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Topic: Re: Image Extraction (CMYK - JPEG)
Conf: (P-PDF) Developers, Msg: 57471
From: LeonardR
Date: 5/29/2002 05:26 PM

At 03:21 AM 11/28/2001 +1100, p-pdf-developer Listmanager wrote:
>I have found that extracting DCT compressed CMYK data will often give a
>negative result.

ONLY if the original CMYK JPEG file was created with Photoshop (or
some other Adobe product).

>This is because of a difference in interpretation of the colours, and is
>not a fault in the PDF or the extraction.


>You have to invert the colours in each channel.

But you should ONLY do it, when you know the file came from an
Adobe product - because otherwise you'd end up with other CMYK images
inverted :(.

>I don't know if this can be done without decompressing and recompressing.

No, you have to pass the data through something like libjpeg to
not only decopress the data - BUT also parse the tags to determine if the
image should be inverted. (and if you do invert the bits of the original
file, you also need to remove the Adobe tag, since otherwise software will
be very confused).

Of course, if the original image was from Photoshop and the user
of your extractor plans to use it again with Photoshop AND you invert the
CMYK space, then they will be VERY unhappy!!!


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