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Date: Tue, 17 Dec 2002 22:14:57 -0500
To: "Dominic Smith" <dominicsmith501@hotmail.com>,
	tiff@olympiakos.com
From: Leonard Rosenthol <leonardr@lazerware.com>
Subject: Re: CMYK JPEG's
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<x-flowed>At 9:47 PM +0000 12/17/02, Dominic Smith wrote:
>I have some CMYK JPEG (.jpg, not TIFF JPEG) files which open ok in 
>several recent PC packages (Paintshop 7 etc). But when I read them 
>with libjpeg they are negative.

	They are most likely created by Adobe software, which writes 
out "inverted" CMYK JPEGs.  This problem is described in quite a lot 
of detail in the libjpeg and Ghostscript documentation - check them 
out!


>I read that JPEG doesn't define the polarity of CMYK data, and that 
>both interpretations (black is zero and white is zero) exist in the 
>wild. Does libjpeg use white is zero?

	Everybody except Adobe uses "white is zero"...


>Also, has black is zero become the de facto (de Adobe?) standard? 
>Should I invert the libjpeg output as a matter of course, and will 
>this cover most/virtually all images I am likely to meet?
>

	No, you need to actually check to see if the data comes from 
an Adobe application (presence of the Adobe APP1 tag) and then 
switch.  HOWEVER, even that isn't 100% since there are some tools 
that will manipulate the data and leave the tags in place even if 
they aren't valid anymore :(.


Leonard
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Leonard Rosenthol                            <mailto:leonardr@lazerware.com>
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