Introduction to image processing - Color transfer

Introduction

For the following exercices, you need Python 3 with some basic librairies (see below). All images necessary for the session are available here.

If you use your own Python 3 install, you should download the images, put them in a convenient directory and update the path in the next cell.

In [1]:
path = '../im/'
In [2]:
import numpy as np
import matplotlib.pyplot as plt
import matplotlib
%matplotlib inline
from mpl_toolkits.mplot3d import Axes3D

Load and display a color image. A color image is made of three channels : red, green and blue. A color image in $\mathbb{R}^{N\times M}$ is stored as a $N\times M\times 3$ matrix.

Be careful with the functions plt.imread() and plt.imshow() of matplotlib.

  • plt.imread() reads png images as numpy arrays of floating points between 0 and 1, but it reads jpg or bmp images as numpy arrays of 8 bit integers.

  • In this practical session, we assume floating point images between 0 and 1, so if you use jpg or bmp images, you should normalize them to $[0,1]$.

  • If 'im' is an image encoded as a double numpy array, plt.imshow(im) will display all values above 1 in white and all values below 0 in black. If the image 'im' is encoded on 8 bits though, plt.imshow(im) will display 0 in black and 255 in white.</span>

In [3]:
imrgb1 = plt.imread(path+'renoir.jpg')/255
imrgb2 = plt.imread(path+'gauguin.jpg')/255
imrgb1=imrgb1[:,:,0:3] # useful if the image is a png with a transparency channel
imrgb2=imrgb2[:,:,0:3] 

#we display the images
fig, axes = plt.subplots(nrows=1, ncols=2, figsize=(15, 7))
axes[0].imshow(imrgb1)
axes[0].set_title('First image')
axes[1].imshow(imrgb2)
axes[1].set_title('Second image')
fig.tight_layout()

Separable color transfer

Separable affine transfer

We write a function affine_transfer which apply an affine transform to an image $u$ such that it has the same mean and the same standard deviation as $v$ on each channel.

In [4]:
def affine_transfer(u,v):
    w = np.zeros(u.shape)
    for i in range(0,3):
        w[:,:,i] = (u[:,:,i] -np.mean(u[:,:,i]))/np.std(u[:,:,i])*np.std(v[:,:,i])+ np.mean(v[:,:,i])
    return w

# We apply this function to the two previous images :
w = affine_transfer(imrgb1,imrgb2)
w = (w>1)+(w<=1)*(w>0)*w      # w should be in [0,1]

# Then we display the result :
plt.figure(figsize=(8, 8))
plt.title('result of color separable affine transfer')
plt.imshow(w)
plt.show()