I am attempting to convert an image in PNG format to a 16-bit PGM format and save it using Python’s PIL library. I’m using Python 3.12.4 in all examples shown.

Using the following test.png image:

Attempting a simple script like this with the Image.save function:

from PIL import Image

image = Image.open("test.png")
image.save("test.pgm")

Can save resave the image as PGM, however it’s always saved as an 8-bit image, as shown by GIMP:

Attempting to specify the amount of pixel bits via the optional bits argument as such:

from PIL import Image

image = Image.open("test.png")
image.save("test.pgm", bits = 16)

Also results in an 8-bit PGM image being saved.

Attempting to manually create a numpy array of the np.uint16 type & then creating an image from it using the Image.fromarray function as such:

from PIL import Image
import numpy as np

image = Image.open("test.png")
imageData = np.array(image.getdata(), dtype = np.uint16)
newImage = Image.fromarray(imageData)
newImage.save("test.pgm", bits = 16)

Results in:

OSError: cannot write mode I;16 as PPM

I also noticed that this appears to break the image, as saving with np.uint32 saves a blank image with a very narrow horizontal size and a very large vertical size.

What is the proper way to save 16-bit PGM images using the Python PIL library?

Thanks for reading my post, any guidance is appreicated.