PIL changes pixel value when saving

Question

This is as simple as it gets. Make RGB image, one pixel big. Set pixel value to (a,b,c). Save. Problem is, the saved image has a different pixel value then (a,b,c). Usually, with +-1 on one of the channels, as if it were random noise, but its consistent. Heres the code:

from PIL import Image
newImg = Image.new('RGB', (1,1), "black")
pixels = newImg.load()
pixels[0,0] = (0,3,0)
newImg.save("point.jpg")
savedImage = Image.open("point.jpg")
pixelsSaved = savedImage.load()

print pixels[0,0]
print pixelsSaved[0,0]

The output from this is: (0, 3, 0) (1, 3, 0)

consistently.

If I use (4, 2, 0) as the pixel to save, the output is: (4, 2, 0) (3, 2, 0)

and (0,10,0) gives: (0, 10, 0) (0, 10, 1)

for example.

Why does the value change upon saving? How do I prevent it? Please help.

Thanks.

Answer 1

Save it as a .png. .jpg is a lossy compressed format.