Add an option for "basic" HRTF rendering

This method is intended to help development by easily testing the quality of
the B-Format encode and B-Format-to-HRTF decode. When used with HRTF, all
sources are renderer using the virtual B-Format output, rather than just
B-Format sources.

Despite the CPU cost savings (only four channels need to be filtered with HRTF,
while sources all render normally), the spatial acuity offered by the B-Format
output is pretty poor since it's only first-order ambisonics, so "full" HRTF
rendering is definitely preferred.

It's /possible/ for some systems to be edge cases that prefer the CPU cost
savings provided by basic over the sharper localization provided by full, and
you do still get 3D positional cues, but this is unlikely to be an actual use-
case in practice.

Alc/ALc.c

@@ -1976,6 +1976,7 @@ static ALCenum UpdateDeviceParams(ALCdevice *device, const ALCint *attrList)
     }
 
     device->Hrtf = NULL;
+    device->Hrtf_Mode = DisabledHrtf;
     if(device->FmtChans != DevFmtStereo)
     {
         free(device->Bs2b);
@@ -1984,20 +1985,35 @@ static ALCenum UpdateDeviceParams(ALCdevice *device, const ALCint *attrList)
     else
     {
         bool headphones = device->IsHeadphones;
+        enum HrtfMode hrtf_mode = FullHrtf;
         const char *mode;
         int bs2blevel;
         int usehrtf;
 
-        if(device->Type != Loopback && ConfigValueStr(NULL, "stereo-mode", &mode))
+        if(device->Type != Loopback)
         {
-            if(strcasecmp(mode, "headphones") == 0)
-                headphones = true;
-            else if(strcasecmp(mode, "speakers") == 0)
-                headphones = false;
-            else if(strcasecmp(mode, "auto") != 0)
-                ERR("Unexpected stereo-mode: %s\n", mode);
+            if(ConfigValueStr(NULL, "stereo-mode", &mode))
+            {
+                if(strcasecmp(mode, "headphones") == 0)
+                    headphones = true;
+                else if(strcasecmp(mode, "speakers") == 0)
+                    headphones = false;
+                else if(strcasecmp(mode, "auto") != 0)
+                    ERR("Unexpected stereo-mode: %s\n", mode);
+            }
+
+            if(ConfigValueStr(NULL, "hrtf-mode", &mode))
+            {
+                if(strcasecmp(mode, "full") == 0)
+                    hrtf_mode = FullHrtf;
+                else if(strcasecmp(mode, "basic") == 0)
+                    hrtf_mode = BasicHrtf;
+                else
+                    ERR("Unexpected hrtf-mode: %s\n", mode);
+            }
         }
 
+
         if(device->Type == Loopback || !ConfigValueBool(NULL, "hrtf", &usehrtf))
             usehrtf = ((device->Flags&DEVICE_HRTF_REQUEST) || headphones);
 
@@ -2005,6 +2021,7 @@ static ALCenum UpdateDeviceParams(ALCdevice *device, const ALCint *attrList)
             device->Hrtf = GetHrtf(device->FmtChans, device->Frequency);
         if(device->Hrtf)
         {
+            device->Hrtf_Mode = hrtf_mode;
             TRACE("HRTF enabled\n");
             free(device->Bs2b);
             device->Bs2b = NULL;
@@ -3273,6 +3290,7 @@ ALC_API ALCdevice* ALC_APIENTRY alcOpenDevice(const ALCchar *deviceName)
 
     device->Flags = 0;
     device->Bs2b = NULL;
+    device->Hrtf_Mode = DisabledHrtf;
     AL_STRING_INIT(device->DeviceName);
     device->DryBuffer = NULL;
 
@@ -3725,6 +3743,7 @@ ALC_API ALCdevice* ALC_APIENTRY alcLoopbackOpenDeviceSOFT(const ALCchar *deviceN
 
     device->Flags = 0;
     device->Bs2b = NULL;
+    device->Hrtf_Mode = DisabledHrtf;
     AL_STRING_INIT(device->DeviceName);
     device->DryBuffer = NULL;
 

Alc/ALu.c

@@ -563,7 +563,7 @@ ALvoid CalcNonAttnSourceParams(ALvoice *voice, const ALsource *ALSource, const A
 
         voice->IsHrtf = AL_FALSE;
     }
-    else if(Device->Hrtf)
+    else if(Device->Hrtf_Mode == FullHrtf)
     {
         voice->Direct.OutBuffer += voice->Direct.OutChannels;
         voice->Direct.OutChannels = 2;
@@ -999,7 +999,7 @@ ALvoid CalcSourceParams(ALvoice *voice, const ALsource *ALSource, const ALCconte
         BufferListItem = BufferListItem->next;
     }
 
-    if(Device->Hrtf)
+    if(Device->Hrtf_Mode == FullHrtf)
     {
         /* Use a binaural HRTF algorithm for stereo headphone playback */
         aluVector dir = {{ 0.0f, 0.0f, -1.0f, 0.0f }};

OpenAL32/Include/alMain.h

@@ -593,6 +593,13 @@ enum DeviceType {
 };
 
 
+enum HrtfMode {
+    DisabledHrtf,
+    BasicHrtf,
+    FullHrtf
+};
+
+
 /* The maximum number of Ambisonics coefficients. For a given order (o), the
  * size needed will be (o+1)**2, thus zero-order has 1, first-order has 4,
  * second-order has 9, and third-order has 16. */
@@ -681,6 +688,7 @@ struct ALCdevice_struct
 
     /* HRTF filter tables */
     const struct Hrtf *Hrtf;
+    enum HrtfMode Hrtf_Mode;
     HrtfState Hrtf_State[MAX_OUTPUT_CHANNELS];
     HrtfParams Hrtf_Params[MAX_OUTPUT_CHANNELS];
     ALuint Hrtf_Offset;