[opus] [PATCH] Support for Channel Mapping 253.

Jean-Marc Valin jmvalin at jmvalin.ca
Wed Nov 1 19:04:35 UTC 2017

>     8) I see there's a "gain" field in the matrix, but I can't see it used
>     anywhere. Did I miss something?
> Gain should be pulled out by the user via
> overall output gain. We assume the mixing matrix gain is always zero and
> that this only matters for the output gain from the demixing matrix.

Yeah OK, I forgot about the feature. Just to make sure I remember
correctly... The (user) encoder code calls
OPUS_PROJECTION_GET_DEMIXING_MATRIX_GAIN() and uses the result to fill
the "gain" field of the header, right?

>     11) So one issue I just noticed is that opus_projection_encode() and
>     opus_projection_encode_float() (same for the decoder) use arbitrarily
>     large amounts of stack memory for "buf". In opus_multistream_encode()
>     that's avoided by converting just two channels at a time, but here it's
>     not quite clear how to do that without duplicating a lot of the
>     multistream code. If we can't address the issue with this patch, the
>     least would be to abort when trying to use these calls with
> Done. Let me know if we should try designing something around this. 

Yes, I think that needs to be addressed. Since the feature isn't enabled
by default and this patch is already complicated enough, I think it
should be addressed in a separate patch. I'm not yet sure what the best
approach is. That'll require some thought.

I'm having another look at your revised patches and I'll get back to you.



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