def tenengrad(img, ksize=3):
    ''''TENG' algorithm (Krotkov86)'''
    Gx = cv2.Sobel(img, ddepth=cv2.CV_64F, dx=1, dy=0, ksize=ksize)
    Gy = cv2.Sobel(img, ddepth=cv2.CV_64F, dx=0, dy=1, ksize=ksize)
    FM = Gx*Gx + Gy*Gy
    mn = cv2.mean(FM)[0]
    if np.isnan(mn):
        return np.nanmean(FM)
    return mn