Ignoring camera shake

A camera sitting on a desk gets bumped — someone leans on it, or types hard enough to shake the surface. Every pixel in the picture shifts together, and to a motion detector that looks like the entire scene moved at once: a recorded event with nothing in it.

With this on, each frame is compared against the one before it at a range of small offsets. If sliding the whole picture a few pixels makes the two frames line up again, the camera moved rather than something in front of it, and the frame is ignored — along with the next few, because a knocked camera rocks before it settles.

Something moving through the scene can't produce that: a person changes one part of the picture while the background stays exactly where it was, and no amount of sliding makes those frames agree. A lighting change can't either, which is handled separately.

It's on unless you turn it off. Sometimes the whole picture moving is the event — the room shaking, or a slammed door jolting the shelf the camera sits on — and with it off, those record like any other motion.

• Leave it on for a camera on a desk, a shelf, or anything that gets touched in the normal course of the day.
• Turn it off if you want to know when the camera itself is disturbed.
• A camera that is panning, rather than being knocked, moves further than this looks for and still registers as motion.