lavf/webvttdec: use private option to specify WebVTT kind

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 private 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.
Signed-off-by: Matthew Heaney <matthewjheaney@google.com>
Signed-off-by: Clément Bœsch <ubitux@gmail.com>

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;
+    int kind;
 } WebVTTContext;
 
 static int webvtt_probe(AVProbeData *p)
@@ -66,6 +69,7 @@ static int webvtt_read_header(AVFormatContext *s)
     avpriv_set_pts_info(st, 64, 1, 1000);
     st->codec->codec_type = AVMEDIA_TYPE_SUBTITLE;
     st->codec->codec_id   = AV_CODEC_ID_WEBVTT;
+    st->disposition |= webvtt->kind;
 
     av_bprint_init(&header, 0, AV_BPRINT_SIZE_UNLIMITED);
     av_bprint_init(&cue,    0, AV_BPRINT_SIZE_UNLIMITED);
@@ -186,6 +190,25 @@ 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", "Set kind of WebVTT track", OFFSET(kind), AV_OPT_TYPE_INT, { .i64 = 0 }, 0, INT_MAX, KIND_FLAGS, "webvtt_kind" },
+        { "subtitles",    "WebVTT subtitles kind",    0, AV_OPT_TYPE_CONST, { .i64 = 0 },                           INT_MIN, INT_MAX, 0, "webvtt_kind" },
+        { "captions",     "WebVTT captions kind",     0, AV_OPT_TYPE_CONST, { .i64 = AV_DISPOSITION_CAPTIONS },     INT_MIN, INT_MAX, 0, "webvtt_kind" },
+        { "descriptions", "WebVTT descriptions kind", 0, AV_OPT_TYPE_CONST, { .i64 = AV_DISPOSITION_DESCRIPTIONS }, INT_MIN, INT_MAX, 0, "webvtt_kind" },
+        { "metadata",     "WebVTT metadata kind",     0, AV_OPT_TYPE_CONST, { .i64 = AV_DISPOSITION_METADATA },     INT_MIN, INT_MAX, 0, "webvtt_kind" },
+    { 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 +219,5 @@ AVInputFormat ff_webvtt_demuxer = {
     .read_seek2     = webvtt_read_seek,
     .read_close     = webvtt_read_close,
     .extensions     = "vtt",
+    .priv_class     = &webvtt_demuxer_class,
 };