HRTF Support
============

Starting with OpenAL Soft 1.14, HRTFs can be used to enable enhanced
spatialization for both 3D (mono) and multi-channel sources, when used with
headphones/stereo output. This can be enabled using the 'hrtf' config option.

For multi-channel sources this creates a virtual speaker effect, making it
sound as if speakers provide a discrete position for each channel around the
listener. For mono sources this provides much more versatility in the perceived
placement of sounds, making it seem as though they are coming from all around,
including above and below the listener, instead of just to the front, back, and
sides.

The default data set is based on the KEMAR HRTF data provided by MIT, which can
be found at <http://sound.media.mit.edu/resources/KEMAR.html>. It's only
available when using 44100hz playback.


Custom HRTF Data Sets
=====================

OpenAL Soft also provides an option to use user-specified data sets, in
addition to or in place of the default set. This allows users to provide their
own data sets, which could be better suited for their heads, or to work with
stereo speakers instead of headphones, or to support more playback sample
rates, for example.

The file format is specified below. It uses little-endian byte order.

==
ALchar   magic[8] = "MinPHR01";
ALuint   sampleRate;

ALubyte hrirSize;  /* Can be 8 to 128 in steps of 8. */
ALubyte evCount;   /* Can be 5 to 128. */

ALubyte azCount[evCount]; /* Each can be 1 to 128. */

/* NOTE: hrirCount is the sum of all azCounts */
ALshort coefficients[hrirCount][hrirSize];
ALubyte delays[hrirCount]; /* Each can be 0 to 63. */
==

The data is described as thus:

The file first starts with the 8-byte marker, "MinPHR01", to identify it as an
HRTF data set. This is followed by an unsigned 32-bit integer, specifying the
sample rate the data set is designed for (OpenAL Soft will not use it if the
output device's playback rate doesn't match).

Afterward, an unsigned 8-bit integer specifies how many sample points (or
finite impulse response filter coefficients) make up each HRIR.

The following unsigned 8-bit integer specifies the number of elevations used
by the data set. The elevations start at the bottom (-90 degrees), and
increment upwards.  Following this is an array of unsigned 8-bit integers, one
for each elevation which specifies the number of azimuths (and thus HRIRs) that
make up each elevation.  Azimuths start clockwise from the front, constructing
a full circle for the left ear only. The right ear uses the same HRIRs but in
reverse (ie, left = angle, right = 360-angle).

The actual coefficients follow. Each coefficient is a signed 16-bit sample,
with each HRIR being a consecutive number of sample points.  The HRIRs must be
minimum-phase.  This allows the use of a smaller filter length, reducing
computation.  For reference, the built-in data set uses a 32-point filter while
even the smallest data set provided by MIT used a 128-sample filter (a 4x
reduction by applying minimum-phase reconstruction). Theoretically, one could
further reduce the minimum-phase version down to a 16-point filter with only a
small reduction in quality.

After the coefficients is an array of unsigned 8-bit delay values, one for
each HRIR. This is the propagation delay (in samples) a signal must wait before
being convolved with the corresponding minimum-phase HRIR filter.
