Store HRTF delays with some fractional precision

Since the delays get bilinearly filtered before use, maintaining a bit of extra
precision can help accuracy even if it ultimately gets rounded to the nearest
integer before use. This should help with resampled HRTFs, which need to scale
the delays that are already rounded, and can also be leveraged by an updated
MHR format.

1 files changed, 23 insertions, 13 deletions

diff --git a/alc/hrtf.cpp b/alc/hrtf.cpp
index 9564df47..ad14717b 100644
--- a/alc/hrtf.cpp
+++ b/alc/hrtf.cpp
@@ -90,6 +90,11 @@ struct LoadedHrtf {
 
 #define MAX_HRIR_DELAY               (HRTF_HISTORY_LENGTH-1)
 
+#define HRIR_DELAY_FRACBITS 2
+#define HRIR_DELAY_FRACONE (1<<HRIR_DELAY_FRACBITS)
+
+static_assert(MAX_HRIR_DELAY*HRIR_DELAY_FRACONE < 256, "MAX_HRIR_DELAY or DELAY_FRAC too large");
+
 constexpr ALchar magicMarker00[8]{'M','i','n','P','H','R','0','0'};
 constexpr ALchar magicMarker01[8]{'M','i','n','P','H','R','0','1'};
 constexpr ALchar magicMarker02[8]{'M','i','n','P','H','R','0','2'};
@@ -249,12 +254,12 @@ void GetHrtfCoeffs(const HrtfStore *Hrtf, float elevation, float azimuth, float
     };
 
     /* Calculate the blended HRIR delays. */
-    delays[0] = fastf2u(
-        Hrtf->delays[idx[0]][0]*blend[0] + Hrtf->delays[idx[1]][0]*blend[1] +
-        Hrtf->delays[idx[2]][0]*blend[2] + Hrtf->delays[idx[3]][0]*blend[3]);
-    delays[1] = fastf2u(
-        Hrtf->delays[idx[0]][1]*blend[0] + Hrtf->delays[idx[1]][1]*blend[1] +
-        Hrtf->delays[idx[2]][1]*blend[2] + Hrtf->delays[idx[3]][1]*blend[3]);
+    float d{Hrtf->delays[idx[0]][0]*blend[0] + Hrtf->delays[idx[1]][0]*blend[1] +
+        Hrtf->delays[idx[2]][0]*blend[2] + Hrtf->delays[idx[3]][0]*blend[3]};
+    delays[0] = fastf2u(d * float{1.0f/HRIR_DELAY_FRACONE});
+    d = Hrtf->delays[idx[0]][1]*blend[0] + Hrtf->delays[idx[1]][1]*blend[1] +
+        Hrtf->delays[idx[2]][1]*blend[2] + Hrtf->delays[idx[3]][1]*blend[1];
+    delays[1] = fastf2u(d * float{1.0f/HRIR_DELAY_FRACONE});
 
     const ALuint irSize{Hrtf->irSize};
     ASSUME(irSize >= MIN_IR_SIZE);
@@ -333,12 +338,12 @@ void BuildBFormatHrtf(const HrtfStore *Hrtf, DirectHrtfState *state,
             (     elev0.blend) * (     az1.blend)};
 
         /* Calculate the blended HRIR delays. */
-        res.ldelay = fastf2u(
-            Hrtf->delays[idx[0]][0]*blend[0] + Hrtf->delays[idx[1]][0]*blend[1] +
-            Hrtf->delays[idx[2]][0]*blend[2] + Hrtf->delays[idx[3]][0]*blend[3]);
-        res.rdelay = fastf2u(
-            Hrtf->delays[idx[0]][1]*blend[0] + Hrtf->delays[idx[1]][1]*blend[1] +
-            Hrtf->delays[idx[2]][1]*blend[2] + Hrtf->delays[idx[3]][1]*blend[3]);
+        float d{Hrtf->delays[idx[0]][0]*blend[0] + Hrtf->delays[idx[1]][0]*blend[1] +
+            Hrtf->delays[idx[2]][0]*blend[2] + Hrtf->delays[idx[3]][0]*blend[3]};
+        res.ldelay = fastf2u(d * float{1.0f/HRIR_DELAY_FRACONE});
+        d = Hrtf->delays[idx[0]][1]*blend[0] + Hrtf->delays[idx[1]][1]*blend[1] +
+            Hrtf->delays[idx[2]][1]*blend[2] + Hrtf->delays[idx[3]][1]*blend[3];
+        res.rdelay = fastf2u(d * float{1.0f/HRIR_DELAY_FRACONE});
 
         /* Calculate the blended HRIR coefficients. */
         double *coeffout{al::assume_aligned<16>(&res.hrir[0][0])};
@@ -689,6 +694,7 @@ std::unique_ptr<HrtfStore> LoadHrtf00(std::istream &data, const char *filename)
             ERR("Invalid delays[%zd]: %d (%d)\n", i, delays[i][0], MAX_HRIR_DELAY);
             failed = AL_TRUE;
         }
+        delays[i][0] <<= HRIR_DELAY_FRACBITS;
     }
     if(failed)
         return nullptr;
@@ -788,6 +794,7 @@ std::unique_ptr<HrtfStore> LoadHrtf01(std::istream &data, const char *filename)
             ERR("Invalid delays[%zd]: %d (%d)\n", i, delays[i][0], MAX_HRIR_DELAY);
             failed = AL_TRUE;
         }
+        delays[i][0] <<= HRIR_DELAY_FRACBITS;
     }
     if(failed)
         return nullptr;
@@ -950,6 +957,7 @@ std::unique_ptr<HrtfStore> LoadHrtf02(std::istream &data, const char *filename)
                 ERR("Invalid delays[%zu][0]: %d (%d)\n", i, delays[i][0], MAX_HRIR_DELAY);
                 failed = AL_TRUE;
             }
+            delays[i][0] <<= HRIR_DELAY_FRACBITS;
         }
     }
     else if(channelType == CHANTYPE_LEFTRIGHT)
@@ -993,6 +1001,8 @@ std::unique_ptr<HrtfStore> LoadHrtf02(std::istream &data, const char *filename)
                 ERR("Invalid delays[%zu][1]: %d (%d)\n", i, delays[i][1], MAX_HRIR_DELAY);
                 failed = AL_TRUE;
             }
+            delays[i][0] <<= HRIR_DELAY_FRACBITS;
+            delays[i][1] <<= HRIR_DELAY_FRACBITS;
         }
     }
     if(failed)
@@ -1362,7 +1372,7 @@ HrtfStore *GetLoadedHrtf(const std::string &name, const char *devname, const ALu
 
             ALubyte (&delays)[2] = const_cast<ALubyte(&)[2]>(hrtf->delays[i]);
             for(size_t j{0};j < 2;++j)
-                delays[j] = static_cast<ALubyte>(minu(MAX_HRIR_DELAY,
+                delays[j] = static_cast<ALubyte>(minu(MAX_HRIR_DELAY*HRIR_DELAY_FRACONE,
                     (delays[j]*devrate + hrtf->sampleRate/2) / hrtf->sampleRate));
         }