[vlc-devel] [RFC]: Reworking the metadata listener API => writting a proper filter API

[Steve Lhomme]

It doesn't say when the filter is called. Is it before, during, after 
rendering ?
In the case of (dynamic) HDR it should at least be just before or at the 
same time the next frame will be displayed/sent. And probably not occur 
if the frame is dropped if it's a state and not sent for each picture 
(which should also be documented).

On 2021-03-05 16:48, Thomas Guillem wrote:
> Hello everyone,
> 
> I recently added a generic metadata listener API into the new player. 
> It's only used to get loudness measurements for now, cf. 
> https://code.videolan.org/tguillem/vlc/-/commit/e8f17308146f446bc46f6a3b787aed2406ce8d31 
> <https://code.videolan.org/tguillem/vlc/-/commit/e8f17308146f446bc46f6a3b787aed2406ce8d31>
> And I tried to port it to libvlc here: 
> https://code.videolan.org/tguillem/vlc/-/commits/loudness-libvlc 
> <https://code.videolan.org/tguillem/vlc/-/commits/loudness-libvlc>
> 
> I received few valid critics with these 2 API (Core + LibVKC) from 
> developers, and LibVLC users:
> - LibVLC users don't like using a new kind a listener API (that is not 
> the classical event manager)
> - New kind of metadata will likely come from a filter.
> - Other kind of metadata (like HDR) can come from es_format_t. We need 
> to find a proper way to forward es_format_t change to LibVLC in that case.
> 
> So, maybe it is a good time to think about a proper filter API in LibVLC 
> and Core. An API to manipulate filters of a given category (splitter, 
> audio, meter, video, sub) from a specific output. This API will let 
> users add, re-order, remove filters and also get notification from them 
> (for the audio meter filter).
> 
> Note that aout and vout are not (yet?) exposed to libvlc, so the libVLC 
> will only be able to handle filters by category on the media_player.
> 
> Here is my proposal for the Core API (in a form of a C header):
> 
> typedef struct vlc_filter_entry_t vlc_filter_entry_t;
> 
> /* All type of filters supported by VLC */
> enum vlc_filter_type
> {
>      VLC_PLAYER_FILTER_AUDIO,
>      /* Such filters are only useful if the user listen to events via
>       * vlc_filter_entry_NewAudioMeter() */
>      VLC_PLAYER_FILTER_AUDIO_METER,
>      VLC_PLAYER_FILTER_VIDEO,
>      VLC_PLAYER_FILTER_VIDEO_SPLITTER,
>      VLC_PLAYER_FILTER_SUBTITLE,
>      VLC_PLAYER_FILTER_SUBTITLE_SOURCE,
> };
> 
> /* Listener, only used with audio_meter filters */
> struct vlc_filter_audio_meter_listener
> {
>      union {
>          /* Generic callback. For the loudness audio_meter, val1 will be a
>           * int64_t date, val2 will be a pointer to the vlc_audio_loudness
>           * struct */
>          void (*on_values_changed)(vlc_value_t val1, val_value_t val2);
>          /* Maybe add some other cb definition later depending on the 
> need */
>      };
> };
> 
> /* Create a new filter entry
> * The filter entry hold necessary informations to start a filter on the aout
> * or the vout.
> */
> VLC_API vlc_filter_entry_t *
> vlc_filter_entry_New(enum vlc_filter_type type, const char *name, const 
> char *option);
> 
> /* Create a new audio meter entry with a listener callback */
> VLC_API vlc_filter_entry_t *
> vlc_filter_entry_NewAudioMeter(const char *name,  const char *option, 
> const struct vlc_filter_audio_meter_listener * cb, void *data);
> 
> /* Return the name of the filter entry */
> VLC_API const char * vlc_filter_entry_GetName(vlc_filter_entry_t *fentry);
> 
> /* Return the option of the filter entry */
> VLC_API const char * vlc_filter_entry_GetOption(vlc_filter_entry_t *fentry);
> 
> /* Release a filter entry. Filter entries are refcounted internally (by 
> the filter_list and the filter entry creator */
> VLC_API void vlc_filter_entry_Release(vlc_filter_entry_t *fentry);
> 
> /* Filter entry list, owned by vout and aout. There is one filter list 
> per filter type */
> 
> typedef struct vlc_filter_list_t vlc_filter_list;
> 
> /* Lock the filter list, read and write operations need to be done locked */
> void vlc_filter_list_Lock(vlc_filter_list_t *list);
> 
> /* Unlock the filter list, and commit the written changes if needed. To 
> commit
> * any changes, the list will change the corresponding filter option (like
> * "video-splitter") on the corresponding vout/aout. */
> void vlc_filter_list_Unlock(vlc_filter_list_t *list);
> 
> /* Return the number of filters */
> size_t vlc_filter_list_GetCount(vlc_filter_list_t *list);
> 
> /* Get the filter entry at the specific index */
> vlc_filter_entry_t * vlc_filter_list_GetFilterAt(vlc_filter_list_t 
> *list, size_t index);
> 
> /* Add a new filter at a specific position or move a filter at the new
> * position. */
> void
> vlc_filter_list_MoveFilterAt(vlc_filter_list_t *list, size_t index, 
> vlc_filter_entry_t *filter);
> 
> /* Append a filter at the end of the list */
> static inline void
> vlc_filter_list_AppendFilter(vlc_filter_list_t *list, vlc_filter_entry_t 
> *filter)
> {
>      vlc_filter_list_MoveFilterAt(list, vlc_filter_list_GetCount(list), 
> filter);
> }
> 
> /* Remove a filter from the list. */
> void vlc_filter_list_RemoveFilter(vlc_filter_list_t *list, 
> vlc_filter_entry_t *filter);
> 
> /* Get the filter list of the aout for the specific type */
> vlc_filter_list_t *
> aout_GetFilterList(aout_output_t *, enum vlc_filter_type type);
> 
> /* Get the filter list of the vout for the specific type */
> vlc_filter_list_t *
> vout_GetFilterList(vout_output_t *, enum vlc_filter_type type);
> 
> Comments are more than welcome!
> 
> Best,
> Thomas
> 
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