[FFmpeg-devel] [PATCH] Use custom option to specify WebVTT kind

[Matthew Heaney]

WebVTT subtitle tracks have four kinds. Certain downstream muxers
(such as for WebM) need to know which WebVTT kind this is, in order to
specify the codec id of the output track.

A new custom input option, "-kind", has been added to the WebVTT
demuxer.  It accepts as a value any of "subtitles" (the default),
"captions", "descriptions", and "metadata".  The kind option value is
used to assign a value to the stream disposition flag, to which four
new values have been added, corresponding the four WebVTT kinds.
---
 libavformat/avformat.h  |  8 ++++++++
 libavformat/webvttdec.c | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/libavformat/avformat.h b/libavformat/avformat.h
index 1d7ba45..a217420 100644
--- a/libavformat/avformat.h
+++ b/libavformat/avformat.h
@@ -636,6 +636,14 @@ typedef struct AVIndexEntry {
 #define AV_DISPOSITION_ATTACHED_PIC      0x0400
 
 /**
+ * To specify the WebVTT kind.
+ */
+#define AV_DISPOSITION_SUBTITLES    0x1000
+#define AV_DISPOSITION_CAPTIONS     0x2000
+#define AV_DISPOSITION_DESCRIPTIONS 0x4000
+#define AV_DISPOSITION_METADATA     0x8000
+
+/**
  * Options for behavior on timestamp wrap detection.
  */
 #define AV_PTS_WRAP_IGNORE      0   ///< ignore the wrap
diff --git a/libavformat/webvttdec.c b/libavformat/webvttdec.c
index 7d9910b..9843b82 100644
--- a/libavformat/webvttdec.c
+++ b/libavformat/webvttdec.c
@@ -29,9 +29,12 @@
 #include "subtitles.h"
 #include "libavutil/bprint.h"
 #include "libavutil/intreadwrite.h"
+#include "libavutil/opt.h"
 
 typedef struct {
+    const AVClass *class;
     FFDemuxSubtitlesQueue q;
+    char* kind;
 } WebVTTContext;
 
 static int webvtt_probe(AVProbeData *p)
@@ -67,6 +70,18 @@ static int webvtt_read_header(AVFormatContext *s)
     st->codec->codec_type = AVMEDIA_TYPE_SUBTITLE;
     st->codec->codec_id   = AV_CODEC_ID_WEBVTT;
 
+    if (!av_strcasecmp(webvtt->kind, "subtitles")) {
+      st->disposition = AV_DISPOSITION_SUBTITLES;
+    } else if (!av_strcasecmp(webvtt->kind, "captions")) {
+      st->disposition = AV_DISPOSITION_CAPTIONS;
+    } else if (!av_strcasecmp(webvtt->kind, "descriptions")) {
+      st->disposition = AV_DISPOSITION_DESCRIPTIONS;
+    } else if (!av_strcasecmp(webvtt->kind, "metadata")) {
+      st->disposition = AV_DISPOSITION_METADATA;
+    } else {
+      return AVERROR(EINVAL);
+    }
+
     av_bprint_init(&header, 0, AV_BPRINT_SIZE_UNLIMITED);
     av_bprint_init(&cue,    0, AV_BPRINT_SIZE_UNLIMITED);
 
@@ -186,6 +201,23 @@ static int webvtt_read_close(AVFormatContext *s)
     return 0;
 }
 
+#define OFFSET(x) offsetof(WebVTTContext, x)
+#define KIND_FLAGS AV_OPT_FLAG_SUBTITLE_PARAM
+
+static const AVOption options[] = {
+    { "kind", "Kind of WebVTT track",
+      OFFSET(kind), AV_OPT_TYPE_STRING, { .str = "subtitles" },
+      0, 0, KIND_FLAGS, NULL },
+    { NULL },
+};
+
+static const AVClass webvtt_demuxer_class = {
+    .class_name  = "WebVTT demuxer",
+    .item_name   = av_default_item_name,
+    .option      = options,
+    .version     = LIBAVUTIL_VERSION_INT,
+};
+
 AVInputFormat ff_webvtt_demuxer = {
     .name           = "webvtt",
     .long_name      = NULL_IF_CONFIG_SMALL("WebVTT subtitle"),
@@ -196,4 +228,5 @@ AVInputFormat ff_webvtt_demuxer = {
     .read_seek2     = webvtt_read_seek,
     .read_close     = webvtt_read_close,
     .extensions     = "vtt",
+    .priv_class     = &webvtt_demuxer_class,
 };
-- 
1.8.3