CODEWITHSUNDEEP
pythonopencv
Maham
Maham

Reputation: 442

Thresholding of a grayscale Image in a range

Does OpenCV cv.InRange function work only for RGB images?Can I do thresholding of grayscale image using this function?

I got an error,Following is my code:

   import cv2
   image=cv2.imread("disparitySGB.jpg")
   thresh=cv2.inRange(image,190,255);

It gives the following error:

thresh=cv2.inRange(image,190,255); TypeError: unknown is not a numpy array

I tried fixing it by:

  thresh=cv2.inRange(image,numpy.array(190),numpy.array(255));

Now there is no error but it produces black image.

Upvotes: 7

Views: 27414

Answers (4)

Zishaan
Zishaan

Reputation: 1

Your cv2.imread is reading a RGB image. To read in grayscale it is 

image = cv2.imread("disparitySGB.jpg", 0)

Upvotes: 0

Bryce
Bryce

Reputation: 1

You just need to 'import numpy as np' and your original code should work fine.

Upvotes: 0

Hannes Ovrén
Hannes Ovrén

Reputation: 21851

For a gray-valued image which has shape (M, N) in numpy and size MxN with one single channel in OpenCV, then cv2.inRange takes scalar bounds:

gray = cv2.imread(filename, cv2.CV_LOAD_IMAGE_GRAYSCALE)
gray_filtered = cv2.inRange(gray, 190, 255)

But for RGB-images which have shape (M, N, 3) in numpy and size MxN with three channels in OpenCV you need to have the bounds match the "channel size".

rgb = cv2.imread(filename, cv2.CV_LOAD_IMAGE_COLOR)
rgb_filtered = cv2.inRange(gray, (190, 190, 190), (255, 255, 255))

This is explained in the documentation, although not very clearly.

Upvotes: 12

Mailerdaimon
Mailerdaimon

Reputation: 6090

cv2.inRange(src, lowerb, upperb[, dst]) → dst

Takes src as array and lowerand upper as array or a scalar, this means you can use it to Threshold Grayscale images. You just have to use scalars for upper and lower.

Example:

myResult = cv2.InRange(myGrayscale, 50, 100)

Upvotes: 1

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