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How would I go about getting the screen coordinates of something that enters frame with motionDetection filter? I'm fairly new to programming, and would prefer a swift answer if possible.

Example - I have the iphone pointing at a wall - monitoring it with the motionDetector. If I bounce a tennis ball against the wall - I want the app to place an image of a tennis ball on the iphone display at the same spot it hit the wall.

To do this, I would need the coordinates of where the motion occurred. I thought maybe the "centroid" argument did this.... but I'm not sure.

JoeC
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I should point out that the motion detector is pretty crude. It works by taking a low-pass filtered version of the video stream (a composite image generated by a weighted average of incoming video frames) and then subtracting that from the current video frame. Pixels that differ above a certain threshold are marked. The number of these pixels, along with the centroid of the marked pixels are provided as a result.

The centroid is a normalized (0.0-1.0) coordinate representing the centroid of all of these differing pixels. A normalized strength gives you the percentage of the pixels that were marked as differing.

Movement in a scene will generally cause a bunch of pixels to differ, and for a single moving object the centroid will generally be the center of that object. However, it's not a reliable measure, as lighting changes, shadows, other moving objects, etc. can also cause pixels to differ.

For true object tracking, you'll want to use feature detection and tracking algorithms. Unfortunately, the framework as published does not have a fully implemented version of any of these at present.

Brad Larson
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  • Thanks for answering Brad. For my purposes, how would I convert normalized coordinates to screen coordinates? An alternative to what I want to do would be to take a picture at the moment that movement occurs- – JoeC Jan 25 '16 at 17:43
  • @JoeC - Multiply them by the size of the image as displayed onscreen. – Brad Larson Jan 25 '16 at 18:16