Wednesday 7 September 2011

iOSDev UK: Programming iOS Sensors

Alasdair Allan, Babilim Light Industries

Magnetometer (it’s not a digital compass!)

  • 4th gen iPod Touch doesn’t have magnetometer
    • so no outside AR apps without markers
  • UIAccelerometer API newly deprecated for CoreMotion in iOS5
  • if you want compass heading but not location, then just start up [locationManager startUpdatingHeading]
    • you won’t get location updates warning
  • however, magnetic north varies across the world
    • there’s a big lookup table in the iPhone that can translate the magnetic heading into true north
    • so you need actual location to work this out
  • watch out for device orientation: magnetometer always reports heading pointing out of top of device
    • need to rotate according to device orientation…
  • local magnetic anomalies cause a fluctuating magnetic field — which is when it tells you to wave your device around
  • Earth’s magnetic pole is a quadrapole, not a bipole — it swaps over every now and then and the north pole goes south

Gyroscope & Accelerometer (CoreMotion)

  • iPhone 4 has more bits in its accelerometer sampler
  • combining gyroscope and accelerometer provides very accurate device attitude
  • if there’s no natural timer in your app, then you may have to use the push API
    • otherwise much easier to use pull API
  • CMMotionManager should be treated as a singleton (but API allows you to create multiples…)
  • monitoring does take a lot of CPU, so remember to stop it when you’re finished
    • device motion at 100 samples/sec uses 65% of iPhone 4 CPU
    • see “Pushing Device Motion” slide
  • you can fetch a frame of reference and then work out attitude relative to that

AR Toolkits

External accessories

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