avfilter/metadata: add intuitive labels for metadata values

doc/filters.texi

@@ -25296,10 +25296,10 @@ The expression is evaluated through the eval API and can contain the following
 constants:
 
 @table @option
-@item VALUE1
+@item VALUE1, FRAMEVAL
 Float representation of @code{value} from metadata key.
 
-@item VALUE2
+@item VALUE2, USERVAL
 Float representation of @code{value} as supplied by user in @code{value} option.
 @end table
 

libavfilter/f_metadata.c

@@ -61,12 +61,16 @@ enum MetadataFunction {
 static const char *const var_names[] = {
     "VALUE1",
     "VALUE2",
+    "FRAMEVAL",
+    "USERVAL",
     NULL
 };
 
 enum var_name {
     VAR_VALUE1,
     VAR_VALUE2,
+    VAR_FRAMEVAL,
+    VAR_USERVAL,
     VAR_VARS_NB
 };
 
@@ -172,8 +176,8 @@ static int parse_expr(MetadataContext *s, const char *value1, const char *value2
     if (sscanf(value1, "%lf", &f1) + sscanf(value2, "%lf", &f2) != 2)
         return 0;
 
-    s->var_values[VAR_VALUE1] = f1;
-    s->var_values[VAR_VALUE2] = f2;
+    s->var_values[VAR_VALUE1] = s->var_values[VAR_FRAMEVAL] = f1;
+    s->var_values[VAR_VALUE2] = s->var_values[VAR_USERVAL]  = f2;
 
     return av_expr_eval(s->expr, s->var_values, NULL);
 }