Opencv convert to grayscale not working properly

Question

I've been trying to convert an image to grayscale using opencv in Python but it converts the image to some kind of thermal camera image. What am I doing wrong? Here is the code for image below:

img =X_tr[9999]
plt.imshow(img)
plt.show()
img = cv2.cvtColor(img.astype(np.uint8), cv2.COLOR_RGB2GRAY)
plt.imshow(img)
plt.show()
img.shape

This image is taken from CIFAR10 dataset. Thanks.

I just saw that there is actually already a similar question (not using opencv though). I will still leave my answer here, as the point about the normalization does not become apparent from the other thread — ImportanceOfBeingErnest, Apr 01 '17 at 14:28

score 20 · Accepted Answer · answered Apr 01 '17 at 14:18

20

Gray scale images, i.e. images with only one colorchannel, are interpreted by imshow as to be plotted using a colormap. You therefore need to specify the colormap you want to use (and the normalization, if it matters).

plt.imshow(img, cmap="gray", vmin=0, vmax=255)

answered Apr 01 '17 at 14:18

ImportanceOfBeingErnest

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I am so stupid. Thanks. – Neeraj Negi Apr 01 '17 at 14:22

Opencv convert to grayscale not working properly

1 Answers1