Implement a lerped lookup of the HRTF coefficients/delays

Code supplied by Christopher Fitzgerald

3 files changed, 93 insertions, 46 deletions

diff --git a/Alc/ALu.c b/Alc/ALu.c
index e7b53763..ebfd0817 100644
--- a/Alc/ALu.c
+++ b/Alc/ALu.c
@@ -269,8 +269,6 @@ ALvoid CalcNonAttnSourceParams(ALsource *ALSource, const ALCcontext *ALContext)
     {
         for(c = 0;c < num_channels;c++)
         {
-            const ALshort *hrtf_left, *hrtf_right;
-
             if(chans[c] == LFE)
             {
                 /* Skip LFE */
@@ -284,17 +282,10 @@ ALvoid CalcNonAttnSourceParams(ALsource *ALSource, const ALCcontext *ALContext)
                 continue;
             }
 
-            GetHrtfCoeffs(0.0, angles[c] * (M_PI/180.0),
-                          &hrtf_left, &hrtf_right,
-                          &ALSource->Params.HrtfDelay[c][0],
-                          &ALSource->Params.HrtfDelay[c][1]);
-            for(i = 0;i < HRIR_LENGTH;i++)
-            {
-                ALSource->Params.HrtfCoeffs[c][i][0] =
-                               hrtf_left[i]*(1.0/32767.0)*DryGain*ListenerGain;
-                ALSource->Params.HrtfCoeffs[c][i][1] =
-                              hrtf_right[i]*(1.0/32767.0)*DryGain*ListenerGain;
-            }
+            GetLerpedHrtfCoeffs(0.0, angles[c] * (M_PI/180.0),
+                                DryGain*ListenerGain,
+                                ALSource->Params.HrtfCoeffs[c],
+                                ALSource->Params.HrtfDelay[c]);
         }
     }
     else
@@ -692,11 +683,9 @@ ALvoid CalcSourceParams(ALsource *ALSource, const ALCcontext *ALContext)
         BufferListItem = BufferListItem->next;
     }
 
-    // Use energy-preserving panning algorithm for multi-speaker playback
     if((Device->Flags&DEVICE_USE_HRTF))
     {
-        const ALshort *hrtf_left, *hrtf_right;
-
+        // Use a binaural HRTF algorithm for stereo headphone playback
         if(Distance > 0.0f)
         {
             ALfloat invlen = 1.0f/Distance;
@@ -705,18 +694,14 @@ ALvoid CalcSourceParams(ALsource *ALSource, const ALCcontext *ALContext)
             Position[2] *= invlen;
         }
 
-        GetHrtfCoeffs(asin(Position[1]), atan2(Position[0], -Position[2]*ZScale),
-                      &hrtf_left, &hrtf_right,
-                      &ALSource->Params.HrtfDelay[0][0],
-                      &ALSource->Params.HrtfDelay[0][1]);
-        for(i = 0;i < HRIR_LENGTH;i++)
-        {
-            ALSource->Params.HrtfCoeffs[0][i][0] = hrtf_left[i]*(1.0/32767.0) * DryGain;
-            ALSource->Params.HrtfCoeffs[0][i][1] = hrtf_right[i]*(1.0/32767.0) * DryGain;
-        }
+        GetLerpedHrtfCoeffs(asin(Position[1]),
+                            atan2(Position[0], -Position[2]*ZScale), DryGain,
+                            ALSource->Params.HrtfCoeffs[0],
+                            ALSource->Params.HrtfDelay[0]);
     }
     else
     {
+        // Use energy-preserving panning algorithm for multi-speaker playback
         ALfloat DirGain, AmbientGain;
         const ALfloat *SpeakerGain;
         ALfloat length;
diff --git a/Alc/hrtf.c b/Alc/hrtf.c
index 801e26b9..a4232079 100644
--- a/Alc/hrtf.c
+++ b/Alc/hrtf.c
@@ -39,34 +39,96 @@ static struct HRTF {
 #include "hrtf_tables.inc"
 };
 
-static ALuint CalcEvIndex(ALdouble ev)
+// Calculate the elevation indices given the polar elevation in radians.
+// This will return two indices between 0 and (evCount - 1) and an
+// interpolation factor between 0.0 and 1.0.
+static void CalcEvIndices (ALfloat ev, ALuint evidx [2], ALfloat * evmu)
 {
-    ev = (M_PI/2.0 + ev) * (evCount-1) / M_PI;
-    return (ALuint)(ev+0.5);
+    ev = (M_PI/2.0f + ev) * (evCount-1) / M_PI;
+    evidx[0] = (ALuint)ev;
+    evidx[1] = __min(evidx[0] + 1, evCount - 1);
+    *evmu = ev - evidx[0];
 }
 
-static ALuint CalcAzIndex(ALint evidx, ALdouble az)
+// Calculate the azimuth indices given the polar azimuth in radians.  This
+// will return two indices between 0 and (azCount [ei] - 1) and an
+// interpolation factor between 0.0 and 1.0.
+static void CalcAzIndices (ALuint evidx, ALfloat az, ALuint azidx [2], ALfloat * azmu)
 {
-    az = (M_PI*2.0 + az) * azCount[evidx] / (M_PI*2.0);
-    return (ALuint)(az+0.5) % azCount[evidx];
+    az = (M_PI*2.0f + az) * azCount[evidx] / (M_PI*2.0f);
+    azidx[0] = (ALuint)az % azCount[evidx];
+    azidx[1] = (azidx[0] + 1) % azCount[evidx];
+    *azmu = az - (ALuint)az;
 }
 
