pyplot.imsave() saves image correctly but cv2.imwrite() saved the same image as black

Question

from scipy.misc import imread
from matplotlib import pyplot

import cv2
from cv2 import cv

from SRM import SRM ## Module for Statistical Regional Segmentation

im = imread("lena.png") 
im2 = cv2.imread("lena.png")
print type(im), type(im2), im.shape, im2.shape 
## Prints <type 'numpy.ndarray'> <type 'numpy.ndarray'> (120, 120, 3) (120, 120, 3)

srm = SRM(im, 256)
segmented = srm.run()

srm2 = SRM(im2, 256)
segmented2 = srm2.run()

pic = segmented/256
pic2 = segmented2/256

pyplot.imshow(pic)
pyplot.imsave("onePic.jpg", pic)

pic = pic.astype('uint8')
cv2.imwrite("onePic2.jpg", pic2)

pyplot.show()

onePic.jpg gives the correct segmented image but onePic2.jpg gives a complete black image. Converting the datatype to uint8 using pic = pic.astype('uint8') did not help. I still gives a black image!

onePic.jpg using pyplot.imsave() :

onePic2.jpg using cv2.imwrite() :

Please help!

Answer 1

在将pic转换为uint8之前，您需要将其乘以255以获取正确的范围。

Answer 2

Although I agree with @sansuiso, in my case I found a possible edge case where my images were being shifted either one bit up in the scale or one bit down.

Since we're dealing with unsigned ints, a single shift means a possible underflow/overflow, and this can corrupt the whole image.

I found cv2's convertScaleAbs with an alpha value of 255.0 to yield better results.

def write_image(path, img):
    # img = img*(2**16-1)
    # img = img.astype(np.uint16)
    # img = img.astype(np.uint8)
    img = cv.convertScaleAbs(img, alpha=(255.0))
    cv.imwrite(path, img)

This answer goes into more detail.

Answer 3

I encountered a similar situation with face detection, I wonder if there is a better way to execute this, here is my solution here as a reference.

from deepface import DeepFace
import cv2 
import matplotlib.pyplot as plt

# import image and output
img_path = "image.jpg"
detected_face = DeepFace.detectFace(img_path, target_size = (128, 128))
plt.imshow(detected_face)

# image color scaling and saving 
detected_face = cv2.cvtColor( detected_face,cv2.COLOR_BGR2RGB)
detected_face = cv2.convertScaleAbs(detected_face, alpha=(255.0))
cv2.imwrite("image_thumbnail.jpg", detected_face)