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Bug 2156

Summary: Android: Accelerometer values fed to joystick driver are inverted for the X axis and wrong for Z
Product: SDL Reporter: Denis Bernard <denis>
Component: joystickAssignee: Sam Lantinga <slouken>
Status: RESOLVED FIXED QA Contact: Sam Lantinga <slouken>
Severity: normal    
Priority: P2 CC: denis
Version: HG 2.0   
Hardware: All   
OS: Android (All)   

Description Denis Bernard 2013-10-15 18:04:05 UTC 
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):

@Override
public void onSensorChanged(SensorEvent event) {
    if (event.sensor.getType() == Sensor.TYPE_ACCELEROMETER) {
        SDLActivity.onNativeAccel(-event.values[0] / SensorManager.GRAVITY_EARTH,
                                  event.values[1] / SensorManager.GRAVITY_EARTH,
                                  event.values[2] / SensorManager.GRAVITY_EARTH - 1);
    }
}

Id'd suggest applying this on top of Bug 2155.
Comment 1 Sam Lantinga 2013-10-18 06:47:25 UTC 
Can you double check this relative to your latest patch?

The code now looks like this:
            float x, y;
            switch (mDisplay.getRotation()) {
                case Surface.ROTATION_90:
                    x = -event.values[1];
                    y = event.values[0];
                    break;
                case Surface.ROTATION_270:
                    x = event.values[1];
                    y = -event.values[0];
                    break;
                case Surface.ROTATION_180:
                    x = -event.values[1];
                    y = -event.values[0];
                    break;
                default:
                    x = event.values[0];
                    y = event.values[1];
                    break;
            }
            SDLActivity.onNativeAccel(x / SensorManager.GRAVITY_EARTH,
                                      y / SensorManager.GRAVITY_EARTH,
                                      event.values[2] / SensorManager.GRAVITY_EARTH);
Comment 2 Denis Bernard 2013-10-18 11:24:25 UTC 
(In reply to Sam Lantinga from comment #1)
> Can you double check this relative to your latest patch?

>             SDLActivity.onNativeAccel(x / SensorManager.GRAVITY_EARTH,
>                                       y / SensorManager.GRAVITY_EARTH,
>                                       event.values[2] /
> SensorManager.GRAVITY_EARTH);

Yes, this still stands. I didn't want to double patch ;) I'll provide a patch on top of 2155 this evening. In the meantime I'd appreciate if someone else could confirm this (wrong?) behaviour on other devices.

Also worth noting, I stumbled upon this bug when playing around with the Accelerometer iOS Demo where the value of the joystick's Y axis is negated:

float ax = SDL_JoystickGetAxis(accelerometer, 0);
float ay = -SDL_JoystickGetAxis(accelerometer, 1);

Is X or Y flipped as well on iOS?
Comment 3 Sam Lantinga 2013-10-21 06:53:07 UTC 
Fixed!
http://hg.libsdl.org/SDL/rev/fdada7e3b3e7

iOS did indeed have an inverted Y axis, which is now also fixed.

Thanks!
Comment 4 Sam Lantinga 2013-10-21 06:53:24 UTC 
Closing this bug.