libavformat/hlsplaylist: add subtitle_varname for naming subtitle streams

If 'sname:*' is set in the var_stream_map variable, use it as
the NAME attribute for subtitles. This improves the naming of
subtitle streams in HTML players, providing clearer and more
descriptive labels for users.

Reviewed-by: Steven Liu <lq@chinaffmpeg.org>

doc/muxers.texi

@@ -2436,13 +2436,14 @@ ffmpeg -re -i in.ts -b:a:0 32k -b:a:1 64k -b:v:0 1000k \
 @item
 Create a single variant stream. Add the @code{#EXT-X-MEDIA} tag with
 @code{TYPE=SUBTITLES} in the master playlist with webvtt subtitle group name
-'subtitle'. Make sure the input file has one text subtitle stream at least.
+'subtitle' and optional subtitle name, e.g. 'English'. Make sure the input
+file has one text subtitle stream at least.
 @example
 ffmpeg -y -i input_with_subtitle.mkv \
  -b:v:0 5250k -c:v h264 -pix_fmt yuv420p -profile:v main -level 4.1 \
  -b:a:0 256k \
  -c:s webvtt -c:a mp2 -ar 48000 -ac 2 -map 0:v -map 0:a:0 -map 0:s:0 \
- -f hls -var_stream_map "v:0,a:0,s:0,sgroup:subtitle" \
+ -f hls -var_stream_map "v:0,a:0,s:0,sgroup:subtitle,sname:English" \
  -master_pl_name master.m3u8 -t 300 -hls_time 10 -hls_init_time 4 -hls_list_size \
  10 -master_pl_publish_rate 10 -hls_flags \
  delete_segments+discont_start+split_by_time ./tmp/video.m3u8

libavformat/hlsenc.c

@@ -189,6 +189,7 @@ typedef struct VariantStream {
     const char *sgroup;   /* subtitle group name */
     const char *ccgroup;  /* closed caption group name */
     const char *varname;  /* variant name */
+    const char *subtitle_varname;  /* subtitle variant name */
 } VariantStream;
 
 typedef struct ClosedCaptionsStream {
@@ -1563,7 +1564,8 @@ static int create_master_playlist(AVFormatContext *s,
                 break;
             }
 
-            ff_hls_write_subtitle_rendition(hls->m3u8_out, sgroup, vtt_m3u8_rel_name, vs->language, i, hls->has_default_key ? vs->is_default : 1);
+            ff_hls_write_subtitle_rendition(hls->m3u8_out, sgroup, vtt_m3u8_rel_name, vs->language,
+                    vs->subtitle_varname, i, hls->has_default_key ? vs->is_default : 1);
         }
 
         if (!hls->has_default_key || !hls->has_video_m3u8) {
@@ -2137,6 +2139,9 @@ static int parse_variant_stream_mapstring(AVFormatContext *s)
             } else if (av_strstart(keyval, "name:", &val)) {
                 vs->varname  = val;
                 continue;
+            } else if (av_strstart(keyval, "sname:", &val)) {
+                vs->subtitle_varname  = val;
+                continue;
             } else if (av_strstart(keyval, "agroup:", &val)) {
                 vs->agroup   = val;
                 continue;

libavformat/hlsplaylist.c

@@ -57,13 +57,18 @@ void ff_hls_write_audio_rendition(AVIOContext *out, const char *agroup,
 
 void ff_hls_write_subtitle_rendition(AVIOContext *out, const char *sgroup,
                                      const char *filename, const char *language,
-                                     int name_id, int is_default)
+                                     const char *sname, int name_id, int is_default)
 {
     if (!out || !filename)
         return;
 
     avio_printf(out, "#EXT-X-MEDIA:TYPE=SUBTITLES,GROUP-ID=\"%s\"", sgroup);
-    avio_printf(out, ",NAME=\"subtitle_%d\",DEFAULT=%s,", name_id, is_default ? "YES" : "NO");
+    if (sname) {
+        avio_printf(out, ",NAME=\"%s\",", sname);
+    } else {
+        avio_printf(out, ",NAME=\"subtitle_%d\",", name_id);
+    }
+    avio_printf(out, "DEFAULT=%s,", is_default ? "YES" : "NO");
     if (language) {
         avio_printf(out, "LANGUAGE=\"%s\",", language);
     }

libavformat/hlsplaylist.h

@@ -41,7 +41,7 @@ void ff_hls_write_audio_rendition(AVIOContext *out, const char *agroup,
                                   int name_id, int is_default, int nb_channels);
 void ff_hls_write_subtitle_rendition(AVIOContext *out, const char *sgroup,
                                      const char *filename, const char *language,
-                                     int name_id, int is_default);
+                                     const char *sname, int name_id, int is_default);
 void ff_hls_write_stream_info(AVStream *st, AVIOContext *out, int bandwidth,
                               int avg_bandwidth,
                               const char *filename, const char *agroup,