CODEWITHSUNDEEP
opencvimshow
Sam
Sam

Reputation: 219

What does cv2.COLOR_GRAY2RGB do?

I happened to encounter this API cv2.COLOR_GRAY2RGB. I found it strange because there should have no way to convert an grey scale image to RGB image. So I tried something like this: I took an image like this:

enter image description here

The image is shown by plt.imshow(img) (with default arguments). Then I convert it into grey scale with cv2.cvtColor(img, cv2.COLOR_BGR2GRAY) and get this:

enter image description here

I know it does not appear grey-scale looking is because imshow() by default is not displaying grey-scale image (more like heat-map I think). So I used cv2.cvtColor(img, cv2.COLOR_GRAY2RGB) and got this:

enter image description here

It appears grey to out eyes despite it has three channels now. So I conclude that cv2.COLOR_GRAY2RGB is a walk-around to display grey-scale image in grey-scale fashion without changing the settings for imshow().

Now my question is, when I use cv2.cvtColor(img, cv2.COLOR_BGR2GRAY) again to convert this three-channel gray image back to one channel, the pixel value is exactly the same as the first time I converted the original image into one channel with cv2.cvtColor(img, cv2.COLOR_BGR2GRAY): enter image description here

In other words, cv2.COLOR_BGR2GRAY can do a many-to-one mapping. I wonder how is that possible.

Upvotes: 1

Views: 12262

Answers (1)

ilke444
ilke444

Reputation: 2741

COLOR_BGR2GRAY color mode estimates a gray value for each pixel using a weighted sum of B, G, R channels, w_R*R+w_G*G+w_B*B=Y per pixel. So any 3 channel image becomes 1 channel.

COLOR_GRAY2BGR color mode basically replaces all B, G, R channels with the gray value Y, so B=Y, G=Y, R=Y. It converts a single channel image to multichannel by replicating.

More documentation about color modes are here.

Upvotes: 4

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