Blurring and Noise

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.image as mpimg

The past lecture covered three related topics: noise, blurs, and edge detection. Let's discuss them and give some examples. Again, we'll work with our favorite cat.

img=mpimg.imread('kitty-cat.jpg')
plt.imshow(img)
plt.show()

It'll be more convenient here if our image is stored in the floating point format, so let's change it to that.

img = img/255

Noise

In lecture we learned how to add different types of noises to grayscale images. There's nothing special about grey scale images. Let's practice these noises by applying them to color images:

Salt and Pepper Noise:

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def salt_pepper(img, p1=0.1, p2=0.1):
    img = img.copy()
    r = np.random.rand(img.shape[0], img.shape[1])
    img[r < p1, :] = 0
    img[r > 1-p2, :] = 1
    return img

plt.imshow(salt_pepper(img))
plt.show()

Gaussian Noise:

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def gauss_noise(img,p=0.5):
    img=img.copy()
    noise=np.random.randn(img.shape[0],img.shape[1],1)*p*np.var(img)**(0.5)
    img = img + noise
    img = np.maximum(img, np.zeros(img.shape))
    img = np.minimum(img, np.ones(img.shape))
    return img

plt.imshow(gauss_noise(img))
plt.show()

Blurring

I was mistaken on how much you covered in lecture, so this isn't what we're going to be doing for this exercise. I'll leave it here because it will make sense later, though!

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If you remember from lecture, to blur an image we take some small matrix (called a filter) and do some special operation (called convolution) with the image matrix. The convolution operation is just that we take our image matrix and multiply each block of the matrix element-wise by our filter and then take the sum.

Let's write the function to apply a filter (the convolution function) ourselves. Normally for something this fundamental, you would want to just use some convolution function from a module. There's two reasons why we aren't going to do that here:

1. Writing something yourself once is good practice and will help you understand how it works.
2. NumPy doesn't actually have a function for this! It does have a convolution function, but it doesn't do what we want (it works on vectors, not matrices). The module SciPy has this function, but we haven't learned SciPy yet.

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What we will actually be doing is implementing a uniform blur function.

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def apply_blur(mat, blur_size):
    new_img = np.zeros([img.shape[0]-2*blur_size,img.shape[1]-2*blur_size,3])
    for i in range(new_img.shape[0]):
        for j in range(new_img.shape[1]):
            grid = img[i:i+2*blur_size+1,j:j+2*blur_size+1,:]
            new_img[i,j,:] = np.mean(np.mean(grid, axis=1), axis=0)
    return new_img

plt.imshow(apply_blur(img, 10))
plt.show()