Properly convert png to npy numpy array (Image to Array)

Question

When I generate an image and then generate a numpy array from it, the original .npy file differs from the new one. I thought new-array.npy would be exactly the same as original-array.npy since they are coming from the same image.

For an example, I used this little image with 4*4 pixels:
original-image.png

Here is a larger version (not the one I'm working with):

The last part of the code is the one that converts the .png to .npy. I think the problem is in here somewhere.

import numpy as np
from PIL import Image
from matplotlib import pyplot as plt

filename = 'image-test'

img = Image.open( filename + '.png' )
data = np.array( img, dtype='uint8' )

np.save( filename + '.npy', data)

# visually testing our output
img_array = np.load(filename + '.npy')
plt.imshow(img_array) 

My simple algorithm:

1. Generate random rgb array and save it as .npy
2. Save a .png file from that numpy array.
3. Load that .png file and save it back to .npy

import numpy as np
from matplotlib import pyplot as plt
import matplotlib

from PIL import Image                                                                                


####create a matrix of random colors
filename = "original-array"

matrix=np.random.random((4,4,3))
nx,ny,nz=np.shape(matrix)
CXY=np.zeros([ny, nx])
for i in range(ny):
    for j in range(nx):
        CXY[i,j]=np.max(matrix[j,i,:])

#Save binary data
np.save(filename + '.npy', CXY)
print(filename + " was saved")

#Load npy
img_array = np.load(filename + '.npy')
plt.imshow(img_array)


####Save npy as png
filename = "original-image"

img_name = filename +".png"
matplotlib.image.imsave(img_name, img_array)
print(filename + " was saved")


#### Convert that png back to numpy array

img = Image.open( filename + '.png' )
data = np.array( img, dtype='uint8' )

#Convert the new npy file to png
filename = "new-array"

np.save( filename + '.npy', data)
print(filename + " was saved")


#Load npy
img_array = np.load(filename + '.npy')

filename = "new-image"
#Save as png
img_name = filename +".png"
matplotlib.image.imsave(img_name, img_array)
print(filename + " was saved")

Here is the result :

When I regenerate an image from new-array.npy I get exactly the same image as original-image.png:

Answer 1

The files are different because the arrays have different data types.

The first time you save the data is when you save the array CXY. This array has a type of np.float64, since that is the default data type returned by np.zeros.

The second array is created by loading the original image, not the saved npy file. This is where the inconsistencies are introduced, since the PNG data is of type np.uint8 (and is cast again to np.uint8 in the next line). This is a smaller data type, hence the smaller overall file size.