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.

2 files changed, 14 insertions, 2 deletions

diff --git a/options/m_option.c b/options/m_option.c
index 522e771356..e777c5a19e 100644
--- a/options/m_option.c
+++ b/options/m_option.c
@@ -2940,6 +2940,8 @@ print_help: ;
         if (desc->print_help)
             desc->print_help(log);
         m_config_print_option_list(config, "*");
+    } else if (list->print_unknown_entry_help) {
+        list->print_unknown_entry_help(log, mp_tprintf(80, "%.*s", BSTR_P(name)));
     } else {
         mp_warn(log, "Option %.*s: item %.*s doesn't exist.\n",
                BSTR_P(opt_name), BSTR_P(name));
@@ -3155,11 +3157,17 @@ static int parse_obj_settings_list(struct mp_log *log, const m_option_t *opt,
             if (!ol->get_desc(&desc, n))
                 break;
             if (!desc.hidden) {
-                mp_info(log, "  %-15s: %s\n",
+                mp_info(log, "  %-16s %s\n",
                        desc.name, desc.description);
             }
         }
         mp_info(log, "\n");
+        if (ol->print_help_list)
+            ol->print_help_list(log);
+        if (!ol->use_global_options) {
+            mp_info(log, "Get help on individual entries via: --%s=entry=help\n",
+                    opt->name);
+        }
         return M_OPT_EXIT;
     }
 
diff --git a/options/m_option.h b/options/m_option.h
index f47dac294e..af082a893d 100644
--- a/options/m_option.h
+++ b/options/m_option.h
@@ -129,7 +129,7 @@ struct m_obj_desc {
     const char *init_options;
     // Don't list entry with "help"
     bool hidden;
-    // Callback to print custom help if "help" is passed
+    // Callback to print custom help if "vf=entry=help" is passed
     void (*print_help)(struct mp_log *log);
     // Callback that allows you to override the static default values. The
     // pointer p points to the struct described by options/priv_size, with
@@ -159,6 +159,10 @@ struct m_obj_list {
     bool disallow_positional_parameters;
     // Each sub-item is backed by global options (for AOs and VOs).
     bool use_global_options;
+    // Callback to print additional custom help if "vf=help" is passed
+    void (*print_help_list)(struct mp_log *log);
+    // Callback to print help for _unknown_ entries with "vf=entry=help"
+    void (*print_unknown_entry_help)(struct mp_log *log, const char *name);
 };
 
 // Find entry by name