Add an option to reverse Z panning of mono sources

Applications that are not built around OpenAL's orientation system need to
convert their given vectors to it. Depending on how this is done, it can lead
to proper stereo (left-right) panning with improper surround sound (front-back)
panning, which thusly sounds correct with stereo output and incorrect with 4+
channel output.

This option is intended to help fix playback of such applications on surround
sound systems, without having to resort to forcing stereo output.

2 files changed, 10 insertions, 3 deletions

diff --git a/Alc/ALc.c b/Alc/ALc.c
index df1a3ec2..f3096b73 100644
--- a/Alc/ALc.c
+++ b/Alc/ALc.c
@@ -393,6 +393,9 @@ static FILE *LogFile;
 // Cone scalar
 ALdouble ConeScale = 0.5;
 
+// Localized Z scalar for mono sources
+ALdouble ZScale = 1.0;
+
 ///////////////////////////////////////////////////////
 
 
@@ -466,6 +469,10 @@ static void alc_init(void)
     if(str && (strcasecmp(str, "true") == 0 || strtol(str, NULL, 0) == 1))
         ConeScale = 1.0;
 
+    str = getenv("__ALSOFT_REVERSE_Z");
+    if(str && (strcasecmp(str, "true") == 0 || strtol(str, NULL, 0) == 1))
+        ZScale = -1.0;
+
     InitializeCriticalSection(&g_csMutex);
     ALTHUNK_INIT();
     ReadALConfig();
diff --git a/Alc/ALu.c b/Alc/ALu.c
index 96fa8579..dc37bcfa 100644
--- a/Alc/ALu.c
+++ b/Alc/ALu.c
@@ -822,8 +822,8 @@ ALvoid CalcSourceParams(ALsource *ALSource, const ALCcontext *ALContext)
     {
         const ALshort *hrtf_left, *hrtf_right;
 
-        GetHrtfCoeffs(atan2(Position[1], -Position[2]) * (180.0/M_PI),
-                      atan2(Position[0], -Position[2]) * (180.0/M_PI),
+        GetHrtfCoeffs(atan2(Position[1], -Position[2]*ZScale) * (180.0/M_PI),
+                      atan2(Position[0], -Position[2]*ZScale) * (180.0/M_PI),
                       &hrtf_left, &hrtf_right);
         for(i = 0;i < HRTF_LENGTH;i++)
         {
@@ -835,7 +835,7 @@ ALvoid CalcSourceParams(ALsource *ALSource, const ALCcontext *ALContext)
     }
     else
     {
-        pos = aluCart2LUTpos(-Position[2], Position[0]);
+        pos = aluCart2LUTpos(-Position[2]*ZScale, Position[0]);
         SpeakerGain = &Device->PanningLUT[MAXCHANNELS * pos];
 
         DirGain = aluSqrt(Position[0]*Position[0] + Position[2]*Position[2]);