- multiple laserbundles ("tripwire") could do the trick, but then you should be able to do a "wave" and have mounting points.
- A webcam from above, compare the image with an "empty room", people will appear as changed pixels.
calculate the distance of the changed pixels to the centre pixel and take the minimum distance
min = 1000000; // big number
pixel = (0,0);
for each pixel of the image
if (pixel != referenceimage.pixel)
d = distance(pixel, center)
if ( d < min)
min = d;
pixel = (x,y);