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How to access and modify pixel value in an image using OpenCV Python?
To access a single pixel value in an image we can use indexing the same as we do to NumPy array indexing. We can use slicing to access a sequence of pixel values. To modify the pixel values we use the simple Python assignment operator ("=").
Steps
To access and modify pixel values in an image we could follow the below steps-
Import the required library. In all the following examples, the required Python library is OpenCV. Make sure you have already installed it.
Read the input RGB image using cv2.imread(). The RGB image read using this method is in BGR format. Optionally assign the read BGR image to img.
To access a single pixel use indexing and to modify a single pixel value use assignment to indexing. Like to modify the pixel value at [200,150] to red color we apply
img[200,150] = (0, 0, 255)
To access a sequence of pixels use slicing and to modify these pixel values use assignment to slicing. Like to modify the pixel values at [100:300,150:350] to red color we apply
img[100:300,150:350] = (0, 0, 255)
We will use this image as the Input File in the following example.
Example
In this Python program, we access the pixel value at a point in the input image. We also find the pixel value for three different color channels and modify the pixel value at that point for the red color channel.
# program to access and modify a pixel value # import required libraries import cv2 # read the input image img = cv2.imread('horizon.jpg') # access pixel values using indexing print("pixel value at [200,150]:", img[200,150]) print("pixel value blue channel at [200,150]:", img[200,150][0]) print("pixel value green channel at [200,150]:", img[200,150][1]) print("pixel value red channel at[200,150]:", img[200,150][2]) # modify the pixel value at [200,150] for red color channel img[200,150] = (0, 0, 255) print("after modifying pixel value at [200,150]:", img[200,150])
Output
When you run the above program, it will produce the following output −
pixel value at [200,150]: [115 192 254] pixel value blue channel at [200,150]: 115 pixel value green channel at [200,150]: 192 pixel value red channel at [200,150]: 254 after modifying pixel value at [200,150]: [ 0 0 255]
Example
In this Python program, we access the pixel values of a region in the input image. We also modify these pixel values to red color.
# program to access and modify the pixel values of a region # import required libraries import cv2 # read the input image img = cv2.imread('horizon.jpg') # access pixel values using indexing and slicing # modify pixel color of a region to red color img[100:300,150:350] = (0, 0, 255) # display the modified image cv2.imshow('Modified Image', img) cv2.waitKey(0) cv2.destroyAllWindows()
Output
When you run the above program, it will produce the following output −
Notice the output image after modifying the pixel values to red color.
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