-void GetHrtfCoeffs(ALfloat elevation, ALfloat angle, const ALshort **left, const ALshort **right, ALuint *ldelay, ALuint *rdelay)
+// Calculates static HRIR coefficients and delays for the given polar
+// elevation and azimuth in radians.  Linear interpolation is used to
+// increase the apparent resolution of the HRIR dataset.  The coefficients
+// are also normalized and attenuated by the specified gain.
+void GetLerpedHrtfCoeffs(ALfloat elevation, ALfloat azimuth, ALfloat gain, ALfloat (*coeffs)[2], ALuint *delays)
 {
-    ALuint lidx, ridx;
-    ALuint evidx, azidx;
-
-    evidx = CalcEvIndex(elevation);
-    azidx = CalcAzIndex(evidx, angle);
-
-    lidx = evOffset[evidx] + azidx;
-    ridx = evOffset[evidx] + ((azCount[evidx]-azidx) % azCount[evidx]);
-
-    *ldelay = Hrtf.delays[lidx];
-    *rdelay = Hrtf.delays[ridx];
+    ALuint evidx[2], azidx[2];
+    ALfloat mu[3];
+    ALuint lidx[4], ridx[4];
+    ALuint i;
+
+    // Claculate elevation indices and interpolation factor.
+    CalcEvIndices(elevation, evidx, &mu[2]);
+
+    // Calculate azimuth indices and interpolation factor for the first
+    // elevation.
+    CalcAzIndices(evidx[0], azimuth, azidx, &mu[0]);
+
+    // Calculate the first set of linear HRIR indices for left and right
+    // channels.
+    lidx[0] = evOffset[evidx[0]] + azidx[0];
+    lidx[1] = evOffset[evidx[0]] + azidx[1];
+    ridx[0] = evOffset[evidx[0]] + ((azCount[evidx[0]]-azidx[0]) % azCount[evidx[0]]);
+    ridx[1] = evOffset[evidx[0]] + ((azCount[evidx[0]]-azidx[1]) % azCount[evidx[0]]);
+
+    // Calculate azimuth indices and interpolation factor for the second
+    // elevation.
+    CalcAzIndices (evidx[1], azimuth, azidx, &mu[1]);
+
+    // Calculate the second set of linear HRIR indices for left and right
+    // channels.
+    lidx[2] = evOffset[evidx[1]] + azidx[0];
+    lidx[3] = evOffset[evidx[1]] + azidx[1];
+    ridx[2] = evOffset[evidx[1]] + ((azCount[evidx[1]]-azidx[0]) % azCount[evidx[1]]);
+    ridx[3] = evOffset[evidx[1]] + ((azCount[evidx[1]]-azidx[1]) % azCount[evidx[1]]);
+
+    // Calculate the normalized and attenuated HRIR coefficients using linear
+    // interpolation when there is enough gain to warrant it.  Zero the
+    // coefficients if gain is too low.
+    if(gain > 0.0001f)
+    {
+        ALdouble scale = gain * (1.0/32767.0);
+        for(i = 0;i < HRIR_LENGTH;i++)
+        {
+            coeffs[i][0] = lerp(lerp(Hrtf.coeffs[lidx[0]][i], Hrtf.coeffs[lidx[1]][i], mu[0]),
+                                lerp(Hrtf.coeffs[lidx[2]][i], Hrtf.coeffs[lidx[3]][i], mu[1]),
+                                mu[2]) * scale;
+            coeffs[i][1] = lerp(lerp(Hrtf.coeffs[ridx[0]][i], Hrtf.coeffs[ridx[1]][i], mu[0]),
+                                lerp(Hrtf.coeffs[ridx[2]][i], Hrtf.coeffs[ridx[3]][i], mu[1]),
+                                mu[2]) * scale;
+        }
+    }
+    else
+    {
+        for(i = 0;i < HRIR_LENGTH;i++)
+        {
+            coeffs[i][0] = 0.0f;
+            coeffs[i][1] = 0.0f;
+        }
+    }
 
-    *left  = Hrtf.coeffs[lidx];
-    *right = Hrtf.coeffs[ridx];
+    // Calculate the HRIR delays using linear interpolation.
+    delays[0] = (ALuint)(lerp(lerp(Hrtf.delays[lidx[0]], Hrtf.delays[lidx[1]], mu[0]),
+                              lerp(Hrtf.delays[lidx[2]], Hrtf.delays[lidx[3]], mu[1]),
+                              mu[2]) + 0.5f);
+    delays[1] = (ALuint)(lerp(lerp(Hrtf.delays[ridx[0]], Hrtf.delays[ridx[1]], mu[0]),
+                              lerp(Hrtf.delays[ridx[2]], Hrtf.delays[ridx[3]], mu[1]),
+                              mu[2]) + 0.5f);
 }
 
 ALCboolean IsHrtfCompatible(ALCdevice *device)
diff --git a/OpenAL32/Include/alMain.h b/OpenAL32/Include/alMain.h
index 26a3bf30..1fb75e18 100644
--- a/OpenAL32/Include/alMain.h
+++ b/OpenAL32/Include/alMain.h
@@ -523,7 +523,7 @@ const ALCchar *DevFmtChannelsString(enum DevFmtChannels chans);
 #define HRIR_MASK        (HRIR_LENGTH-1)
 void InitHrtf(void);
 ALCboolean IsHrtfCompatible(ALCdevice *device);
-void GetHrtfCoeffs(ALfloat elevation, ALfloat angle, const ALshort **left, const ALshort **right, ALuint *ldelay, ALuint *rdelay);
+void GetLerpedHrtfCoeffs(ALfloat elevation, ALfloat azimuth, ALfloat gain, ALfloat (*coeffs)[2], ALuint *delays);
 
 void al_print(const char *fname, unsigned int line, const char *fmt, ...)
              PRINTF_STYLE(3,4);