[openal] AL_SOFT_source_spatialize proposal

[Chris Robinson]

Here's the updated extension proposal, now including the 1/N scaling 
when down-mixing a panned multi-channel source. If there's no 
objections, it can be considered complete.
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Name

    AL_SOFT_source_spatialize

Contributors

    Chris Robinson

Contact

    Chris Robinson (chris.kcat 'at' gmail.com)

Status

    In Progress

Dependencies

    This extension is for OpenAL 1.1.

Overview

    This extension provides more control over the spatialization behavior of
    sources. By default, OpenAL will spatialize sources playing a mono buffer
    and not spatialize sources playing a multi-channel buffer. Spatialization
    features include positional panning, distance attenuation, cone-related
    attenuation, and doppler effects. This extension allows such features to
    be enabled and disabled per-source independently of the buffer format.

Issues

    Q: Most 3D audio systems don't allow panning non-mono sources due to
       problems with correctly rendering the soundfield in 3D space. How is
       this dealt with?
    A: When a spatialized source plays a non-mono buffer and has a non-0
       distance from the listener, the channels are down-mixed to mono and
       panned as appropriate (that is, all channels occupy the same spot in
       3D space). An exception is the LFE channel of buffer formats that
       include one. The LFE input is sent separately to LFE output as normal,
       rather than being down-mixed and panned. It is still influenced by
       distance and cone attenuation however.

       When such a source occupies the same position as the listener, it is
       played with separated channels as if it was not spatialized (including
       being unaffected by listener orientation). But it still receives
       doppler effects and cone-related attenuation.

    Q: When a multi-channel source is down-mixed for spatialization, is any
       extra attenuation applied from multiple channels occupying the same
       space?
    A: Typically down-mixing multi-channel to mono sees a gain correction of
       1/num_channels to preserve peak amplitude, or sqrt(1/num_channels) to
       preserve the apparent loudness. However, not applying the given
       correction gives full control to the application to do what it wants
       without OpenAL in the way.

       This extension applies a gain correction of 1/num_channels when down-
       mixing a panned multi-channel source. For input sounds that don't clip,
       this ensures no clipping when the source is panned. Be aware that the
       number of channels excludes LFE input since it's sent separately to LFE
       output, i.e. it's not included in the down-mix.

    Q: What happens to a mono source that is played without spatizalization?
    A: A mono source without spatialization will not have distance or cone
       attenuation, directional panning, or doppler effects, as expected. Its
       perceived location may either be centered (i.e. equal energy from all
       directions) or front-center, but its perceived location should be
       consistent with a 0-distance spatialized mono source.

New Procedures and Functions

        None.

New Tokens

    Accepted as the <param> parameter of alSourcei, alSourceiv, alGetSourcei,
    and alGetSourceiv:

        AL_SOURCE_SPATIALIZE_SOFT                0x1214

    Accepted as the <value> of the AL_SOURCE_SPATIALIZE_SOFT property:

        AL_AUTO_SOFT                             0x0002

Additions to Specification

    Controlling Source Spatialization

    A source may have spatialization features applied depending on its
    AL_SOURCE_SPATIALIZE_SOFT property. Valid values for the property are
    AL_FALSE, AL_TRUE, and AL_AUTO_SOFT. When set to AL_FALSE, spatialization
    features are never applied to the source. When set to AL_TRUE, they are
    always applied. When set to AL_AUTO_SOFT, they are applied if playing a
    buffer with one channel, and not if the buffer has more than one channel.
    The default is AL_AUTO_SOFT.

    When spatialization features are applied on a source playing a non-mono
    buffer, if the source occupies the same 3D position as the listener the
    buffer's channels are placed around the listener according to the format
    (e.g. a stereo buffer has the left channel 30 degrees left of front, and
    the right channel 30 degrees right of front). This panning is NOT effected
    by the source direction or listener orientation.

    If the source does not occupy the same 3D position as the listener, the
    proper panning direction is calculated and all channels of the buffer will
    be panned to that direction (effectively down-mixing the buffer to mono
    dynamically). The buffer's channels will also receive a gain correction of
    1/num_channels when panned, to preserve peak amplitude of the mixed
    channels. An exception is the LFE channel in buffer formats that include
    one. The LFE channel may be sent to the LFE output as normal and not be
    part of the panned mix, however it is still attenuated according to the
    source distance and cone.

Errors

    An AL_INVALID_VALUE error is generated if the value specified for
    AL_SOURCE_SPATIALIZE_SOFT is not AL_FALSE, AL_TRUE, or AL_AUTO_SOFT.