options: slightly improve filter help output for lavfi bridge

--vf=help will now list libavfilter filters, and e.g. --vf=yadif=help
will list libavfilter filter options.

The latter is rather bare, because the AVOption API is really awful
(holy shit how is it so bad), and would require us to handle _every_
option type manually.

Alternatively we could call av_opt_show2(), which ffmpeg uses for help
output in its CLI tools and which is much more detailed. But it's rather
foreign and forces output through av_log(), so I don't really want to
use it.

1 files changed, 47 insertions, 4 deletions

diff --git a/filters/f_lavfi.c b/filters/f_lavfi.c
index b3f74b508b..745d1699a7 100644
--- a/filters/f_lavfi.c
+++ b/filters/f_lavfi.c
@@ -903,15 +903,58 @@ static bool is_usable(const AVFilter *filter, int media_type)
            is_single_media_only(filter->outputs, media_type);
 }
 
-static void print_help(struct mp_log *log, int mediatype, char *name, char *ex)
+static void dump_list(struct mp_log *log, int media_type)
 {
-    mp_info(log, "List of libavfilter filters:\n");
+    mp_info(log, "Available libavfilter filters:\n");
     for (const AVFilter *filter = avfilter_next(NULL); filter;
          filter = avfilter_next(filter))
     {
-        if (is_usable(filter, mediatype))
-            mp_info(log, " %-16s %s\n", filter->name, filter->description);
+        if (is_usable(filter, media_type))
+            mp_info(log, "  %-16s %s\n", filter->name, filter->description);
     }
+}
+
+void print_lavfi_help(struct mp_log *log, const char *name, int media_type)
+{
+    const AVFilter *f = avfilter_get_by_name(name);
+    if (!f) {
+        mp_err(log, "Filter '%s' not found.\n", name);
+        return;
+    }
+    if (!is_usable(f, media_type)) {
+        mp_err(log, "Filter '%s' is not usable in this context (wrong media \n"
+               "types or wrong number of inputs/outputs).\n", name);
+    }
+    mp_info(log, "Options:\n\n");
+    const AVClass *class = f->priv_class;
+    // av_opt_next() requires this for some retarded incomprehensible reason.
+    const AVClass **c = &class;
+    int offset= -1;
+    int count = 0;
+    for (const AVOption *o = av_opt_next(c, 0); o; o = av_opt_next(c, o)) {
+        // This is how libavfilter (at the time) decided to assign positional
+        // options (called "shorthand" in the libavfilter code). So we
+        // duplicate it exactly.
+        if (o->type == AV_OPT_TYPE_CONST || o->offset == offset)
+            continue;
+        offset = o->offset;
+
+        mp_info(log, " %s\n", o->name);
+        count++;
+    }
+    mp_info(log, "\nTotal: %d options\n", count);
+}
+
+void print_lavfi_help_list(struct mp_log *log, int media_type)
+{
+    dump_list(log, media_type);
+    mp_info(log, "\nIf libavfilter filters clash with builtin mpv filters,\n"
+            "prefix them with lavfi- to select the libavfilter one.\n\n");
+}
+
+static void print_help(struct mp_log *log, int mediatype, char *name, char *ex)
+{
+    dump_list(log, mediatype);
     mp_info(log, "\n"
         "This lists %s->%s filters only. Refer to\n"
         "\n"