[MPlayer-dev-eng] Dynamically load audio/video filter at runtime through commands

[Jehan Pagès]

Hi,

I would love to be able to load and unload an audio filter at runtime
(hence listen to a song by adding/removing filters on the fly without
having to stop the song and rewrite the command line by changing -af
options, but either by a slave command or simply by binding specific
commands to a key). So I went on the irc channel a few hours ago, and
I have been confirmed this is not possible currently (someone said it
is possible with something called ffdshow used with mplayer but
searching this on the web, it appears to be Windows only, which I
don't have).

So I made a first patch to add such fonctionnality. As far as my tests
went, it works pretty well. So before going further, here is what I
added:
-------
./mplayer -input cmdlist |grep af
af_add               String
af_del               String
af_clr
-------

1/ So "af_add filter_name" adds an audio filter. For instance the
command "af_add karaoke" will add the karaoke filter. You can also
give filter options the same way as the command line, for instance:
"af_add scaletempo=speed=tempo".
2/ "af_del filter_name" will unload the first audio filter with the
given name (for instance "ad_del scaletempo" will remove the first
instance of scaletempo filter).
3/ "af_clr" unloads all loaded filter.

Notes:
I) af_add and af_del can be given only one filter at a time. It would
be interesting to be able to load several filters at the same time,
like in the command line. I may enable this as a second step.

II) I wanted to have an af_add command to append to the filter list
and an af_pre to prepend (like on the command line) but the functions
to append and prepend audio filters are private and the only public
function to add a filter (af_add in libaf/af.h) states:

* The filter will be inserted somewhere nice in the
 * list of filters (i.e. at the beginning unless the
 * first filter is the format filter (why??).

So couldn't it be interesting to make af_append and af_preprend (from
libaf/af.c) public as well?

III) I was thinking it could be interesting to have a bare "af"
command which would override current audio filters with a new list,
like in the command line, (as in command line, both method --
add/remove and af methods -- could be available).

IV) As I was only interested in audio filter, I implemented this only
for audio filters, but if the video filters are implemented a similar
way (I have not looked the code for video filters), I can easily make
commands for video filters as well.

As a conclusion, the diff is attached to this email. I hope you are
interested to add this upstream because for me this functionnality is
interesting (and hopefully I am not alone) and I really won't maintain
any kind of fork-like of mplayer... :-)
Thanks for this great software anyway.

Jehan
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