Normally we don't even try this, but in corner cases it can happen. For
example when inserting lavcac3enc at runtime, and display-sync-resample
was active.
The audio format neogitation code was pretty complicated, although the
idea was simple: when the format changes (or on the first audio frame),
filter only the new frame through the entire filter chain, discard the
resulting frame, but use the format to initialize the AO.
This was useful for "fudging" the channel remix behavior (upmix or
downmix), and moving it before other filters. Apparently this was useful
for things like DRC filters, which might work better in stereo, and
which also can only achieve the desired volume levels by doing it before
a downmix, which would modify the volume. This mechanism was introduced
in commit 60048b7eb9 (which the commit message also describes as
"idiotic heuristic"). Knowing the output format is inherently necessary
for this, because otherwise we can't know what the hell the user defined
filters will do.
There were problems with robustness. Some filters needed more than one
frame. Resampling in particular would discard initial audio at high
resampling ratios. Some filters might drop audio intentionally (like
clipping data on timestamp ranges). There were also allegations that
some decoders output 0 length frames (although that is invalid in
libavcodec). The state machine was excessively complex and hard to
understand too.
There are 3 things that could have been done:
1. Fix robustness problems by doing more heuristics, like repeating
audio frames or simply decoding several frames. Since filters can
behave differently, this would have added lots of complexity.
2. Make use of libavfilter's format negotiation, and add the same to
mpv builtin filters. This is sort of annoying, because the format
negotiation in libavfilter changes the state of the filters. It also
reports only some parameters (mostly all for audio, but a lot of
holes for video). It would remove some of the state machine, but not
all.
3. Drop the channel remix fudging, and do the same as the video chain.
This would not require format negotiation, but instead you can just
filter the audio frames, and look what comes out of it. If nothing
comes out, simply never create an AO.
This commit selects option 3. It removes the remix fudging, which means
the loss of a feature. Users can instead add "--af=format=channels=2"
before their DRC filter, or something. I'm also considering changing the
default for --audio-channels back to stereo, and downmix in the decoder
or at the start of the filter chain, which would give the same results,
except requiring more configuration.
Implementation-wise, this is still a bit different from the video path.
The VO always remains the same instance, while the AO might have to be
recreated on configuration changes. This still requires explicit format
change handling + draining old data, but by putting it into
f_autoconvert, not much new code is needed.
This tried to avoid running the audio/video functions depending on
whether any of the audio or video related format restrictions were
called (so the filter would show an error if a mismatching media type
was passed in). It was a shit idea anyway, so fuck it.
Get rid of the old vf.c code. Replace it with a generic filtering
framework, which can potentially handle more than just --vf. At least
reimplementing --af with this code is planned.
This changes some --vf semantics (including runtime behavior and the
"vf" command). The most important ones are listed in interface-changes.
vf_convert.c is renamed to f_swscale.c. It is now an internal filter
that can not be inserted by the user manually.
f_lavfi.c is a refactor of player/lavfi.c. The latter will be removed
once --lavfi-complex is reimplemented on top of f_lavfi.c. (which is
conceptually easy, but a big mess due to the data flow changes).
The existing filters are all changed heavily. The data flow of the new
filter framework is different. Especially EOF handling changes - EOF is
now a "frame" rather than a state, and must be passed through exactly
once.
Another major thing is that all filters must support dynamic format
changes. The filter reconfig() function goes away. (This sounds complex,
but since all filters need to handle EOF draining anyway, they can use
the same code, and it removes the mess with reconfig() having to predict
the output format, which completely breaks with libavfilter anyway.)
In addition, there is no automatic format negotiation or conversion.
libavfilter's primitive and insufficient API simply doesn't allow us to
do this in a reasonable way. Instead, filters can use f_autoconvert as
sub-filter, and tell it which formats they support. This filter will in
turn add actual conversion filters, such as f_swscale, to perform
necessary format changes.
vf_vapoursynth.c uses the same basic principle of operation as before,
but with worryingly different details in data flow. Still appears to
work.
The hardware deint filters (vf_vavpp.c, vf_d3d11vpp.c, vf_vdpaupp.c) are
heavily changed. Fortunately, they all used refqueue.c, which is for
sharing the data flow logic (especially for managing future/past
surfaces and such). It turns out it can be used to factor out most of
the data flow. Some of these filters accepted software input. Instead of
having ad-hoc upload code in each filter, surface upload is now
delegated to f_autoconvert, which can use f_hwupload to perform this.
Exporting VO capabilities is still a big mess (mp_stream_info stuff).
The D3D11 code drops the redundant image formats, and all code uses the
hw_subfmt (sw_format in FFmpeg) instead. Although that too seems to be a
big mess for now.
f_async_queue is unused.
wm4