff_ttml_muxer = {
     .write_header      = ttml_write_header,
     .write_packet      = ttml_write_packet,
     .write_trailer     = ttml_write_trailer,
+    .deinit            = ttml_free,
 };
-- 
2.30.2

