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February 1998

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1998.02.11 12:41 "AW: Tiff JPEG compression for CMYK images", by Than Olaf
1998.02.11 14:58 "Re: AW: Tiff JPEG compression for CMYK images", by Tom Lane

1998.02.11 14:58 "Re: AW: Tiff JPEG compression for CMYK images", by Tom Lane

"Than.Olaf" <Than.Olaf@GrunerDruck.de> writes:
> I think CMYK has 4 channels.

So it does, but that's irrelevant.  The problem discussed in the message
you quoted had to do with asking the JPEG codec to do RGB->YCbCr
conversion when the input data didn't have three channels.  (If I recall
correctly, the upshot was that Bill was trying to apply JPEG compression
to indexed-color data, which is not going to work in any case.)  For CMYK
data, the code is not going to do any colorspace conversion, and it will
support any number of channels.

> Can i write a class "JPEG" image using libtiff and IJG jpeg-6a with
> PhotometricInterpretation = 5 (CMYK)

Yes.

> Commpression               = 6 (JPEG)
> JPEGProc                     = 1
> JPEGquality                  = xx

No.  libtiff/libjpeg support the newer Compression=7 JPEG scheme defined
by TIFF Tech Note #2.  Please read the Tech Note to find out why
Compression=6 is no good.

			regards, tom lane
			organizer, Independent JPEG Group