4 Create a simple image

Objectives

• synthesize an RGB image (and know, for example, that yellow = green + red)

• manipulate the value / color correspondence

• use the : operator

Image synthesis

As usual, do not forget the modules:

import numpy as np
import matplotlib.pyplot as plt

The image is composed of blocks of homogeneous color with size 20 × 20 pixels, so as to form an image of 40 × 80 pixels. Also, it contains three bands as it is an RGB image. Therefore we create an array f of size 40 × 80 × 3:

f = np.zeros((40,80,3))

We use Additive color to create the colors. Thus, yellow is obtained by combining green and red.

f[ 0:20 ,   :   , 0 ] = 1   # Red band = band 0   (top row: pixels 0 to 19)
f[  :   ,  0:40 , 1 ] = 1   # Green band = band 1 (the two columns on the left)
f[  :   , 20:60 , 2 ] = 1   # Blue band = band 2  (the two columns at the center)

The bands can be displayed separately:

plt.figure(figsize=(15,7))

plt.subplot(1,3,1)
plt.imshow(f[:,:,0], cmap="gray")
plt.title('Red band')

plt.subplot(1,3,2)
plt.imshow(f[:,:,1], cmap="gray")
plt.title('Green band')

plt.subplot(1,3,3)
plt.imshow(f[:,:,2], cmap="gray")
plt.title('Blue band')

plt.show()

And finally the color image:

plt.imshow(f)
plt.show()