I have some experiments with JPEG, [the doc](http://effbot.org/imagingbook/format-jpeg.htm) 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](http://php.net/manual/en/function.imagejpeg.php) 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