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I am trying to build an app for the iPhone 4 which enables the user to "point" at a hardcoded destination and a dot appears where the destination is located.

First, i use the compass to make a horizontal compass(this will cover the left/right rotation):

// Heading
nowHeading = heading.trueHeading;

// Shift image (horizontal compass)
float shift = bearing - nowHeading;
destinationImage.center = CGPointMake(shift+160, destinationImage.center.y);

I shift the dot 160 pixels because the screen is 320 pixels width. My question is now, how can I expand this code to handle up and down? Meaning that if i point the phone down in the table, the dot wont show.. I have to point (like taking a picture) at the destination in order for it to be drawn on the screen. I've already implemented the accelerator. But i don't know how to unite these components to solve my problem.

2 Answers2

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Bearing should depend on the field of vision of the camera. For iPhone 4 the horizontal angular view is 47.5 so 320 points/47.5 = xxx points per degree, use that to shift horizontally. You also have to add an adaptive filter to the accelerometers, you can get one from the AccelerometerGraph project from Apple.

You have the rotation in one axis (bearing) you should get the rotation on the other two from the accelerometers. The atan2 of two axis give you the rotation on the third. Go to UIAcceleration and imagine an axis physically piercing the device if that helps and do double xAngle = atan2(acceleration.y, acceleration.z); So once you have the rotation upside down you can repeat what you did for the horizontal with the vertical field of view, eg: 60 for the iPhone.

That is going to be one rough implementation :) but achieving smooth movement is difficult. One thing you can do is use the gyros to get a faster response and correct their signal periodically with the accelerometers. See this talk for the troubles ahead: Sensor Fusion on Android Devices. Here is a website dedicated to the Kalman Filter. If you dare with Quaternions I recommend "Visualizing Quaternions" from Andrew J. Hanson.

Jano
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It sounds like you are trying to do a style of Augmented Reality. If that. Is the case there are several libraries and sample code suggested here:

Augmented Reality

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dredful
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  • You are right, these are the first steps towards an AR app. But I think an AR library would be overkill for my simple app. Thanks anyway! –  May 21 '11 at 09:30