Detecting presence
Based on the knowledge about those pixels that have changed between the stored image and the current video image, we can detect the presence of a foreground subject in front of a static background. Point your webcam in front of a relatively static background; click on the bang box, which is next to the Store comment, to store the background image in the pix_buffer object. Anything that appears in front of the background will be shown in the GEM window. Now we can ask the question: how can we know if there is anything present in front of the background? The answer will be in the pix_blob object:
The pix_blob object calculates the centroid of an image.
Note
The centroid (http://en.wikipedia.org/wiki/Centroid) of an image is its center of mass. Imagine that you cut out the shape of the image in a cardboard. The centroid is the center of mass of that piece of cardboard. You can balance the cardboard by using one finger to hold it as the center of mass.
In our example, the image...
