Greatly improves the plausibility of the transonic flight simulation (#4652)

* Transonic flight now induces yawing and pitching of the nose of the bullet instead of modifying the bullet velocity directly
* The ballistic coefficient now starts to decreases once the bullet goes transonic

extensions/advanced_ballistics/AdvancedBallistics.cpp

@@ -48,6 +48,7 @@ struct Bullet {
     double lastFrame;
     double hDeflection;
     double spinDrift;
+    double bcDegradation;
     unsigned randSeed;
     std::default_random_engine randGenerator;
 };
@@ -68,7 +69,6 @@ Map* map = &mapDatabase[""];
 double calculateRoughnessLength(double posX, double posY) {
     // Source: http://es.ucsc.edu/~jnoble/wind/extrap/index.html
     double roughness_lengths[10] = {0.0002, 0.0005, 0.0024, 0.03, 0.055, 0.1, 0.2, 0.4, 0.8, 1.6};
-    double roughnessLength = 0.0024;
 
     int gridX = (int)floor(posX / 50);
     int gridY = (int)floor(posY / 50);
@@ -482,6 +482,7 @@ void __stdcall RVExtension(char *output, int outputSize, const char *function)
         bulletDatabase[index].lastFrame = tickTime;
         bulletDatabase[index].hDeflection = 0.0;
         bulletDatabase[index].spinDrift = 0.0;
+        bulletDatabase[index].bcDegradation = 1.0;
         bulletDatabase[index].speed = 0.0;
         bulletDatabase[index].frames = 0.0;
         bulletDatabase[index].randSeed = 0;
@@ -617,6 +618,24 @@ void __stdcall RVExtension(char *output, int outputSize, const char *function)
         trueVelocity[2] = velocity[2] - wind[2];
         trueSpeed = sqrt(pow(trueVelocity[0], 2) + pow(trueVelocity[1], 2) + pow(trueVelocity[2], 2));
 
+        double speedOfSound = 331.3 + (0.6 * temperature);
+        double transonicSpeed = 394 + (0.6 * temperature);
+        if (bulletDatabase[index].transonicStabilityCoef < 1.0f && bulletSpeed < transonicSpeed && bulletSpeed > speedOfSound) {
+            std::uniform_real_distribution<double> distribution(-10.0, 10.0);
+            double coef = 1.0f - bulletDatabase[index].transonicStabilityCoef;
+
+            trueVelocity[0] += distribution(bulletDatabase[index].randGenerator) * coef;
+            trueVelocity[1] += distribution(bulletDatabase[index].randGenerator) * coef;
+            trueVelocity[2] += distribution(bulletDatabase[index].randGenerator) * coef;
+            double speed = sqrt(pow(trueVelocity[0], 2) + pow(trueVelocity[1], 2) + pow(trueVelocity[2], 2));
+
+            trueVelocity[0] *= trueSpeed / speed;
+            trueVelocity[1] *= trueSpeed / speed;
+            trueVelocity[2] *= trueSpeed / speed;
+
+            bulletDatabase[index].bcDegradation *= pow(0.993, coef);
+        };
+
         temperature = bulletDatabase[index].temperature - 0.0065 * position[2];
         pressure = (1013.25 - 10 * bulletDatabase[index].overcast) * pow(1 - (0.0065 * (bulletDatabase[index].altitude + position[2])) / (273.15 + temperature + 0.0065 * bulletDatabase[index].altitude), 5.255754495);
 
@@ -640,6 +659,7 @@ void __stdcall RVExtension(char *output, int outputSize, const char *function)
             }
 
             ballisticCoefficient = calculateAtmosphericCorrection(ballisticCoefficient, temperature, pressure, bulletDatabase[index].humidity, bulletDatabase[index].atmosphereModel);
+            ballisticCoefficient *= bulletDatabase[index].bcDegradation;
             drag = deltaT * calculateRetard(bulletDatabase[index].dragModel, ballisticCoefficient, trueSpeed);
             accel[0] = (trueVelocity[0] / trueSpeed) * drag;
             accel[1] = (trueVelocity[1] / trueSpeed) * drag;
@@ -694,16 +714,6 @@ void __stdcall RVExtension(char *output, int outputSize, const char *function)
         positionOffset[0] += sin(bulletDir + M_PI / 2) * spinDriftPartial;
         positionOffset[1] += cos(bulletDir + M_PI / 2) * spinDriftPartial;
 
-        double speedOfSound = 331.3 + (0.6 * temperature);
-        if (bulletSpeed < (speedOfSound + 5) && bulletSpeedAvg > speedOfSound && bulletSpeed > (speedOfSound - 5)) {
-            std::uniform_real_distribution<double> distribution(0.0, 1.0);
-            double coef = 1.0f - bulletDatabase[index].transonicStabilityCoef;
-
-            velocityOffset[0] += (distribution(bulletDatabase[index].randGenerator) * 0.8 - 0.4) * coef;
-            velocityOffset[1] += (distribution(bulletDatabase[index].randGenerator) * 0.8 - 0.4) * coef;
-            velocityOffset[2] += (distribution(bulletDatabase[index].randGenerator) * 0.8 - 0.4) * coef;
-        };
-
         outputStr << "_bullet setVelocity (_bulletVelocity vectorAdd [" << velocityOffset[0] << "," << velocityOffset[1] << "," << velocityOffset[2] << "]); _bullet setPosASL (_bulletPosition vectorAdd [" << positionOffset[0] << "," << positionOffset[1] << "," << positionOffset[2] << "]);";
         strncpy_s(output, outputSize, outputStr.str().c_str(), _TRUNCATE);
         EXTENSION_RETURN();
@@ -725,7 +735,7 @@ void __stdcall RVExtension(char *output, int outputSize, const char *function)
     } else if (!strcmp(mode, "init")) {
         int mapSize = 0;
         int mapGrids = 0;
-        int gridCells = 0;
+        unsigned int gridCells = 0;
 
         worldName = strtok_s(NULL, ":", &next_token);
         mapSize = strtol(strtok_s(NULL, ":", &next_token), NULL, 10);