Rework HRTF coefficient fading

This improves fading between HRIRs as sources pan around. In particular, it
improves the issue with individual coefficients having various rounding errors
in the stepping values, as well as issues with interpolating delay values.

It does this by doing two mixing passes for each source. First using the last
coefficients that fade to silence, and then again using the new coefficients
that fade from silence. When added together, it creates a linear fade from one
to the other. Additionally, the gain is applied separately so the individual
coefficients don't step with rounding errors. Although this does increase CPU
cost since it's doing two mixes per source, each mix is a bit cheaper now since
the stepping is simplified to a single gain value, and the overall quality is
improved.

2 files changed, 9 insertions, 11 deletions

diff --git a/OpenAL32/Include/alMain.h b/OpenAL32/Include/alMain.h
index 04fb7239..7663e58d 100644
--- a/OpenAL32/Include/alMain.h
+++ b/OpenAL32/Include/alMain.h
@@ -620,6 +620,7 @@ typedef struct HrtfState {
 typedef struct HrtfParams {
     alignas(16) ALfloat Coeffs[HRIR_LENGTH][2];
     ALsizei Delay[2];
+    ALfloat Gain;
 } HrtfParams;
 
 typedef struct DirectHrtfState {
diff --git a/OpenAL32/Include/alu.h b/OpenAL32/Include/alu.h
index 8888bdc0..72f33be2 100644
--- a/OpenAL32/Include/alu.h
+++ b/OpenAL32/Include/alu.h
@@ -116,12 +116,9 @@ enum ActiveFilters {
 
 
 typedef struct MixHrtfParams {
-    const HrtfParams *Target;
-    HrtfParams *Current;
-    struct {
-        alignas(16) ALfloat Coeffs[HRIR_LENGTH][2];
-        ALsizei Delay[2];
-    } Steps;
+    const HrtfParams *Current;
+    ALfloat Gain;
+    ALfloat GainStep;
 } MixHrtfParams;
 
 
@@ -133,7 +130,7 @@ typedef struct DirectParams {
     NfcFilter NFCtrlFilter[MAX_AMBI_ORDER];
 
     struct {
-        HrtfParams Current;
+        HrtfParams Old;
         HrtfParams Target;
         HrtfState State;
     } Hrtf;
@@ -225,13 +222,13 @@ typedef void (*RowMixerFunc)(ALfloat *OutBuffer, const ALfloat *gains,
                              const ALfloat (*restrict data)[BUFFERSIZE], ALsizei InChans,
                              ALsizei InPos, ALsizei BufferSize);
 typedef void (*HrtfMixerFunc)(ALfloat *restrict LeftOut, ALfloat *restrict RightOut,
-                              const ALfloat *data, ALsizei Counter, ALsizei Offset, ALsizei OutPos,
-                              const ALsizei IrSize, const MixHrtfParams *hrtfparams,
+                              const ALfloat *data, ALsizei Offset, ALsizei OutPos,
+                              const ALsizei IrSize, MixHrtfParams *hrtfparams,
                               HrtfState *hrtfstate, ALsizei BufferSize);
 typedef void (*HrtfDirectMixerFunc)(ALfloat *restrict LeftOut, ALfloat *restrict RightOut,
                                     const ALfloat *data, ALsizei Offset, const ALsizei IrSize,
-                                    ALfloat (*restrict Coeffs)[2], ALfloat (*restrict Values)[2],
-                                    ALsizei BufferSize);
+                                    const ALfloat (*restrict Coeffs)[2],
+                                    ALfloat (*restrict Values)[2], ALsizei BufferSize);
 
 
 #define GAIN_MIX_MAX  (16.0f) /* +24dB */