From dfffe9c9249cb6ad011fbbb1796733042f97da47 Mon Sep 17 00:00:00 2001
From: andy <andy>
Date: Sat, 10 Jan 2004 22:34:49 +0000
Subject: [PATCH] Move the turbulence "airmass" according to the local wind.
 Clean up and better document the magic numbers in Turbulence.cpp

---
 src/FDM/YASim/Model.cpp      |  7 ++++++
 src/FDM/YASim/Turbulence.cpp | 43 +++++++++++++++++++++++++++---------
 src/FDM/YASim/Turbulence.hpp |  6 +++--
 3 files changed, 44 insertions(+), 12 deletions(-)

diff --git a/src/FDM/YASim/Model.cpp b/src/FDM/YASim/Model.cpp
index ebcb89fba..c91fe7df6 100644
--- a/src/FDM/YASim/Model.cpp
+++ b/src/FDM/YASim/Model.cpp
@@ -97,6 +97,13 @@ void Model::initIteration()
 	t->getGyro(v);
 	Math::add3(v, _gyro, _gyro);
     }
+
+    // Displace the turbulence coordinates according to the local wind.
+    if(_turb) {
+        float toff[3];
+        Math::mul3(_integrator.getInterval(), _wind, toff);
+        _turb->offset(toff);
+    }
 }
 
 // FIXME: This method looks to me like it's doing *integration*, not
diff --git a/src/FDM/YASim/Turbulence.cpp b/src/FDM/YASim/Turbulence.cpp
index c61e5ec13..8862e5503 100644
--- a/src/FDM/YASim/Turbulence.cpp
+++ b/src/FDM/YASim/Turbulence.cpp
@@ -8,10 +8,24 @@ namespace yasim {
 // generated turbulence fields top out at about 70% of this number.
 const float MAX_TURBULENCE = 20;
 
+// Rate, in "meters" per second, of the default time axis motion.
+// This will be multiplied by the rate-hz property to get the actual
+// time axis offset.  A setting of 2.0 causes the maximum frequency
+// component to arrive at 1 Hz.
+const float BASE_RATE = 2.0;
+
+// Power to which the input magnitude (always in the range [0:1]) is
+// raised to get a coefficient for the turbulence velocity.  Setting
+// this to 1.0 makes the scale linear.  Increasing it makes it
+// curvier, with a sharp increase at the high end of the scale.
+const double MAGNITUDE_EXP = 2.0;
+
 // How many generations are "meaningful" (i.e., not part of the normal
 // wind computation).  Decreasing this number will reallocate
-// bandwidth to the higher frequency components.
-const int MEANINGFUL_GENS = 9;
+// bandwidth to the higher frequency components.  A turbulence field
+// will swing between maximal values over a distance of approximately
+// 2^(MEANINGFUL_GENS-1).
+const int MEANINGFUL_GENS = 8;
 
 static const float FT2M = 0.3048;
 
@@ -100,19 +114,27 @@ inline void Turbulence::turblut(int x, int y, float* out)
     out[2] = c2fu(turb[2]) * (_z1 - _z0) + _z0;
 }
 
+void Turbulence::setMagnitude(double mag)
+{
+    _mag = Math::pow(mag, MAGNITUDE_EXP);
+}
+
 void Turbulence::update(double dt, double rate)
 {
-    // Assume a normal rate is 2 unit/sec.  This will cause the
-    // highest frequency turbulence component to arrive at 1 Hz.
-    _currTime += 2 * dt * rate;
-    if(_currTime > _sz) _currTime -= _sz;
+    _timeOff += BASE_RATE * dt * rate;
+}
+
+void Turbulence::offset(float* offset)
+{
+    for(int i=0; i<3; i++)
+        _off[i] += offset[i];
 }
 
 void Turbulence::getTurbulence(double* loc, float* turbOut)
 {
     // Convert to integer 2D coordinates; wrap to [0:_sz].
-    double a = loc[0] + loc[2];
-    double b = loc[1] + _currTime;
+    double a = (loc[0] + _off[0]) + (loc[2] + _off[2]);
+    double b = (loc[1] + _off[1]) + _timeOff;
     a -= _sz * Math::floor(a * (1.0/_sz));
     b -= _sz * Math::floor(b * (1.0/_sz));
     int x = (int)Math::floor(a);
@@ -130,7 +152,7 @@ void Turbulence::getTurbulence(double* loc, float* turbOut)
     turblut(x+1, y+1, turb11);
 
     // Interpolate, add in units
-    float mag = _mag * _mag * MAX_TURBULENCE;
+    float mag = _mag * MAX_TURBULENCE;
     for(int i=0; i<3; i++) {
         float avg0 = (1-a)*turb00[i] + a*turb01[i];
         float avg1 = (1-a)*turb10[i] + a*turb11[i];
@@ -170,7 +192,8 @@ Turbulence::Turbulence(int gens, int seed)
     _seed = seed;
     _mag = 1;
     _x0 = _x1 = _y0 = _y1 = _z0 = _z1 = 0;
-    _currTime = 0;
+    _timeOff = 0;
+    _off[0] = _off[1] = _off[2] = 0;
 
     float* xbuf = new float[_sz*_sz];
     float* ybuf = new float[_sz*_sz];
diff --git a/src/FDM/YASim/Turbulence.hpp b/src/FDM/YASim/Turbulence.hpp
index a615d028f..606298dd4 100644
--- a/src/FDM/YASim/Turbulence.hpp
+++ b/src/FDM/YASim/Turbulence.hpp
@@ -5,8 +5,9 @@ public:
     Turbulence(int gens, int seed);
     ~Turbulence();
     void update(double dt, double rate);
-    void setMagnitude(double mag) { _mag = mag; }
+    void setMagnitude(double mag);
     void getTurbulence(double* loc, float* turbOut);
+    void offset(float* dist);
 
 private:
     unsigned int hashrand(unsigned int i);
@@ -21,7 +22,8 @@ private:
     int _sz;
     int _seed;
 
-    double _currTime;
+    double _off[3];
+    double _timeOff;
     double _mag;
     float _x0, _x1, _y0, _y1, _z0, _z1;
     unsigned char* _data;