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Date: Mon, 18 Dec 2000 07:08:22 +0100 (CET)
From: cbednare@t-online.de (Christian Bednarek)
To: TIFF MailingList <tiff@olympiakos.com>
Subject: Re: Stripes in thumbnail
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On Mon, 18 Dec 2000, Joris Van Damme wrote:

> The published original G4 tiff file is in turn compressed in some format
> with the extension bz2, which is unknow to me, and for which I do not
> have the tools to decompress. So I can't even look at it.

Oh sorry, didn't think of that. I put the original
back (~4.8MB).

> scanlines in the original g4 tiff(s) cannot be correctly interpretted by
> LibTiff, and are simply (partially) skipped, leaving a (partially) blank
> line. Warning and error behaviour of the g3/g4 decompressors has been

Hm. Now as you mention it... there are warnings. But as I'm an absolute
newbie, I didn't put them in relation to the stripes...

> - code to get LibTiff to read the original grayscale files
> - code to convert an image from grayscale to cmyk
> - code to get LibTiff to write the cmyk image
> 
> You should be able to handle the first and third with the aid of the
> LibTiff docs. If you have any additional questions (preferably more
> specific, if you expect an answer), just ask 'em.

Yes, I think this should be possible for me, in case I get enough time to
learn about TIFF :)

> I can elaborate on one way to get you started on the second task.
> Photoshop can save .ast files, which contain a lut to convert between
> lab and cmyk. The Photoshop SDK docs contain more info about this. So,
> one way would be to convert grayscale (which is essentially RGB, with
> r=g=b) to Y (XYZ with X=0 and Z=0), Y to L (Lab with a=0 and b=0) and
> finally L to CMYK using an .ast file/lut.

wow... it's _that_ complicated? But I did forget to mention, that those
thumbnails don't have to be accurate. It's enough for me - and the task
it's needed for - to find that a part of the picture is green and not red.
So red=255-cyan, green=255-magenta... is quite good enough.

Christian

