[openal] HRTF queries

[Chris Robinson]

On 06/20/2014 04:44 PM, Ethan Lee wrote:
> You might be able to design it similarly to the OpenGL shader API, but
> without the need for programs/linking on top of shaders. For instance,
> alcCreateHRTFs(ALint, ALint*), alcBindHRTF(ALint),
> alcAttachHRTFSource(ALchar*), alcParseHRTF(), etc.

The problem with this is that there's no standard HRTF data format for 
an app to provide. OpenAL Soft has its own format(s) for its HRTF files, 
but they're implementation-specific. Having something like this for 
pre-supplied profiles that only handles string names seems like overkill.


Perhaps instead enumeration could provide a set of integer IDs, done 
through alcGetIntegerv. For example...

ALCint num_profiles;
alcGetIntegerv(device, ALC_NUM_HRTF_PROFILES_SOFT, 1, &num_profiles);

// Each profile gives 3 ints: ID, channel config, and sample rate
ALCint profiles[num_profiles * 3];
alcGetIntegerv(device, ALC_HRTF_PROFILES_SOFT, num_profiles * 3,
                profiles);

for(int i = 0;i < num_profiles;i++)
{
     profiles[i*3    ]; // ID for this HRTF profile
     profiles[i*3 + 1]; // Channel config, e.g. ALC_STEREO_SOFT
     profiles[i*3 + 2]; // Sample rate
}

Then to set one, you create a context with an ID in the attribute list 
(for example, if using profile 'p'):

ALCint attr[] = { ALC_HRTF_SOFT, profiles[p*3], 0 };
context = alcCreateContext(device, attr);

You can then check to see which is being used:

ALCint id;
alcGetIntegerv(device, ALC_HRTF_SOFT, 1, &id);

An ID of 0 would mean no HRTF.

Of course, then the issue becomes, how to get a string to display to the 
user from an ID. alcGetString is unsuited for doing that. Perhaps a new 
alcGetStringFromID function, which would work for other ID types as well 
(if a future extension were to add more device-level IDs).