<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def image(self, captcha_str):
 """Generate a greyscale captcha image representing number string Parameters captcha_str: str string a characters for captcha image Returns ------- numpy.ndarray Generated greyscale image in np.ndarray float type with values normalized to [0, 1] """

        img = self.captcha.generate(captcha_str)
        img = np.fromstring(img.getvalue(), dtype='uint8')
        img = cv2.imdecode(img, cv2.IMREAD_GRAYSCALE)
        img = cv2.resize(img, (self.h, self.w))
        img = img.transpose(1, 0)
        img = np.multiply(img, 1 / 255.0)
        return img