Alter tha curve of the density-related delay scale

The delay scale is roughly linear with respect to room size, however the
density is not linear with room size. The density is calculated by taking the
room size cubed, then normalized by some factor. Unnormalizing the density and
taking the cube root restores the original room size to use as a delay scale.

The patch also alters the delay and all-pass line lengths to be based on a 1
meter room size, so the the room size recovered from the density acts as a
direct multiple for the desired target length.

Note that the room scale range is unchanged (5m to 50m), so the minimum and
maximum delays are the same. It should also be noted that 50m may not be the
correct room size for a density value of 1. A density value of 1 corresponds to
an environment size of roughly 2.52m when converted from EAX (DENSITY_SCALE
should be 16 rather than 125000), but sizes that low result in undesirable
resonance in the feedback, indicating other changes are necessary for that to
work.

1 files changed, 33 insertions, 22 deletions

diff --git a/Alc/effects/reverb.c b/Alc/effects/reverb.c
index 864163be..9fba183a 100644
--- a/Alc/effects/reverb.c
+++ b/Alc/effects/reverb.c
@@ -82,14 +82,21 @@ static const aluMatrixf A2B = {{
 static const ALfloat FadeStep = 1.0f / FADE_SAMPLES;
 
 /* The all-pass and delay lines have a variable length dependent on the
- * effect's density parameter.  The resulting density multiplier is:
+ * effect's density parameter, which helps alter the perceived environment
+ * size. The size-to-density conversion is a cubed scale:
  *
- *     multiplier = 1 + (density * LINE_MULTIPLIER)
+ * density = min(1.0, pow(size, 3.0) / DENSITY_SCALE);
  *
- * Thus the line multiplier below will result in a maximum density multiplier
- * of 10.
+ * The line lengths scale linearly with room size, so the inverse density
+ * conversion is needed, taking the cube root of the re-scaled density to
+ * calculate the line length multiplier:
+ *
+ *     length_mult = max(5.0, cbrtf(density*DENSITY_SCALE));
+ *
+ * The density scale below will result in a max line multiplier of 50, for an
+ * effective size range of 5m to 50m.
  */
-static const ALfloat LINE_MULTIPLIER = 9.0f;
+static const ALfloat DENSITY_SCALE = 125000.0f;
 
 /* All delay line lengths are specified in seconds.
  *
@@ -133,12 +140,11 @@ static const ALfloat LINE_MULTIPLIER = 9.0f;
  *     T_i = R_i - r_0
  *         = (2^(i / (2 N - 1)) - 1) r_d
  *
- * Assuming an average of 5m (up to 50m with the density multiplier), we get
- * the following taps:
+ * Assuming an average of 1m, we get the following taps:
  */
 static const ALfloat EARLY_TAP_LENGTHS[NUM_LINES] =
 {
-    0.000000e+0f, 1.010676e-3f, 2.126553e-3f, 3.358580e-3f
+    0.0000000e+0f, 2.0213520e-4f, 4.2531060e-4f, 6.7171600e-4f
 };
 
 /* The early all-pass filter lengths are based on the early tap lengths:
@@ -149,7 +155,7 @@ static const ALfloat EARLY_TAP_LENGTHS[NUM_LINES] =
  */
 static const ALfloat EARLY_ALLPASS_LENGTHS[NUM_LINES] =
 {
-    4.854840e-4f, 5.360178e-4f, 5.918117e-4f, 6.534130e-4f
+    9.7096800e-5f, 1.0720356e-4f, 1.1836234e-4f, 1.3068260e-4f
 };
 
 /* The early delay lines are used to transform the primary reflections into
@@ -172,11 +178,11 @@ static const ALfloat EARLY_ALLPASS_LENGTHS[NUM_LINES] =
  *         = 2 (r_a - T_(N-i-1) - r_0)
  *         = 2 r_a (1 - (2 / 3) 2^((N - i - 1) / (2 N - 1)))
  *
- * Using an average dimension of 5m, we get:
+ * Using an average dimension of 1m, we get:
  */
 static const ALfloat EARLY_LINE_LENGTHS[NUM_LINES] =
 {
-    2.992520e-3f, 5.456575e-3f, 7.688329e-3f, 9.709681e-3f
+    5.9850400e-4f, 1.0913150e-3f, 1.5376658e-3f, 1.9419362e-3f
 };
 
 /* The late all-pass filter lengths are based on the late line lengths:
@@ -185,7 +191,7 @@ static const ALfloat EARLY_LINE_LENGTHS[NUM_LINES] =
  */
 static const ALfloat LATE_ALLPASS_LENGTHS[NUM_LINES] =
 {
-    8.091400e-4f, 1.019453e-3f, 1.407968e-3f, 1.618280e-3f
+    1.6182800e-4f, 2.0389060e-4f, 2.8159360e-4f, 3.2365600e-4f
 };
 
