I observed some strange behavior when saving and loading data using matplotlibs imsave() and imread() functions. The RGB values I save are different to the ones I get when loading the picture back in again.
import numpy as np
from matplotlib import image
s_pic = np.zeros((1, 1, 3))
s_pic[0,0,0] = 0.123
s_pic[0,0,1] = 0.456
s_pic[0,0,2] = 0.789
image.imsave('pic.png', s_pic)
l_pic = image.imread('pic.png')
print(l_pic[0,0,0])
print(l_pic[0,0,1])
print(l_pic[0,0,2])
The output I get is:
0.12156863
0.45490196
0.7882353
Can somebody explain why the RGB values change in this process? I have checked the matplotlib documentation but couldn't find an answer to this question.
Can somebody explain why the RGB values change in this process?
RGB values are integers in the range 0-255. Your float is interpreted as:
>>> .123 * 255
31.365
>>> int(.123 * 255)
31
Thirty-one is being written to that pixel. Then the reverse..
>>>
>>> 31 / 255
0.12156862745098039
>>>
Delving into the source for imsave() the array passed to imsave() is converted to RGBA values using matplotlib.cm.ScalarMappable().to_rgba(bytes=True)
>>> from matplotlib import cm
>>> sm = cm.ScalarMappable()
>>> rgba = sm.to_rgba(s_pic, bytes=True)
>>> rgba
array([[[ 31, 116, 201, 255]]], dtype=uint8)
>>>