Digital compass

All information pertains to the N900.

Why a compass is needed

GPS gives location of device; compass gives orientation
Allows pointing the device and getting information relevant to the pointed direction
E.g. Layar, Wikitude

Optical (use one of the cameras, plus image processing)
- Best to use user-facing camera, so the world-facing can be used for capturing the world image for AR
USB
- Unknown if USB host possible.
- Unknown if power is available from USB.
IrDA
- Is unsupported at this point. The device only has an CIR transmitter.
Bluetooth
- Bluetooth data
- Bluetooth audio - sending analog signals via Bluetooth microphone for digital signal processing on the N900
FM radio
- Requires headphones be connected
- Signal processing of audio in software

Honeywell HMC5843
- I2C interface
- Magnetometer requires vector math on client side for interpretation, see OpenMoko i2c compass project for more info
Dinsmore analog

Use GPS motion to determine direction
- Only useful for navigation, not AR
- Does not work if standing still
Use image processing for AR
- E.g. artoolkit
- Still you cannot get absolute orientation of device
Initialized orientation + delta orientations (dead reckoning)
- Maybe you can initialize the device to a known orientation (e.g. straight north), then use optical flow and/or accelerometer data to extrapolate subsequent orientations
Optical compass + manually dragging an on-screen compass to match
- Not automatic so some AR apps like games might not work
- But for navigation or rotating maps, it could work: just glance at the optical compass, then drag the map icon to match the orientation.
Combined GPS/optical compass for daytime use:
- Use GPS location and time info to calculate current sun location
- Let user optically align to visible sun