 /* The late lines are used to approximate the decaying cycle of recursive
@@ -201,19 +207,19 @@ static const ALfloat LATE_ALLPASS_LENGTHS[NUM_LINES] =
  *     L_i = 2 r_a - L_(i-N/2)
  *         = 2 r_a - 2^((i - N / 2) / (N - 1)) r_d
  *
- * For our 5m average room, we get:
+ * For our 1m average room, we get:
  */
 static const ALfloat LATE_LINE_LENGTHS[NUM_LINES] =
 {
-    9.709681e-3f, 1.223343e-2f, 1.689561e-2f, 1.941936e-2f
+    1.9419362e-3f, 2.4466860e-3f, 3.3791220e-3f, 3.8838720e-3f
 };
 
 /* This coefficient is used to define the delay scale from the sinus, according
  * to the modulation depth property. This value must be below the shortest late
- * line length (0.0097), otherwise with certain parameters (high mod time, low
- * density) the downswing can sample before the input.
+ * line length, otherwise with certain parameters (high mod time, low density)
+ * the downswing can sample before the input.
  */
-static const ALfloat MODULATION_DEPTH_COEFF = 0.0032f;
+static const ALfloat MODULATION_DEPTH_COEFF = 0.001f;
 
 
 typedef struct DelayLineI {
@@ -464,6 +470,11 @@ static ALvoid ALreverbState_Destruct(ALreverbState *State)
  *  Device Update                     *
  **************************************/
 
+static inline ALfloat CalcDelayLengthMult(ALfloat density)
+{
+    return maxf(5.0f, cbrtf(density*DENSITY_SCALE));
+}
+
 /* Given the allocated sample buffer, this function updates each delay line
  * offset.
  */
@@ -514,7 +525,7 @@ static ALboolean AllocLines(const ALuint frequency, ALreverbState *State)
     /* Multiplier for the maximum density value, i.e. density=1, which is
      * actually the least density...
      */
-    multiplier = 1.0f + AL_EAXREVERB_MAX_DENSITY*LINE_MULTIPLIER;
+    multiplier = CalcDelayLengthMult(AL_EAXREVERB_MAX_DENSITY);
 
     /* The main delay length includes the maximum early reflection delay, the
      * largest early tap width, the maximum late reverb delay, and the
@@ -588,7 +599,7 @@ static ALboolean ALreverbState_deviceUpdate(ALreverbState *State, ALCdevice *Dev
     if(!AllocLines(frequency, State))
         return AL_FALSE;
 
-    multiplier = 1.0f + AL_EAXREVERB_MAX_DENSITY*LINE_MULTIPLIER;
+    multiplier = CalcDelayLengthMult(AL_EAXREVERB_MAX_DENSITY);
 
     /* The late feed taps are set a fixed position past the latest delay tap. */
     State->LateFeedTap = fastf2i((AL_EAXREVERB_MAX_REFLECTIONS_DELAY +
@@ -993,7 +1004,7 @@ static ALvoid UpdateDelayLine(const ALfloat earlyDelay, const ALfloat lateDelay,
     ALfloat multiplier, length;
     ALuint i;
 
-    multiplier = 1.0f + density*LINE_MULTIPLIER;
+    multiplier = CalcDelayLengthMult(density);
 
     /* Early reflection taps are decorrelated by means of an average room
      * reflection approximation described above the definition of the taps.
@@ -1024,7 +1035,7 @@ static ALvoid UpdateEarlyLines(const ALfloat density, const ALfloat decayTime, c
     ALfloat multiplier, length;
     ALsizei i;
 
-    multiplier = 1.0f + density*LINE_MULTIPLIER;
+    multiplier = CalcDelayLengthMult(density);
 
     for(i = 0;i < NUM_LINES;i++)
     {
@@ -1059,7 +1070,7 @@ static ALvoid UpdateLateLines(const ALfloat density, const ALfloat diffusion, co
      * The average length of the delay lines is used to calculate the
      * attenuation coefficient.
      */
-    multiplier = 1.0f + density*LINE_MULTIPLIER;
+    multiplier = CalcDelayLengthMult(density);
     length = (LATE_LINE_LENGTHS[0] + LATE_LINE_LENGTHS[1] +
               LATE_LINE_LENGTHS[2] + LATE_LINE_LENGTHS[3]) / 4.0f * multiplier;
     /* Include the echo transformation (see below). */