Normalizing images in OpenCV produces black image?

I wrote the following code to normalize an image using NORM_L1 in OpenCV. But the output image was just black. How to solve this?

import cv2
import numpy as np
import Image

img = cv2.imread('img7.jpg')
gray_image = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
a = np.asarray(gray_image)


dst = np.zeros(shape=(5,2))

b=cv2.normalize(a,dst,0,255,cv2.NORM_L1)


im = Image.fromarray(b)

im.save("img50.jpg")

cv2.waitKey(0)
cv2.destroyAllWindows()

Answer

If you want to change the range to [0, 1], make sure the output data type is float.

image = cv2.imread("lenacolor512.tiff", cv2.IMREAD_COLOR)  # uint8 image
norm_image = cv2.normalize(image, None, alpha=0, beta=1, norm_type=cv2.NORM_MINMAX, dtype=cv2.CV_32F)