Make the dithering depth configurable

4 files changed, 44 insertions, 24 deletions

diff --git a/Alc/ALc.c b/Alc/ALc.c
index b31c4f08..7f6cf279 100644
--- a/Alc/ALc.c
+++ b/Alc/ALc.c
@@ -2222,6 +2222,35 @@ static ALCenum UpdateDeviceParams(ALCdevice *device, const ALCint *attrList)
           device->SourcesMax, device->NumMonoSources, device->NumStereoSources,
           device->AuxiliaryEffectSlotMax, device->NumAuxSends);
 
+    if(!GetConfigValueBool(alstr_get_cstr(device->DeviceName), NULL, "dither", 1))
+        device->DitherDepth = 0.0f;
+    else
+    {
+        ALint depth = 0;
+        ConfigValueInt(alstr_get_cstr(device->DeviceName), NULL, "dither-depth", &depth);
+        if(depth <= 0)
+        {
+            switch(device->FmtType)
+            {
+                case DevFmtByte:
+                case DevFmtUByte:
+                    depth = 8;
+                    break;
+                case DevFmtShort:
+                case DevFmtUShort:
+                    depth = 16;
+                    break;
+                case DevFmtInt:
+                case DevFmtUInt:
+                case DevFmtFloat:
+                    break;
+            }
+        }
+        else if(depth > 24)
+            depth = 24;
+        device->DitherDepth = (depth > 0) ? powf(2.0f, (ALfloat)(depth-1)) : 0.0f;
+    }
+
     if(ConfigValueBool(alstr_get_cstr(device->DeviceName), NULL, "output-limiter", &val))
         gainLimiter = val ? ALC_TRUE : ALC_FALSE;
     /* Valid values for gainLimiter are ALC_DONT_CARE_SOFT, ALC_TRUE, and
@@ -4026,10 +4055,6 @@ ALC_API ALCdevice* ALC_APIENTRY alcOpenDevice(const ALCchar *deviceName)
             ERR("Unsupported ambi-format: %s\n", fmt);
     }
 
-    device->DitherEnabled = GetConfigValueBool(
-        alstr_get_cstr(device->DeviceName), NULL, "dither", 1
-    );
-
     device->Limiter = CreateDeviceLimiter(device);
 
     if(DefaultEffect.type != AL_EFFECT_NULL)
@@ -4450,8 +4475,6 @@ ALC_API ALCdevice* ALC_APIENTRY alcLoopbackOpenDeviceSOFT(const ALCchar *deviceN
     // Open the "backend"
     V(device->Backend,open)("Loopback");
 
-    device->DitherEnabled = GetConfigValueBool(NULL, NULL, "dither", 1);
-
     device->Limiter = CreateDeviceLimiter(device);
 
     {
diff --git a/Alc/ALu.c b/Alc/ALu.c
index c398dc2b..f6e5cb12 100644
--- a/Alc/ALu.c
+++ b/Alc/ALu.c
@@ -1786,12 +1786,10 @@ void aluMixData(ALCdevice *device, ALvoid *buffer, ALsizei size)
             if(Limiter)
                 ApplyCompression(Limiter, OutChannels, SamplesToDo, OutBuffer);
 
-            /* Dither only applies to 8- and 16-bit output for now, as 32-bit
-             * int and float are at the limits of the rendered sample depth.
-             * This can change if the dithering bit depth becomes configurable
-             * (effectively quantizing to a lower bit depth than the output is
-             * capable of).
-             */
+            if(device->DitherDepth > 0.0f)
+                ApplyDither(OutBuffer, &device->DitherSeed, device->DitherDepth, SamplesToDo,
+                            OutChannels);
+
 #define WRITE(T, a, b, c, d) do {                                             \
     Write##T(SAFE_CONST(ALfloatBUFFERSIZE*,(a)), (b), (c), (d));              \
     buffer = (T*)buffer + (c)*(d);                                            \
@@ -1799,23 +1797,15 @@ void aluMixData(ALCdevice *device, ALvoid *buffer, ALsizei size)
             switch(device->FmtType)
             {
                 case DevFmtByte:
-                    if(device->DitherEnabled)
-                        ApplyDither(OutBuffer, &device->DitherSeed, 128.0f, SamplesToDo, OutChannels);
                     WRITE(ALbyte, OutBuffer, buffer, SamplesToDo, OutChannels);
                     break;
                 case DevFmtUByte:
-                    if(device->DitherEnabled)
-                        ApplyDither(OutBuffer, &device->DitherSeed, 128.0f, SamplesToDo, OutChannels);
                     WRITE(ALubyte, OutBuffer, buffer, SamplesToDo, OutChannels);
                     break;
                 case DevFmtShort:
-                    if(device->DitherEnabled)
-                        ApplyDither(OutBuffer, &device->DitherSeed, 32768.0f, SamplesToDo, OutChannels);
                     WRITE(ALshort, OutBuffer, buffer, SamplesToDo, OutChannels);
                     break;
                 case DevFmtUShort:
-                    if(device->DitherEnabled)
-                        ApplyDither(OutBuffer, &device->DitherSeed, 32768.0f, SamplesToDo, OutChannels);
                     WRITE(ALushort, OutBuffer, buffer, SamplesToDo, OutChannels);
                     break;
                 case DevFmtInt:
diff --git a/OpenAL32/Include/alMain.h b/OpenAL32/Include/alMain.h
index 6e17651f..e5dd8c26 100644
--- a/OpenAL32/Include/alMain.h
+++ b/OpenAL32/Include/alMain.h
@@ -794,7 +794,7 @@ struct ALCdevice_struct
     DistanceComp ChannelDelay[MAX_OUTPUT_CHANNELS];
 
     /* Dithering control. */
-    bool DitherEnabled;
+    ALfloat DitherDepth;
     ALuint DitherSeed;
 
     /* Running count of the mixer invocations, in 31.1 fixed point. This
diff --git a/alsoftrc.sample b/alsoftrc.sample
index 26d494f9..3e7d0eec 100644
--- a/alsoftrc.sample
+++ b/alsoftrc.sample
@@ -188,11 +188,18 @@
 #output-limiter = true
 
 ## dither:
-#  Applies dithering on the final mix for 8- and 16-bit output. This replaces
-#  the distortion created by nearest-value quantization with low-level
-#  whitenoise.
+#  Applies dithering on the final mix, for 8- and 16-bit output by default.
+#  This replaces the distortion created by nearest-value quantization with low-
+#  level whitenoise.
 #dither = true
 
+## dither-depth:
+#  Quantization bit-depth for dithered output. A value of 0 (or less) will
+#  match the output sample depth. For int32, uint32, and float32 output, 0 will
+#  disable dithering because they're at or beyond the rendered precision. The
+#  maximum dither depth is 24.
+#dither-depth = 0
+
 ## volume-adjust:
 #  A global volume adjustment for source output, expressed in decibels. The
 #  value is logarithmic, so +6 will be a scale of (approximately) 2x, +12 will