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

Before converting pic to uint8, you need to multiply it by 255 to get the correct range.

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.