By default, stereo outputs using UHJ, which renders to a B-Format buffer that
gets run through a UHJ encoder for output. It may also output using pair-wise
panning, which pans between -30 and +30 degrees with the speakers at the two
ends. In both cases, the stereo coefficients are ignored.
Mono, having only one output channel, can realistically only attenuate its
channel. Turning the volume up and down accomplishes the same result.
Could really do with some optimizations to the mixing gain calculations. For
ambisonic targets, the coefficients will only have 1 non-0 entry for each
output, so the double loop in unnecessarily wasteful. Similarly, most uses
won't need a full height encoding either, so a horizontal-only or mixed-order
target could reduce the number of channels.
This uses a virtual B-Format buffer for mixing, and then uses a dual-band
decoder for improved positional quality. This currently only works with first-
order output since first-order input (from the AL_EXT_BFROMAT extension) would
not sound correct when fed through a second- or third-order decoder.
This also does not currently implement near-field compensation since near-field
rendering effects are not implemented.
There were phase issues caused by applying HRTF directly to the B-Format
channels, since the HRIR delays were all averaged which removed the inter-aural
time-delay, which in turn removed significant spatial information.
This mixes to a 4-channel first-order ambisonics buffer. With ACN ordering and
N3D scaling, this makes it easy to remain compatible with effects that only
care about mono input since channel 0 is an unattenuated mono signal.
Note that it still uses FuMa scalings internally. Coefficients loaded from
config files specify if they're FuMa (in both ordering and scaling) or N3D,
and will get reordered or rescaled as needed.
It seems a simple scaling on the coefficients will allow first-order content to
work with second- and third-order coefficients, although obviously not with any
improved locality. That may be something to look into for the future, but this
is good enough for now.
It's possible to calculate HRTF coefficients for full third-order ambisonics
now, but it's still not possible to use them here without upmixing first-order
content.
This adds the ability to directly decode B-Format with HRTF, though only first-
order (WXYZ) for now. Second- and third-order would be easilly doable, however
we'd need to be able to up-mix first-order content (from the BFORMAT2D and
BFORMAT3D buffer formats) since it would be inappropriate to decode lower-order
content with a higher-order decoder.
