Add a head-dampening option

This simulates occlusion of the player's head for sounds coming from behind,
when outputing to mono or stereo

Alc/ALc.c

@@ -1711,6 +1711,15 @@ ALCAPI ALCdevice* ALCAPIENTRY alcOpenDevice(const ALCchar *deviceName)
 
         device->Bs2bLevel = GetConfigValueInt(NULL, "cf_level", 0);
 
+        if(aluChannelsFromFormat(device->Format) <= 2)
+        {
+            device->HeadDampen = GetConfigValueFloat(NULL, "head_dampen", DEFAULT_HEAD_DAMPEN);
+            device->HeadDampen = __min(device->HeadDampen, 1.0f);
+            device->HeadDampen = __max(device->HeadDampen, 0.0f);
+        }
+        else
+            device->HeadDampen = 0.0f;
+
         // Find a playback device to open
         SuspendContext(NULL);
         for(i = 0;BackendList[i].Init;i++)

Alc/ALu.c

@@ -685,17 +685,11 @@ static ALvoid CalcSourceParams(const ALCcontext *ALContext, ALsource *ALSource,
         ALfloat scale = (Angle-InnerAngle) / (OuterAngle-InnerAngle);
         ConeVolume = (1.0f+(ALSource->flOuterGain-1.0f)*scale);
         ConeHF = (1.0f+(OuterGainHF-1.0f)*scale);
-        DryMix *= ConeVolume;
-        if(ALSource->DryGainHFAuto)
-            DryGainHF *= ConeHF;
     }
     else if(Angle > OuterAngle)
     {
         ConeVolume = (1.0f+(ALSource->flOuterGain-1.0f));
         ConeHF = (1.0f+(OuterGainHF-1.0f));
-        DryMix *= ConeVolume;
-        if(ALSource->DryGainHFAuto)
-            DryGainHF *= ConeHF;
     }
     else
     {
@@ -703,6 +697,24 @@ static ALvoid CalcSourceParams(const ALCcontext *ALContext, ALsource *ALSource,
         ConeHF = 1.0f;
     }
 
+    // Apply some high-frequency attenuation for sources behind the listener
+    // NOTE: This should be aluDotproduct({0,0,-1}, ListenerToSource), however
+    // that is equivalent to aluDotproduct({0,0,1}, SourceToListener), which is
+    // the same as SourceToListener[2]
+    Angle = aluAcos(SourceToListener[2]) * 180.0f/M_PI;
+    // Sources within the minimum distance attenuate less
+    if(OrigDist < MinDist)
+        Angle *= OrigDist/MinDist;
+    if(Angle > 90.0f)
+    {
+        ALfloat scale = (Angle-90.0f) / (180.1f-90.0f); // .1 to account for fp errors
+        ConeHF *= 1.0f - (ALContext->Device->HeadDampen*scale);
+    }
+
+    DryMix *= ConeVolume;
+    if(ALSource->DryGainHFAuto)
+        DryGainHF *= ConeHF;
+
     // Clamp to Min/Max Gain
     DryMix = __min(DryMix,MaxVolume);
     DryMix = __max(DryMix,MinVolume);

OpenAL32/Include/alMain.h

@@ -163,6 +163,8 @@ static __inline void al_print(const char *fname, unsigned int line, const char *
 
 #define LOWPASSFREQCUTOFF          (5000)
 
+#define DEFAULT_HEAD_DAMPEN        (0.25f)
+
 
 // Find the next power-of-2 for non-power-of-2 numbers.
 static __inline ALuint NextPowerOf2(ALuint value)
@@ -269,6 +271,9 @@ struct ALCdevice_struct
     struct bs2b *Bs2b;
     ALCint       Bs2bLevel;
 
+    // Simulated dampening from head occlusion
+    ALfloat      HeadDampen;
+
     // Dry path buffer mix
     float DryBuffer[BUFFERSIZE][OUTPUTCHANNELS];
 

alsoftrc.sample

@@ -48,6 +48,14 @@
 #  stereo modes.
 #cf_level = 0
 
+## head_dampen:
+#  Sets the amount of dampening on sounds emanating from behind the listener.
+#  This is used to simulate the natural occlusion of the head, which is
+#  typically missing with mono and stereo output, and as such, only works on
+#  mono and stereo output modes. Valid values range from 0 to 1 (inclusive),
+#  and higher values provide a stronger effect.
+#head_dampen = 0.25
+
 ## frequency:
 #  Sets the output frequency.
 #frequency = 44100