On 9/22/22 08:43, Anton Khirnov wrote:
Quoting Scott Theisen (2022-09-22 03:04:16)
On 9/21/22 15:51, James Almer wrote:
On 9/21/2022 4:44 PM, Rémi Denis-Courmont wrote:
Le keskiviikkona 21. syyskuuta 2022, 22.26.11 EEST Scott Theisen a
écrit :
diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
index 7db5d1b1c5..bcf3a845a8 100644
--- a/libavcodec/avcodec.h
+++ b/libavcodec/avcodec.h
@@ -2076,6 +2076,17 @@ typedef struct AVCodecContext {
* The decoder can then override during decoding
as needed.
*/
AVChannelLayout ch_layout;
+
+ /**
+ * Audio only. This flag is set when MPEG audio mode dual
channel has
been detected. + * This signals that the audio is two
independent mono
channels. + *
+ * 0 normally, 1 if dual channel flag is set.
+ *
+ * - encoding: currently unused (functionally equivalent to
stereo,
patch welcome) + * - decoding: set by lavc
+ */
+ int mpeg_audio_mode_dual_channel;
} AVCodecContext;
I agree that the dual mono flag should be exposed to the application
somehow,
but isn't this a slient ABI break?
It's not a break, but it's overkill for what's essentially a flag.
This is how MythTV customized FFmpeg (in 2006) for this and this way was
probably the easiest. It /is/ a flag, so maybe adding an int as a
bitset instead of as a bool so other flags could be added if necessary?
The proper way to do this would be to signal such a layout as an
actual channel layout, using a custom order one where both channels
are set as Front Center. But i don't know if until the old channel
layout API fields are gone we should have decoders setting something
only new API users will understand. Old API field users would look at
channels and see a 2, and channel_layout and see it's empty, but then
old and new API values would technically conflict, so I'd like to hear
some opinions.
I'm not very familiar with either channel layout API, but the audio is
encoded identically to stereo other than the flag, so could we add
another entry, e.g. AV_CHANNEL_ORDER_MONO, to the `enum AVChannelOrder`
to signify that each channel is independent, but is otherwise identical
to AV_CHANNEL_ORDER_NATIVE?
The whole point is that it's NOT identical to AV_CHANNEL_ORDER_NATIVE.
ORDER_CUSTOM with two FC channels is exactly the right way to handle
this IMO.
I don't really disagree that AV_CHANNEL_ORDER_CUSTOM with two Front
Center channels is one way to do it, I was just hoping for an easier way.
I think that would require modification to the code calling
ff_mpa_decode_header. But I'm not sure how to create that custom order
layout and I would like consensus that the custom order is the way to go
before doing it.
libavcodec/audiotoolboxdec.c just sets an unspecified order.
libavcodec/mpegaudio_parser.c uses av_channel_layout_default.
Also, should we concern ourselves with the old API? It didn't look like
either of the above files did.
Regards,
Scott Theisen
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