[Openal] Linux positional audio (resend)

[Jason Daly]

Stephen J Baker wrote:

> jpweber at imonk.com wrote:
>
>> Before I get too far with this, does positional audio work
>> on Linux?  Right now, I get distance attenuation, but just
>> mono effect.
>
>
> I have recently being doing this very thing - and I can
> definitely confirm that stereo position, range attenuation
> and doppler all work in Linux using the CVS checkout tagged
> 'Linux_Spec1-0'. 

It works, but the effect is less convincing than the Windows 
implementations, where a considerably greater amount of effort has no 
doubt been spent.  I use the Linux version a lot, but I've always found 
the separation between channels a bit lacking.

>> If I understand, proper stereo has much more to do with phase
>> versus just fiddling the balance.  I'm guessing this is
>> a special feature of some audio chipsets.
>
>
> Phase certainly plays a part in getting the front-versus-back
> thing right.  I don't think the Linux version (at least) does
> *that* right. 

With a 4-speaker arrangement, if you configure OpenAL (in .openalrc) to 
use ALSA and 4 speakers, it will do front/back as well as it does 
left/right.  There are no HRTF's done, though, so headphones will 
definitely not give you any front/back effects.

>> I've tried several sounds, specifically trying 1 channel
>> sounds, since the docs say 2-channel sounds are implied
>> non-positional.  Is there more at issue with
>> positional-ready sounds?
>
>
> I had no problem with stereo sounds being positioned
> as if they were monophonic. 

Under Windows, stereo sounds are not spatialized.  Under Linux, I 
believe they are.  This will change in the 1.1 version, however.  I 
believe 1.1 specifies that multi-channel sounds are not spatialized.

>> Does the alcOpenDevice device name matter, or is NULL
>> the best route? 
>

NULL definitely works, and you can specify which devices are used in 
your .openalrc file in the user's home directory.  I'm not sure if the 
argument to alcOpenDevice() even works in Linux or not.  I think it's 
supposed to (the code is in place for it), and it will certainly be 
working before the Linux implementation is considered 1.1 compliant.

In Linux, some of the devices you might see are "native", "alsa", "sdl", 
"waveout".  Most of the time just "native" will be available.  If you 
configure with --enable-alsa, then the "alsa" device would be available 
as well.  At least that's the way it used to work :-)

--"J"