PIL open, save changes brightness of PNG

Question

I'm using Pillow to open and save PNG image without any modifications. Images on output are displayed darker than the original:

Here is my code:

from PIL import Image
x = Image.open("cat.png")
x.save("cat-after.png","PNG")

If I open "cat-after.png", it will have same pixels as "cat.png".

I also noticed, that cat.png has altered gamma:

x.info
>> {'aspect': (1, 1),
>> 'chromaticity': (0.3127, 0.329, 0.64, 0.33, 0.3, 0.6, 0.15, 0.06),
>> 'gamma': 0.50994}

And in the reopened image, there is no metadata yet:

x2 = Image.open("cat-after.png")
x2.info
>> {}

And I think, this is because Pillow doesn't preserve gamma. How to make Pillow save the same image?

Answer 1

I encountered the same problem (with ffmpeg-generated PNGs) and I found a way to do this, although quite low-level. I got inspired by this github issue and this code.

from PIL import Image
from PIL import PngImagePlugin

x = Image.open("cat.png")

metadata = PngImagePlugin.PngInfo()
metadata.add(bytes("gAMA", "ascii"), round(x.info["gamma"] * 100_000).to_bytes(4, 'big'))
chromaticity_bytes = b""
for chrm in x.info["chromaticity"]:
    chromaticity_bytes += round(chrm * 100_000).to_bytes(4, 'big')
metadata.add(bytes("cHRM", "ascii"), chromaticity_bytes)

x.save("cat-after.png", pnginfo=metadata)