[Image-SIG] Problem with JPEG and CMYK color space

Cesare Leonardi ced at bernispa.com
Fri Apr 14 09:38:16 CEST 2006

kevin at cazabon.com ha scritto:
> When saving a CMYK file to JPG, PIL seems to invert the colors.  I've 
> submitted a patch to Fredrik already for this, and it should be in the 
> next major build.
> Adding a simple invert to the image before saving will help.

Thanks all for your responses.

Kevin, i've made some tests following what you've suggested and here is 
the results.
To obtain the color inversion i've used the function invert() in 
ImageChops module. I've added the code and the new sample images in this 
page (see "part 2"):

As you can see your suggestion works partially.
Images 02 and 03 (that was particular in "part 1") are the only that 
after the inversion looks ok.
All the other images looks less dark, but the color are always different 
than the original.
The particular thing to note is that the images produced by the 
inversion looks the same as Image.show(). Seems that show() already do 
the inversion.

There is another thing that i have noted only now: the images saved by 
PIL are often less big than the original. Sometimes much less. For example:
test01: 1435 KB   test01-dst: 198 KB   test01-dst-invert: 198 KB
test04: 1237 KB   test01-dst: 405 KB   test01-dst-invert: 405 KB
test08: 1647 KB   test01-dst: 777 KB   test01-dst-invert: 776 KB
test09: 1715 KB   test01-dst: 665 KB   test01-dst-invert: 665 KB
test10: 2032 KB   test01-dst: 659 KB   test01-dst-invert: 659 KB
I can think that PIL optimize the compression, but that difference are 
really big. Isn't it suspicious? I expect that saving an image in a new 
file without modifications produces a file very similar to the original, 
isn't it?
The original images was not produced by me so the author can have used 
low compression. I cannot make tests since i haven't a program that can 
save in CMYK (Gimp seems not able).



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