[FFmpeg-devel] [PATCH] fftools/ffmpeg_demux: gracefully ignore mismatching channel layouts for -channel_layout option

[FFmpeg-devel] [PATCH] fftools/ffmpeg_demux: gracefully ignore mismatching channel layouts for -channel_layout option

[Marton Balint]

The very old behaviour of -channel_layout was to simply warn the user about
channel layouts which does not have a matching channel count, and ignore them,
instead of reporting an error.

The recent fix re-added support for overriding -channel_layout, but it rejected
mismatching layouts. There is no easy way for the user to specify a channel
layout only for streams with matching number of channels, so this patch
restores the very old behaviour of ignoring mismatching layouts. See the
discussion in ticket #11016.

Signed-off-by: Marton Balint <cus@passwd.hu>
---
 fftools/ffmpeg_demux.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/fftools/ffmpeg_demux.c b/fftools/ffmpeg_demux.c
index 1ca8d804ae..2e77210237 100644
--- a/fftools/ffmpeg_demux.c
+++ b/fftools/ffmpeg_demux.c
@@ -1399,13 +1399,14 @@ static int ist_add(const OptionsContext *o, Demuxer *d, AVStream *st)
                 av_channel_layout_uninit(&par->ch_layout);
                 par->ch_layout = ch_layout;
             } else {
-                av_log(ist, AV_LOG_ERROR,
-                    "Specified channel layout '%s' has %d channels, but input has %d channels.\n",
+                av_log(ist, AV_LOG_WARNING,
+                    "Specified channel layout '%s' has %d channels, but input has %d channels. Ignoring specified layout.\n",
                     ch_layout_str, ch_layout.nb_channels, par->ch_layout.nb_channels);
                 av_channel_layout_uninit(&ch_layout);
-                return AVERROR(EINVAL);
+                ch_layout_str = NULL;
             }
-        } else {
+        }
+        if (!ch_layout_str) {
             int guess_layout_max = INT_MAX;
             MATCH_PER_STREAM_OPT(guess_layout_max, i, guess_layout_max, ic, st);
             guess_input_channel_layout(ist, par, guess_layout_max);
-- 
2.35.3

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Re: [FFmpeg-devel] [PATCH] fftools/ffmpeg_demux: gracefully ignore mismatching channel layouts for -channel_layout option

[Anton Khirnov]

Quoting Marton Balint (2024-06-03 23:48:47)
> The very old behaviour of -channel_layout was to simply warn the user about
> channel layouts which does not have a matching channel count, and ignore them,
> instead of reporting an error.
> 
> The recent fix re-added support for overriding -channel_layout, but it rejected
> mismatching layouts. There is no easy way for the user to specify a channel
> layout only for streams with matching number of channels, so this patch
> restores the very old behaviour of ignoring mismatching layouts. See the
> discussion in ticket #11016.

I'm ambivalent about this. On one hand it probably doesn't hurt, for now
at least, on the other it seems quite ad-hoc. Previously it worked this
way mostly by accident, whereas if we now restore this behaviour
deliberately we'll be committing to supporting it for the foreseeable
future.

-- 
Anton Khirnov
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Re: [FFmpeg-devel] [PATCH] fftools/ffmpeg_demux: gracefully ignore mismatching channel layouts for -channel_layout option

[Marton Balint]

On Wed, 5 Jun 2024, Anton Khirnov wrote:

> Quoting Marton Balint (2024-06-03 23:48:47)
>> The very old behaviour of -channel_layout was to simply warn the user about
>> channel layouts which does not have a matching channel count, and ignore them,
>> instead of reporting an error.
>>
>> The recent fix re-added support for overriding -channel_layout, but it rejected
>> mismatching layouts. There is no easy way for the user to specify a channel
>> layout only for streams with matching number of channels, so this patch
>> restores the very old behaviour of ignoring mismatching layouts. See the
>> discussion in ticket #11016.
>
> I'm ambivalent about this. On one hand it probably doesn't hurt, for now
> at least, on the other it seems quite ad-hoc. Previously it worked this
> way mostly by accident, whereas if we now restore this behaviour
> deliberately we'll be committing to supporting it for the foreseeable
> future.
>

Yeah, I am a bit unsure about it as well. What made me implement this is 
that there is really no easy way I could think of to simulate the old 
behaviour. An alternative idea might be to introduce a new option 
"-try_channel_map" which only gives a warning.

Ideally if we could use a stream specifier something like

-channel_map:a:eval:eq(st.codecpar.ch_layout.nb_channels,1) mono

that should solve this in a generic way, but this requires massive new 
features in the eval API and in the opt API as well.

Regards,
Marton
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