Fixed bug 2156 - Android: Accelerometer values fed to joystick driver are inverted for the X axis and wrong for Z

Denis Bernard

Background information: http://developer.android.com/reference/android/hardware/SensorEvent.html#values

Steps to reproduce: compile testjoystick.c as an android app (change screen size according to your device). While running the app, also run:

adb logcat -c; adb logcat -s 'SDL:*' 'SDL/APP:*'

When tilting the device left/right, the joystick moves in the opposite direction of what one would expect. Or at least, the behaviour is not consistent with the Y axis.

Also when the device sits on a table (obviously not moving), the Z axis value oscillates between -32000 and +32000 (by overflow):

I/SDL/APP ( 1994): Joystick 0 axis 2 value: 32511
I/SDL/APP ( 1994): Joystick 0 axis 2 value: 32575
I/SDL/APP ( 1994): Joystick 0 axis 2 value: 32383
I/SDL/APP ( 1994): Joystick 0 axis 2 value: -32386
I/SDL/APP ( 1994): Joystick 0 axis 2 value: -32450
I/SDL/APP ( 1994): Joystick 0 axis 2 value: -32578

This is caused by the accelerometer yielding a constant value around 9.81 for Z and feeding something like 0.9 to 1.1 to the joystick driver, resulting in the overflow.

Proposed fix in SDLActivity.java (swap X and subtract G from Z reading)

android-project/src/org/libsdl/app/SDLActivity.java

@@ -642,9 +642,9 @@ class SDLSurface extends SurfaceView implements SurfaceHolder.Callback,
                     y = event.values[1];
                     break;
             }
-            SDLActivity.onNativeAccel(x / SensorManager.GRAVITY_EARTH,
+            SDLActivity.onNativeAccel(-x / SensorManager.GRAVITY_EARTH,
                                       y / SensorManager.GRAVITY_EARTH,
-                                      event.values[2] / SensorManager.GRAVITY_EARTH);
+                                      event.values[2] / SensorManager.GRAVITY_EARTH - 1);
         }
     }