Why does pyrtools imshow() print a value range that's different from np.min() and np.max()?

quick problem description:

pyrtools imshow function giving me different and negative ranges

details:

im following the tutorial at https://pyrtools.readthedocs.io/en/latest/tutorials/01_tools.html

since i dont have .pgm image, i'm using the below .jpg image.

here is the modified python code

import matplotlib.pyplot as plt
import pyrtools as pt
import numpy as np
import cv2

# Load the JPG image
oim = cv2.imread('/content/ein.jpg')
print(f"input image shape: {oim.shape}")

# Convert to grayscale if it's an RGB image
if len(oim.shape) == 3:  # Check if the image has 3 channels (RGB)
    oim_gray = cv2.cvtColor(oim, cv2.COLOR_BGR2GRAY)
else:
    oim_gray = oim  # Already grayscale

print(f"grayscale image shape: {oim_gray.shape}")

# Check the range of the oim_gray
print(f"value range of oim_gray: {np.min(oim_gray), np.max(oim_gray)}")

# Subsampling
imSubSample = 1
im = pt.blurDn(oim_gray, n_levels=imSubSample, filt='qmf9')
# Check the range of the subsampled image
print(f"value range of im: {np.min(im), np.max(im)}")

# Display the original and subsampled images
pt.imshow([oim_gray, im], title=['original (grayscale)', 'subsampled'], vrange='auto2', col_wrap=2);

and here is the output

input image shape: (256, 256, 3)
grayscale image shape: (256, 256)
value range of oim_gray: (4, 245)
value range of im: (5.43152380173187, 251.90158197570906)

with the given image

as you can see, from the printouts, both the images oim_gray and im are in positive range. there aren't any negative values on neither im and oim_gray.

but when checking the output image, i see the range -38 & 170 (please check the text over the output image). this doesn't make any sense, and don't understand. can you help on this to understand?

That is behavior of pyrtools, specifically decided by vrange='auto2'.

'auto2': all images have same vmin/vmax, which are the mean (across all images) minus/plus 2 std dev (across all images)

(documentation in source)

If you don't like that library's behavior, you can file a bug at https://github.com/LabForComputationalVision/pyrtools/issues