avfilter/vf_drawtext: add optional default value to metadata function

Signed-off-by: Tobias Rapp <t.rapp@noa-archive.com>

doc/filters.texi

@@ -6682,7 +6682,12 @@ The time at which the filter is running, expressed in the local time zone.
 It can accept an argument: a strftime() format string.
 
 @item metadata
-Frame metadata. It must take one argument specifying metadata key.
+Frame metadata. Takes one or two arguments.
+
+The first argument is mandatory and specifies the metadata key.
+
+The second argument is optional and specifies a default value, used when the
+metadata key is not found or empty.
 
 @item n, frame_num
 The frame number, starting from 0.

libavfilter/version.h

@@ -31,7 +31,7 @@
 
 #define LIBAVFILTER_VERSION_MAJOR   6
 #define LIBAVFILTER_VERSION_MINOR  41
-#define LIBAVFILTER_VERSION_MICRO 100
+#define LIBAVFILTER_VERSION_MICRO 101
 
 #define LIBAVFILTER_VERSION_INT AV_VERSION_INT(LIBAVFILTER_VERSION_MAJOR, \
                                                LIBAVFILTER_VERSION_MINOR, \

libavfilter/vf_drawtext.c

@@ -860,6 +860,8 @@ static int func_metadata(AVFilterContext *ctx, AVBPrint *bp,
 
     if (e && e->value)
         av_bprintf(bp, "%s", e->value);
+    else if (argc >= 2)
+        av_bprintf(bp, "%s", argv[1]);
     return 0;
 }
 
@@ -975,7 +977,7 @@ static const struct drawtext_function {
     { "localtime", 0, 1, 'L', func_strftime },
     { "frame_num", 0, 0, 0,   func_frame_num },
     { "n",         0, 0, 0,   func_frame_num },
-    { "metadata",  1, 1, 0,   func_metadata },
+    { "metadata",  1, 2, 0,   func_metadata },
 };
 
 static int eval_function(AVFilterContext *ctx, AVBPrint *bp, char *fct,