Using the Android Accelerometer without having the device flat

Question

I've been playing around with the Android Accelerometer of late using the Android SDK and the Adobe AIR for Android SDK on my Motorola Droid. What I've noticed is that the accelerometer works just fine, but I was wondering if it is possible to compensate in some fashion so that I don't have to use the device in a horizontal position. In other words, I want to use the accelerometer to control my visual display, but initialize it(or modify in some way) so that I don't have to hold it perfectly flat (not much fun having to lean over the phone).

Does anyone know how I can hold the device comfortably in my hand, say 45 degrees, and still utilize the accelerometer to provide forward/backwards readings? Is this possible? Any examples of this this available?

Answer 1

You'll need some simple matrix multiplication math for that. "Calibrate" the rotation by taking the current matrix when you start the app and invert it, then multiply all subsequent matrices with it - that will give you the delta to the starting position.

Answer 2

I had written an application long long ago which dealt with relative rotations. I've forgotten what the code did, but from what I can see, it seems like -

1) I get the initial rotation matrix using - SensorManager.getRotationMatrix(R, I, gravity.clone(), geomagnetic.clone()); (gravity and geomagnetic are the respective accleration and geomagnetic matrices. Dunno why I use clones but there must be some reason.) 2) Now at any stage, get the current rotation matrix and call it R. 3)Calculate the inverse of the initial matrix and call it "initialInverse". 4)Multiply R with initialInverse (or the other way round, you'll have to figure it out). 5) Get your final orientation using SensorManager.getOrientation(delta, values)

I'm sorry but I've totally forgotten what the above code does. I think I remember reading the words Euler Transform somewhere when I wrote this app, so thats what it might be doing. Unfortunately I cannot give you the complete code since I'll probably release this app in the market. However, if you need some more information, please let me know - I'll look into the code and get back to you.

Answer 3

I am working in a project with similar nature where the accelerometer function is not restricted by the position. My way of handling it is very simple, initialize the accelerometer with the current reading as the default. In other words, you have a button that you press once you have the phone in the proper position, upon pressing the button, the current readings of the accelerometer (measures of G) will be your reference (zero values), and make changes when you go above or below those readings... Hope this helps anyone... cheers