PIL jpeg, how to preserve the pixel color

Question

I have some experiments with JPEG, the doc said "100 completely disables the JPEG quantization stage."

However, I still got some pixel modification during saving. Here is my code:

import Image
red = [20,30,40,50,60,70];
img = Image.new("RGB", [1, len(red)], (255,255,255))
pix = img.load()

for x in range(0,len(red)):
    pix[0,x] = (red[x],255,255)

img.save('test.jpg',quality=100)

img = Image.open('test.jpg')
pix = img.load()

for x in range(0,len(red)):
    print pix[0,x][0],

I got unexpected output: 22 25 42 45 62 65 What should I do to preserve the pixel value ? Please note that I also tried with PHP using imagejpeg and It gives me the correct value when quality=100.

I can use png to preserve, but I want to know the reason behind this and if there is any option to avoid

Answer 1

JPEG consists of many different steps, many of which introduce some loss. By using a sample image containing only red, you've probably run across the worst offender - downsampling or chroma subsampling. Half of the color information is thrown away because the eye is more sensitive to brightness changes than color changes.

Some JPEG encoders can be configured to turn off subsampling, including PIL and Pillow by setting subsampling=0. In any case it won't give you a completely lossless file since there are still other steps that introduce a loss.

Answer 2

JPEG will always carry risk of lossyness, see Is Jpeg lossless when quality is set to 100?.

Your best bet is to use another format, especially if your experiments are for science :) Even if you're forced to start with JPEG (which seems unlikely) you should immediately convert to a lossless format for any kind of analysis and modification.

If you really want to try lossless JPEG work with python you can try jpegtran, "the lossless jpeg image transformation software from the Independent Jpeg Group", but as @Mark notes, this won't get you very far.

By the way, quantization is used in lossy or lossless compression alike, so my guess is that

...100 completely disables the JPEG quantization stage.^[1]

simply means that it's not compressed at all.