Fix IMU orientations for 8bitdo wireless 2 controller in bluetooth mode

Fix IMU timing stamp - simulate the timing stamp from the controller hardware's perspective, as opposed to the receiving computer's perspective. Do this by advancing a time stamp by a fixed rate, based on observation of average rate over bluetooth.

src/joystick/hidapi/SDL_hidapi_8bitdo.c

@@ -42,9 +42,8 @@ enum
     SDL_GAMEPAD_NUM_8BITDO_BUTTONS,
 };
 
-#define ABITDO_GYRO_SCALE  14.2842f
 #define ABITDO_ACCEL_SCALE 4096.f
-
+#define SENSOR_INTERVAL_NS 8000000ULL
 
 typedef struct
 {
@@ -63,6 +62,7 @@ typedef struct
     float accelScale;
     float gyroScale;
     Uint8 last_state[USB_PACKET_LENGTH];
+    Uint64 sensor_timestamp; // Microseconds. Simulate onboard clock. Advance by known rate: SENSOR_INTERVAL_NS == 8ms = 125 Hz
 } SDL_Driver8BitDo_Context;
 
 #pragma pack(push,1)
@@ -149,6 +149,10 @@ static void HIDAPI_Driver8BitDo_SetDevicePlayerIndex(SDL_HIDAPI_Device *device,
 {
 }
 
+#ifndef DEG2RAD
+#define DEG2RAD(x) ((float)(x) * (float)(SDL_PI_F / 180.f))
+#endif
+
 static bool HIDAPI_Driver8BitDo_OpenJoystick(SDL_HIDAPI_Device *device, SDL_Joystick *joystick)
 {
     SDL_Driver8BitDo_Context *ctx = (SDL_Driver8BitDo_Context *)device->context;
@@ -163,12 +167,12 @@ static bool HIDAPI_Driver8BitDo_OpenJoystick(SDL_HIDAPI_Device *device, SDL_Joys
     joystick->nhats = 1;
 
     if (ctx->sensors_supported) {
-        SDL_PrivateJoystickAddSensor(joystick, SDL_SENSOR_GYRO, 250.0f);
+        SDL_PrivateJoystickAddSensor(joystick, SDL_SENSOR_GYRO, 125.0f);
-        SDL_PrivateJoystickAddSensor(joystick, SDL_SENSOR_ACCEL, 250.0f);
+        SDL_PrivateJoystickAddSensor(joystick, SDL_SENSOR_ACCEL, 125.0f);
 
 
         ctx->accelScale = SDL_STANDARD_GRAVITY / ABITDO_ACCEL_SCALE;
-        ctx->gyroScale = SDL_PI_F / 180.0f / ABITDO_GYRO_SCALE;
+        ctx->gyroScale = DEG2RAD(2048) / INT16_MAX; // Hardware senses  +/- 2048 Degrees per second mapped to +/- INT16_MAX
     }
 
     return true;
@@ -357,15 +361,32 @@ static void HIDAPI_Driver8BitDo_HandleStatePacket(SDL_Joystick *joystick, SDL_Dr
         float values[3];
         ABITDO_SENSORS *sensors = (ABITDO_SENSORS *)&data[15];
 
-        sensor_timestamp = timestamp;
+        // Note: we cannot use the time stamp of the receiving computer due to packet delay creating "spiky" timings.
-        values[0] = (sensors->sGyroX) * (ctx->gyroScale);
+        // The imu time stamp is intended to be the sample time of the on-board hardware.
-        values[1] = (sensors->sGyroZ) * (ctx->gyroScale);
+        // In the absence of time stamp data from the data[], we can simulate that by
-        values[2] = (sensors->sGyroY) * (ctx->gyroScale);
+        // advancing a time stamp by the observed/known imu clock rate. This is 8ms = 125 Hz
+        sensor_timestamp = ctx->sensor_timestamp;
+        ctx->sensor_timestamp += SENSOR_INTERVAL_NS;
+
+        // This device's IMU values are reported differently from SDL
+        // Thus we perform a rotation of the coordinate system to match the SDL standard.
+
+        // By observation of this device:
+        // Hardware x is reporting roll (rotation about the power jack's axis)
+        // Hardware y is reporting pitch (rotation about the horizontal axis)
+        // Hardware z is reporting yaw (rotation about the joysticks' center axis)
+        values[0] = -sensors->sGyroY * ctx->gyroScale;  // Rotation around pitch axis
+        values[1] = sensors->sGyroZ * ctx->gyroScale;   // Rotation around yaw axis
+        values[2] = -sensors->sGyroX * ctx->gyroScale;  // Rotation around roll axis
         SDL_SendJoystickSensor(timestamp, joystick, SDL_SENSOR_GYRO, sensor_timestamp, values, 3);
 
-        values[0] = (sensors->sAccelX) * (ctx->accelScale);
+        // By observation of this device:
-        values[1] = (sensors->sAccelZ) * (ctx->accelScale);
+        // Accelerometer X is positive when front of the controller points toward the sky.
-        values[2] = (sensors->sAccelY) * (ctx->accelScale);
+        // Accelerometer y is positive when left side of the controller points toward the sky.
+        // Accelerometer Z is positive when sticks point toward the sky.
+        values[0] = -sensors->sAccelY * ctx->accelScale; // Acceleration along pitch axis
+        values[1] = sensors->sAccelZ * ctx->accelScale;  // Acceleration along yaw axis
+        values[2] = -sensors->sAccelX * ctx->accelScale; // Acceleration along roll axis
         SDL_SendJoystickSensor(timestamp, joystick, SDL_SENSOR_ACCEL, sensor_timestamp, values, 3);
     }