lavfi/ass: extend syntax for ass filter

Make the filter accept named options for the first argument, and update
documentation accordingly.

doc/filters.texi

@@ -1230,26 +1230,33 @@ using the libass library.
 To enable compilation of this filter you need to configure FFmpeg with
 @code{--enable-libass}.
 
-This filter accepts the syntax: @var{ass_filename}[:@var{options}],
-where @var{ass_filename} is the filename of the ASS file to read, and
-@var{options} is an optional sequence of @var{key}=@var{value} pairs,
-separated by ":".
-
-A description of the accepted options follows.
+This filter accepts the following named options, expressed as a
+sequence of @var{key}=@var{value} pairs, separated by ":".
 
 @table @option
+@item filename, f
+Set the filename of the ASS file to read. It must be specified.
+
 @item original_size
-Specifies the size of the original video, the video for which the ASS file
+Specify the size of the original video, the video for which the ASS file
 was composed. Due to a misdesign in ASS aspect ratio arithmetic, this is
 necessary to correctly scale the fonts if the aspect ratio has been changed.
 @end table
 
+If the first key is not specified, it is assumed that the first value
+specifies the @option{filename}.
+
 For example, to render the file @file{sub.ass} on top of the input
 video, use the command:
 @example
 ass=sub.ass
 @end example
 
+which is equivalent to:
+@example
+ass=filename=sub.ass
+@end example
+
 @section bbox
 
 Compute the bounding box for the non-black pixels in the input frame

libavfilter/version.h

@@ -30,7 +30,7 @@
 
 #define LIBAVFILTER_VERSION_MAJOR  3
 #define LIBAVFILTER_VERSION_MINOR  19
-#define LIBAVFILTER_VERSION_MICRO 102
+#define LIBAVFILTER_VERSION_MICRO 103
 
 #define LIBAVFILTER_VERSION_INT AV_VERSION_INT(LIBAVFILTER_VERSION_MAJOR, \
                                                LIBAVFILTER_VERSION_MINOR, \

libavfilter/vf_ass.c

@@ -54,6 +54,8 @@ typedef struct {
 #define FLAGS AV_OPT_FLAG_FILTERING_PARAM|AV_OPT_FLAG_VIDEO_PARAM
 
 static const AVOption ass_options[] = {
+    {"filename",       "set the filename of the ASS file to read",                 OFFSET(filename),   AV_OPT_TYPE_STRING,     {.str = NULL},  CHAR_MIN, CHAR_MAX, FLAGS },
+    {"f",              "set the filename of the ASS file to read",                 OFFSET(filename),   AV_OPT_TYPE_STRING,     {.str = NULL},  CHAR_MIN, CHAR_MAX, FLAGS },
     {"original_size",  "set the size of the original video (used to scale fonts)", OFFSET(original_w), AV_OPT_TYPE_IMAGE_SIZE, {.str = NULL},  CHAR_MIN, CHAR_MAX, FLAGS },
     {NULL},
 };
@@ -83,21 +85,20 @@ static void ass_log(int ass_level, const char *fmt, va_list args, void *ctx)
 static av_cold int init(AVFilterContext *ctx, const char *args)
 {
     AssContext *ass = ctx->priv;
+    static const char *shorthand[] = { "filename", NULL };
     int ret;
 
     ass->class = &ass_class;
     av_opt_set_defaults(ass);
 
-    if (args)
-        ass->filename = av_get_token(&args, ":");
-    if (!ass->filename || !*ass->filename) {
+    if ((ret = av_opt_set_from_string(ass, args, shorthand, "=", ":")) < 0)
+        return ret;
+
+    if (!ass->filename) {
         av_log(ctx, AV_LOG_ERROR, "No filename provided!\n");
         return AVERROR(EINVAL);
     }
 
-    if (*args++ == ':' && (ret = av_set_options_string(ass, args, "=", ":")) < 0)
-        return ret;
-
     ass->library = ass_library_init();
     if (!ass->library) {
         av_log(ctx, AV_LOG_ERROR, "Could not initialize libass.\n");
@@ -127,7 +128,7 @@ static av_cold void uninit(AVFilterContext *ctx)
 {
     AssContext *ass = ctx->priv;
 
-    av_freep(&ass->filename);
+    av_opt_free(ass);
     if (ass->track)
         ass_free_track(ass->track);
     if (ass->renderer)