Feature extraction of images in Python
In images, some frequently used techniques for feature extraction are binarizing and blurring

Binarizing: converts the image array into 1s and 0s. This is done while converting the image to a 2D image. Even gray-scaling can also be used. It gives you a numerical matrix of the image. Grayscale takes much lesser space when stored on Disc.

This is how you do it in Python:

from PIL import Image

%matplotlib inline  

#Import an image
image = Image.open("xyz.jpg")

image


Example Image: 



Now, convert into gray-scale:

im = image.convert('L')

im


will return you this image:



And the matrix can be seen by running this:

array(im)


The array would look something like this:

array([[213, 213, 213, ..., 176, 176, 176],
       [213, 213, 213, ..., 176, 176, 176],
       [213, 213, 213, ..., 175, 175, 175],
       ..., 
       [173, 173, 173, ..., 204, 204, 204],
       [173, 173, 173, ..., 205, 205, 204],
       [173, 173, 173, ..., 205, 205, 205]], dtype=uint8)


Now, use a histogram plot and/or a contour plot to have a look at the image features:

from pylab import *

# create a new figure
figure()
gray()
# show contours with origin upper left corner
contour(im, origin='image')
axis('equal')
axis('off')


figure()


hist(im_array.flatten(), 128)

show()


This would return you a plot, which looks something like this:




Blurring: Blurring algorithm takes weighted average of neighbouring pixels to incorporate surroundings color into every pixel. It enhances the contours better and helps in understanding the features and their importance better.

And this is how you do it in Python:

from PIL import *


figure()
p = image.convert("L").filter(ImageFilter.GaussianBlur(radius = 2))
p.show()


And the blurred image is:



So, these are some ways in which you can do feature engineering. And for advanced methods, you have to understand the basics of Computer Vision and neural networks, and also the different types of filters and their significance and the math behind them.