:$=,+$ Code $+$ Text

Q $vv$ MotionDetection class

def $($self$,$ num$\_$sigmas$=4.,$ discount$=0\mathrm{.}96)$:

$"""$Motion detection implemented via averagerator.

@param num$\_$sigmas: by how many standard deviations should a pixel

differ from the average for motion to be detected. This is

the $\backslash$kappa of the above explanation.

@param discount: discount factor for the averagerator.

$""""$

self.num$\_$sigmas $=$ num$\_$sigmas

self.discount $=$ discount

self. average $=$ None

def detect$\_$motion$($self$,$ img$)$:

$"""$Detects motion.

@param img: an $hw3$ image.

areturns: an $hw$ boolean matrix, indicating where motion occurred.

A pixel is considered a motion pixel if one of its color bands deviates

by more than num$\_$sigmas standard deviations from the average.""."

if self.average is None:

self. average $=np.f$ loat$32($img$)$

return np$.$zeros$\_$like$($img$,$ dtype$=$bool$)$

diff $=np.$ abs$($np$.$float$32($img$)-$ self.average$)$

diff$\_$threshold $=$ self.num$\_$sigmas $**$ np$.$std$($diff$,$ axis$=2,$ keepdims$=$True$)$

motion$\_$mask $=np.$any $($diff $>$ diff$\_$threshold, axis $=2$

self.average $=$ self.discount $**$ self.average self.discount