Motion detection using iPhone

Question

I saw at least 6 apps in AppStore that take photos when detect motion (i.e. a kind of Spy stuff). Does anybody know what is a general way to do such thing using iPhone SDK?

I guess their apps take photos each X seconds and compare current image with previous to determine if there any difference (read "motion"). Any better ideas?

Thank you!

Answer 1

You could probably also use the microphone to detect noise. That's actually how many security system motion detectors work - but they listen in on ultrasonic sound waves. The success of this greatly depends on the iPhone's mic sensitivity and what sort of API access you have to the signal. If the mic's not sensitive enough, listening for regular human-hearing-range noise might be good enough for your needs (although this isn't "true" motion-detection).

As for images - look into using some sort of string-edit-distance algorithm, but for images. Something that takes a picture every X amount of time, and compares it to the previous image taken. If the images are too different (edit distance too big), then the alarm sounds. This will account for slow changes in daylight, and will probably work better than taking a single reference image at the beginning of the surveillance period and then comparing all other images to that.

If you combine these two methods (image and sound), it may get you what you need.

Answer 2

You could have the phone detect light changes meaning using the sensor at the top front of the phone. I just don't know how you would access that part of the phone

Answer 3

I think you've about got it figured out- the phone probably keeps images where the delta between image B and image A is over some predefined threshold.

You'd have to find an image library written in Objective-C in order to do the analysis.

Answer 4

I have this application. I wrote a library for Delphi 10 years ago but the analysis is the same.

The point is to make a matrix from whole the screen, e.g. 25x25, and then make an average color for each cell. After that, compare the R,G,B,H,S,V of average color from one picture to another and, if the difference is more than set, you have motion.

In my application I use fragment shader to show motion in real time. Any question, feel free to ask ;)