Make supportsThrottleModeCenterZero a plugin option

src/FirmwarePlugin/APM/ArduSubFirmwarePlugin.cc

@@ -52,3 +52,8 @@ int ArduSubFirmwarePlugin::manualControlReservedButtonCount(void)
 {
     return 0;
 }
+
+bool ArduSubFirmwarePlugin::supportsThrottleModeCenterZero(void)
+{
+    return false;
+}

src/FirmwarePlugin/APM/ArduSubFirmwarePlugin.h

@@ -69,6 +69,8 @@ public:
     // Overrides from FirmwarePlugin
     int manualControlReservedButtonCount(void);
 
+    bool supportsThrottleModeCenterZero(void);
+
 };
 
 #endif

src/FirmwarePlugin/FirmwarePlugin.cc

@@ -83,6 +83,12 @@ int FirmwarePlugin::manualControlReservedButtonCount(void)
     return -1;
 }
 
+bool FirmwarePlugin::supportsThrottleModeCenterZero(void)
+{
+    // By default, this is supported
+    return true;
+}
+
 bool FirmwarePlugin::adjustIncomingMavlinkMessage(Vehicle* vehicle, mavlink_message_t* message)
 {
     Q_UNUSED(vehicle);

src/FirmwarePlugin/FirmwarePlugin.h

@@ -128,6 +128,11 @@ public:
     /// @return -1: reserver all buttons, >0 number of buttons to reserve
     virtual int manualControlReservedButtonCount(void);
 
+    /// Returns true if the vehicle and firmware supports the use of a throttle joystick that
+    /// is zero when centered. Typically not supported on vehicles that have bidirectional
+    /// throttle.
+    virtual bool supportsThrottleModeCenterZero(void);
+
     /// Called before any mavlink message is processed by Vehicle such that the firmwre plugin
     /// can adjust any message characteristics. This is handy to adjust or differences in mavlink
     /// spec implementations such that the base code can remain mavlink generic.

src/Joystick/Joystick.cc

@@ -284,7 +284,7 @@ void Joystick::run(void)
             throttle =  std::max(-1.0f, std::min(tanf(asinf(throttle_limited)), 1.0f));
 
             // Adjust throttle to 0:1 range
-            if (_throttleMode == ThrottleModeCenterZero && !_activeVehicle->sub()) {
+            if (_throttleMode == ThrottleModeCenterZero && _activeVehicle->supportsThrottleModeCenterZero()) {
                 throttle = std::max(0.0f, throttle);
             } else {
                 throttle = (throttle + 1.0f) / 2.0f;

src/Vehicle/Vehicle.cc

@@ -1528,6 +1528,11 @@ bool Vehicle::supportsManualControl(void) const
     return false;
 }
 
+bool Vehicle::supportsThrottleModeCenterZero(void) const
+{
+    return _firmwarePlugin->supportsThrottleModeCenterZero();
+}
+
 void Vehicle::_setCoordinateValid(bool coordinateValid)
 {
     if (coordinateValid != _coordinateValid) {

src/Vehicle/Vehicle.h

@@ -276,6 +276,7 @@ public:
     Q_PROPERTY(bool                 vtol                    READ vtol                                                   CONSTANT)
     Q_PROPERTY(bool                 rover                   READ rover                                                  CONSTANT)
     Q_PROPERTY(bool                 supportsManualControl   READ supportsManualControl                                  CONSTANT)
+    Q_PROPERTY(bool        supportsThrottleModeCenterZero   READ supportsThrottleModeCenterZero                         CONSTANT)
     Q_PROPERTY(bool                 sub                     READ sub                                                    CONSTANT)
     Q_PROPERTY(bool                 autoDisconnect          MEMBER _autoDisconnect                                      NOTIFY autoDisconnectChanged)
     Q_PROPERTY(QString              prearmError             READ prearmError            WRITE setPrearmError            NOTIFY prearmErrorChanged)
@@ -473,6 +474,7 @@ public:
     bool sub(void) const;
 
     bool supportsManualControl(void) const;
+    bool supportsThrottleModeCenterZero(void) const;
 
     void setFlying(bool flying);
     void setGuidedMode(bool guidedMode);