Use a step counter for gain stepping

This should provide more stable stepping, preventing floating-point errors from
accumulating on each step/sample.

1 files changed, 22 insertions, 17 deletions

diff --git a/Alc/mixer/hrtf_inc.c b/Alc/mixer/hrtf_inc.c
index d6bd8042..3ef22f24 100644
--- a/Alc/mixer/hrtf_inc.c
+++ b/Alc/mixer/hrtf_inc.c
@@ -22,8 +22,9 @@ void MixHrtf(ALfloat *restrict LeftOut, ALfloat *restrict RightOut,
 {
     const ALfloat (*Coeffs)[2] = ASSUME_ALIGNED(hrtfparams->Coeffs, 16);
     const ALsizei Delay[2] = { hrtfparams->Delay[0], hrtfparams->Delay[1] };
-    ALfloat gainstep = hrtfparams->GainStep;
-    ALfloat gain = hrtfparams->Gain;
+    const ALfloat gainstep = hrtfparams->GainStep;
+    const ALfloat gain = hrtfparams->Gain;
+    ALfloat g, stepcount = 0.0f;
     ALfloat left, right;
     ALsizei i;
 
@@ -35,8 +36,10 @@ void MixHrtf(ALfloat *restrict LeftOut, ALfloat *restrict RightOut,
     for(i = 0;i < BufferSize;i++)
     {
         hrtfstate->History[Offset&HRTF_HISTORY_MASK] = *(data++);
-        left = hrtfstate->History[(Offset-Delay[0])&HRTF_HISTORY_MASK]*gain;
-        right = hrtfstate->History[(Offset-Delay[1])&HRTF_HISTORY_MASK]*gain;
+
+        g = gain + gainstep*stepcount;
+        left = hrtfstate->History[(Offset-Delay[0])&HRTF_HISTORY_MASK]*g;
+        right = hrtfstate->History[(Offset-Delay[1])&HRTF_HISTORY_MASK]*g;
 
         hrtfstate->Values[(Offset+IrSize-1)&HRIR_MASK][0] = 0.0f;
         hrtfstate->Values[(Offset+IrSize-1)&HRIR_MASK][1] = 0.0f;
@@ -45,10 +48,10 @@ void MixHrtf(ALfloat *restrict LeftOut, ALfloat *restrict RightOut,
         *(LeftOut++)  += hrtfstate->Values[Offset&HRIR_MASK][0];
         *(RightOut++) += hrtfstate->Values[Offset&HRIR_MASK][1];
 
-        gain += gainstep;
+        stepcount += 1.0f;
         Offset++;
     }
-    hrtfparams->Gain = gain;
+    hrtfparams->Gain = gain + gainstep*stepcount;
 }
 
 void MixHrtfBlend(ALfloat *restrict LeftOut, ALfloat *restrict RightOut,
@@ -59,12 +62,13 @@ void MixHrtfBlend(ALfloat *restrict LeftOut, ALfloat *restrict RightOut,
 {
     const ALfloat (*OldCoeffs)[2] = ASSUME_ALIGNED(oldparams->Coeffs, 16);
     const ALsizei OldDelay[2] = { oldparams->Delay[0], oldparams->Delay[1] };
-    ALfloat oldGain = oldparams->Gain;
-    ALfloat oldGainStep = -oldGain / (ALfloat)BufferSize;
+    const ALfloat oldGain = oldparams->Gain;
+    const ALfloat oldGainStep = -oldGain / (ALfloat)BufferSize;
     const ALfloat (*NewCoeffs)[2] = ASSUME_ALIGNED(newparams->Coeffs, 16);
     const ALsizei NewDelay[2] = { newparams->Delay[0], newparams->Delay[1] };
-    ALfloat newGain = newparams->Gain;
-    ALfloat newGainStep = newparams->GainStep;
+    const ALfloat newGain = newparams->Gain;
+    const ALfloat newGainStep = newparams->GainStep;
+    ALfloat g, stepcount = 0.0f;
     ALfloat left, right;
     ALsizei i;
 
@@ -80,22 +84,23 @@ void MixHrtfBlend(ALfloat *restrict LeftOut, ALfloat *restrict RightOut,
 
         hrtfstate->History[Offset&HRTF_HISTORY_MASK] = *(data++);
 
-        left = hrtfstate->History[(Offset-OldDelay[0])&HRTF_HISTORY_MASK]*oldGain;
-        right = hrtfstate->History[(Offset-OldDelay[1])&HRTF_HISTORY_MASK]*oldGain;
+        g = oldGain + oldGainStep*stepcount;
+        left = hrtfstate->History[(Offset-OldDelay[0])&HRTF_HISTORY_MASK]*g;
+        right = hrtfstate->History[(Offset-OldDelay[1])&HRTF_HISTORY_MASK]*g;
         ApplyCoeffs(Offset, hrtfstate->Values, IrSize, OldCoeffs, left, right);
 
-        left = hrtfstate->History[(Offset-NewDelay[0])&HRTF_HISTORY_MASK]*newGain;
-        right = hrtfstate->History[(Offset-NewDelay[1])&HRTF_HISTORY_MASK]*newGain;
+        g = newGain + newGainStep*stepcount;
+        left = hrtfstate->History[(Offset-NewDelay[0])&HRTF_HISTORY_MASK]*g;
+        right = hrtfstate->History[(Offset-NewDelay[1])&HRTF_HISTORY_MASK]*g;
         ApplyCoeffs(Offset, hrtfstate->Values, IrSize, NewCoeffs, left, right);
 
         *(LeftOut++)  += hrtfstate->Values[Offset&HRIR_MASK][0];
         *(RightOut++) += hrtfstate->Values[Offset&HRIR_MASK][1];
 
-        oldGain += oldGainStep;
-        newGain += newGainStep;
+        stepcount += 1.0f;
         Offset++;
     }
-    newparams->Gain = newGain;
+    newparams->Gain = newGain + newGainStep*stepcount;
 }
 
 void MixDirectHrtf(ALfloat *restrict LeftOut, ALfloat *restrict RightOut,