iOSDev UK: Programming iOS Sensors

Alasdair Allan, Babilim Light Industries

http://programmingiphonesensors.com/

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

• Marker based:
  • ARToolKit for iOS
  • Qualcomm AR Toolkit
• Location-aware
  • doPanic Toolkit
  • ARView — available from Programming iOS Sensors

External accessories

• Arduino for iPhone!!!!
  • Redpark Breakout Pack for iPhone
  • been going through Made for iPhone accreditation for the last 3 years