Use a different method for HRTF mixing

This new method mixes sources normally into a 14-channel buffer with the
channels placed all around the listener. HRTF is then applied to the channels
given their positions and written to a 2-channel buffer, which gets written out
to the device.

This method has the benefit that HRTF processing becomes more scalable. The
costly HRTF filters are applied to the 14-channel buffer after the mix is done,
turning it into a post-process with a fixed overhead. Mixing sources is done
with normal non-HRTF methods, so increasing the number of playing sources only
incurs normal mixing costs.

Another benefit is that it improves B-Format playback since the soundfield gets
mixed into speakers covering all three dimensions, which then get filtered
based on their locations.

The main downside to this is that the spatial resolution of the HRTF dataset
does not play a big role anymore. However, the hope is that with ambisonics-
based panning, the perceptual position of panned sounds will still be good. It
is also an option to increase the number of virtual channels for systems that
can handle it, or maybe even decrease it for weaker systems.

1 files changed, 11 insertions, 3 deletions

diff --git a/Alc/ALc.c b/Alc/ALc.c
index 43f3cdc8..538ed4c3 100644
--- a/Alc/ALc.c
+++ b/Alc/ALc.c
@@ -1712,6 +1712,7 @@ static ALCenum UpdateDeviceParams(ALCdevice *device, const ALCint *attrList)
     enum DevFmtType oldType;
     ALCuint oldFreq;
     FPUCtl oldMode;
+    size_t size;
 
     // Check for attributes
     if(device->Type == Loopback)
@@ -1885,6 +1886,9 @@ static ALCenum UpdateDeviceParams(ALCdevice *device, const ALCint *attrList)
     if((device->Flags&DEVICE_RUNNING))
         return ALC_NO_ERROR;
 
+    al_free(device->DryBuffer);
+    device->DryBuffer = NULL;
+
     UpdateClockBase(device);
 
     if(device->Type != Loopback)
@@ -1992,11 +1996,15 @@ static ALCenum UpdateDeviceParams(ALCdevice *device, const ALCint *attrList)
 
     aluInitPanning(device);
 
-    al_free(device->DryBuffer);
-    device->DryBuffer = al_calloc(16, sizeof(device->DryBuffer[0]) * device->NumChannels);
+    /* With HRTF enabled, the channels are virtual and get positioned around
+     * the virtual listener. Two extra channels are allocated for the actual
+     * HRTF-filtered output.
+     */
+    size = sizeof(device->DryBuffer[0]) * (device->NumChannels + (device->Hrtf ? 2 : 0));
+    device->DryBuffer = al_calloc(16, size);
     if(!device->DryBuffer)
     {
-        ERR("Failed to allocate "SZFMT" bytes for mix buffer\n", sizeof(device->DryBuffer[0]) * device->NumChannels);
+        ERR("Failed to allocate "SZFMT" bytes for mix buffer\n", size);
         return ALC_INVALID_DEVICE;
     }