[openal] AL_SOFT_source_spatialize proposal
chris.kcat at gmail.com
Sat May 6 16:18:12 EDT 2017
Here's one that's a little more involved with modifying existing spec
behavior. The main intention of this is to allow apps to play a stereo
sound localized when emanating from the listener, and play the same
sound non-localized when emanating from elsewhere.
Normally in that scenario, the app would be required to create two
versions of the sound to handle this, the original stereo one to play
when localized and a second mono one to play when non-local. This
increases the amount of work the app has to do to generate and manage
audio buffers, and increases the amount of memory used (about 50% more
per stereo buffer used this way). To say nothing of the difficulty in
dealing with a source that may move between being local and non-local
The only thing I'm really unsure of, and would like feedback on, is what
kind of attenuation (if any) should be applied to a down-mixed sound.
Normally you see an attenuation of 1/channels or sqrt(1/channels) when
down-mixing to mono, but there's a case to make for letting the app deal
with it and not try to invisibly correct it.
Also, I'm not too thrilled with the new name AL_AUTO_SOFT to indicate
preexisting behavior (automatically allow spatialization or not
depending if playing a mono buffer or not, respectively). The
alternative I thought of is AL_DONT_CARE_SOFT, but to me that makes it
seem like the lib is more free to select one than it really does.
Thoughts, opinions, questions?
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Chris Robinson (chris.kcat 'at' gmail.com)
This extension is for OpenAL 1.1.
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.
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
A: Undetermined. Typically down-mixing a multi-channel sound to mono sees
gain correction in the form of 1/num_channels or sqrt(1/num_channels).
However, not applying the given correction gives full control to the
application to do what it wants without OpenAL in the way.
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
Accepted as the <param> parameter of alSourcei, alSourceiv, alGetSourcei,
Accepted as the <value> of the AL_SOURCE_SPATIALIZE_SOFT property:
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). 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 panned, however it is still attenuated.
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.
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