Read 16-bit PNG image file using Python

Question

I'm trying to read a PNG image file written in 16-bit data type. The data should be converted to a NumPy array. But I have no idea how to read the file in '16-bit'. I tried with PIL and SciPy, but they converted the 16-bit data to 8-bit when they load it. Could anyone please let me know how to read data from a 16-bit PNG file and convert it to NumPy array without changing the datatype?

The following is the script that I used.

from scipy import misc
import numpy as np
from PIL import Image
#make a png file    
a = np.zeros((1304,960), dtype=np.uint16)
a[:] = np.arange(960)
misc.imsave('16bit.png',a)

#read the png file using scipy
b = misc.imread('16bit.png')
print "scipy:" ,b.dtype

#read the png file using PIL
c = Image.open('16bit.png')   
d = np.array(c)
print "PIL:", d.dtype
               

Answer 1

I have the same problem here. I tested it even with 16 bit images i created by my own. All of them were opened correctly when i loaded them with the png package. Also the output of 'file ' looked okay.

Opening them with PIL always led to 8-bit numpy-arrays.

Working with Python 2.7.6 on Linux btw.

Like this it works for me:

import png
import numpy as np

reader = png.Reader( path-to-16bit-png )
pngdata = reader.read()
px_array = np.array( map( np.uint16, pngdata[2] ) 
print( px_array.dtype )

Maybe someone can give more information under which circumstances the former approach worked? (as this one is pretty slow)

Thanks in advance.

Answer 2

The simplest solution I've found:

When I open a 16 bit monochrome PNG Pillow it doesn't open correctly as I;16 mode. Image.mode is opened as I (32 bits)

So, the best way to convert to numpy array. It is dtype="int32" so we will convert it to dtype="uint16".

import numpy as np
from PIL import Image

im = Image.fromarray(np.array(Image.open(name)).astype("uint16"))
print("Image mode: ", im.mode)

Tested in Python 3.6.8 with Pillow 6.1.0