convert images

[wilson]

hi
i converted some P5 type .pgm images to .jpg using
x=Image.open("oldimage.pgm")
imsz=x.size
newimg=Image.new('L',imsz)
newimg.putdata(x.getdata())
newimg.save("newimg.jpg")

when i again check the pixel data for these images using getdata()
method i,I find that they are slightly different

ie if oldimage.pgm has pixels
[29 31 38 ..., 10 4 18]
then the corresponding jpg image has
[29 31 38 ..., 10 3 17]

why this difference? shouldn't they be identical?can someone pls
explain this?

[Marc 'BlackJack' Rintsch]

On Sun, 27 Apr 2008 06:42:13 -0700, wilson wrote:

i converted some P5 type .pgm images to .jpg using
[…]
ie if oldimage.pgm has pixels
[29 31 38 ..., 10 4 18]
then the corresponding jpg image has
[29 31 38 ..., 10 3 17]

why this difference? shouldn't they be identical?can someone pls
explain this?

JPEG uses a lossy compression algorithm, i.e. the image loses quality and
therefore the pixel values are not exactly the same as before saving. If
you want get the exact values back you have to use another image format.
For RGB or RGBA data PNG is a good format.

Ciao,
Marc 'BlackJack' Rintsch