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Fast approximation of the azimuth parallax correction

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This commit is contained in:
Ken Cooke 2018-02-28 04:44:20 -08:00
parent 41ccf6afd8
commit bc7b983f48
1 changed files with 44 additions and 6 deletions
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50
libraries/audio/src/AudioHRTF.cpp
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@ -71,6 +71,25 @@ ALIGN32 static const float crossfadeTable[HRTF_BLOCK] = {
0.0029059408f, 0.0022253666f, 0.0016352853f, 0.0011358041f, 0.0007270137f, 0.0004089886f, 0.0001817865f, 0.0000454487f,
};
//
// Fast approximation of the azimuth parallax correction,
// for azimuth = [-pi,pi] and distance = [0.125,2].
//
// Correction becomes 0 at distance = 2.
// Correction is continuous and smooth.
//
static const int NAZIMUTH = 8;
static const float azimuthTable[NAZIMUTH][3] = {
{ 0.018719007f, 0.097263971f, 0.080748954f }, // [-4pi/4,-3pi/4]
{ 0.066995833f, 0.319754290f, 0.336963269f }, // [-3pi/4,-2pi/4]
{ -0.066989851f, -0.101178847f, 0.006359474f }, // [-2pi/4,-1pi/4]
{ -0.018727343f, -0.020357568f, 0.040065626f }, // [-1pi/4,-0pi/4]
{ -0.005519051f, -0.018744412f, 0.040065629f }, // [ 0pi/4, 1pi/4]
{ -0.001201296f, -0.025103593f, 0.042396711f }, // [ 1pi/4, 2pi/4]
{ 0.001198959f, -0.032642381f, 0.048316220f }, // [ 2pi/4, 3pi/4]
{ 0.005519640f, -0.053424870f, 0.073296888f }, // [ 3pi/4, 4pi/4]
};
//
// Model the normalized near-field Distance Variation Filter.
//
@ -777,20 +796,39 @@ static void distanceBiquad(float distance, float& b0, float& b1, float& b2, floa
//
static void nearFieldAzimuthCorrection(float azimuth, float distance, float& azimuthL, float& azimuthR) {
#ifdef HRTF_AZIMUTH_EXACT
float dx = distance * cosf(azimuth);
float dy = distance * sinf(azimuth);
// at reference distance, the azimuth parallax is correct
float dx0 = HRTF_AZIMUTH_REF * cosf(azimuth);
float dy0 = HRTF_AZIMUTH_REF * sinf(azimuth);
float deltaL = atan2f(dy + HRTF_HEAD_RADIUS, dx) - atan2f(dy0 + HRTF_HEAD_RADIUS, dx0);
float deltaR = atan2f(dy - HRTF_HEAD_RADIUS, dx) - atan2f(dy0 - HRTF_HEAD_RADIUS, dx0);
azimuthL += atan2f(dy + HRTF_HEAD_RADIUS, dx) - atan2f(dy0 + HRTF_HEAD_RADIUS, dx0);
azimuthR += atan2f(dy - HRTF_HEAD_RADIUS, dx) - atan2f(dy0 - HRTF_HEAD_RADIUS, dx0);
#else
// at reference distance, the azimuth parallax is correct
float fy = (HRTF_AZIMUTH_REF - distance) / distance;
// add the azimuth correction
float x0 = +azimuth;
float x1 = -azimuth; // compute using symmetry
const float RAD_TO_INDEX = 1.2732394f; // 8/(2*pi), rounded down
int k0 = (int)(RAD_TO_INDEX * x0 + 4.0f);
int k1 = (NAZIMUTH-1) - k0;
assert(k0 >= 0);
assert(k1 >= 0);
assert(k0 < NAZIMUTH);
assert(k1 < NAZIMUTH);
// piecewise polynomial approximation over azimuth=[-pi,pi]
float fx0 = (azimuthTable[k0][0] * x0 + azimuthTable[k0][1]) * x0 + azimuthTable[k0][2];
float fx1 = (azimuthTable[k1][0] * x1 + azimuthTable[k1][1]) * x1 + azimuthTable[k1][2];
// approximate the azimuth correction
// NOTE: must converge to 0 when distance = HRTF_AZIMUTH_REF
azimuthL += deltaL;
azimuthR += deltaR;
azimuthL += fx0 * fy;
azimuthR -= fx1 * fy;
#endif
}
//