Reconciling ConvolverNode's output channel dependencies with the mixing rules in the spec

Currently in the spec most nodes have a notion of input channel count,
which means that the processing code can have assumptions about the number
of input channels.  This is not the case about the number of output
channels though.

According to <
https://dvcs.w3.org/hg/audio/raw-file/tip/webaudio/specification.html#UpMix>,
the number of channels that an AudioNode produces on its output doesn't
have anything to do with the number of channels that the nodes connected to
it will see in their output, as the up/down-mixing happens at the input to
each node.  In other words, the implementation of a node is free to to
choose the number of output channels that it wants without needing to worry
about what other nodes expect.  (Of course, assuming that the chosen number
of output channels makes sense, but let's grant that assumption for now.)

ConvolverNode, however, deviates from this.  In <
https://dvcs.w3.org/hg/audio/raw-file/tip/webaudio/specification.html#Convolution-reverb-effect>,
the spec denotes a matrix of different types of processing that needs to
happen based on the number of input channels, number of channels in the
impulse response buffer, and the number of output channels.  I find that
incompatible with the mixing rules for AudioNodes in the spec.  An
AudioNode cannot make assumptions about the number of output channels in
any meaningful way.  For example, we can connect a ConvolverNode to two
AudioNodes, one with channelCount=2 and channelCountMode="explicit" and one
with channelCount=1 and channelCountMode="explicit".  In this case, it's
not clear what number should be used as the number of output channels for
ConvolverNode.

I think instead, we need to specify the number of output channels to be
expressed as a function of the number of input channels, and the number of
channels in the impulse response buffer, here's my proposal:

Given K being the number of channels in the impulse response buffer, M the
number of output channels will be defined as below:

M = K if K = 1 or K = 2
M = 2 if K = 4
M = 0 otherwise

This formula is compatible with all of the existing modes in <
https://dvcs.w3.org/hg/audio/raw-file/tip/webaudio/specification.html#Convolution-reverb-effect>,
perhaps with the exception of true stereo, where the current spec does not
clearly specify the down-mixing rules at the output.

Does this change make sense?

Thanks!
--
Ehsan
<http://ehsanakhgari.org/>