CODEWITHSUNDEEP
pythonmatplotlib
Maximus
Maximus

Reputation: 669

Matplotlib reads jpg into int8 and png into normalized float

Why do Matplotlib load .png into float32 (from 0 to 1):

img = mpimg.imread('some.png')
print(img.dtype)
>>> float32

and .jpg to int8 (from 0 to 255):

img = mpimg.imread('some.jpg')
print(img.dtype)
>>> int8

Why? On the basis of what considerations is it realized this way?

Upvotes: 12

Views: 7896

Answers (2)

Aaron
Aaron

Reputation: 37

In Matplotlib 3.xx, there is no read_png_int() method any more. But we can use following way to handle png file.

from PIL import Image
from numpy import asarray
test_pic="image.png"
image=Image.open(test_pic)
img=asarray(image) 

print(img)

[[ 91 108  98]
 [ 99 116 106]
 [107 124 116]
 ...
 [  1   5   8]
 [  1   5   8]
 [  1   5   8]]

Upvotes: 2

ImportanceOfBeingErnest
ImportanceOfBeingErnest

Reputation: 339340

As the imread documenation states:

matplotlib can only read PNGs natively, but if PIL is installed, it will use it to load the image and return an array (if possible) [...]

So for png images, matplotlib has its own code to read the image, while for jpg it relies on PIL. 

import matplotlib.pyplot as plt

im = plt.imread("house.png")
print im.dtype               # float32

im = plt.imread("house.jpg")
print im.dtype               # uint8

The internal _png module which is responsible for loading the png image, has two functions, read_png_float and read_png_int. While by default read_png_float is used, you may as well manually use read_png_int to obtain an integer image.

import matplotlib._png as png

im = png.read_png_float("house.png")
print im.dtype              # float32

im = png.read_png_int("house.png")
print im.dtype              # uint8

Finally, for a consistent behaviour you may use PIL for both, png and jpg images instead of relying on matplotlib.

from PIL import Image
import numpy as np

im = np.asarray(Image.open("house.png"))
print im.dtype              # uint8

im = np.asarray(Image.open("house.jpg"))
print im.dtype              # uint8

Upvotes: 19

Related Questions