'Create an running background difference.
alpha - the update weighting where:
accumulator = ((1-alpha)input_image)+((alpha)accumulator)
threshold - the foreground background difference threshold.'

def __init__(self, alpha=0.7, thresh=(20, 20, 20)):

    self.mError = False
    self.mReady = False
    self.mAlpha = alpha
    self.mThresh = thresh
    self.mModelImg = None
    self.mDiffImg = None
    self.mColorImg = None
    self.mBlobMaker = BlobMaker()
