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From: "Joris" <joris.at.lebbeke@skynet.be>
To: "Ross Presser" <rpresser@gmail.com>
References: <656f8fef0610161334v4d6176c6k2e71b083d70c25f0@mail.gmail.com>
Subject: Re: [Tiff] Tag 346
Date: Mon, 16 Oct 2006 23:24:41 +0200
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Status:   

Ross,

Ross Presser wrote:
> Some extensive googling has turned up that Tag 346 was proposed in
> 1995 to make it possible to create paletted images using colorspaces
> other than RGB (much in the way that PDF supports it). Cf the
> following:
>
> http://partners.adobe.com/public/developer/en/tiff/TIFFPM6.pdf
> http://www.awaresystems.be/imaging/tiff/tifftags/indexed.html
> http://www.asmail.be/msg0055062731.html
> http://tools.ietf.org/html/draft-ietf-fax-tiff-fx-13
>
> Is there any extant software anywhere that implements this, either for
> writing or reading? (I have some hints that Pagemaker supported it at
> some point.)
>
> Also, are there any sample images anywhere?

To the best of my knowledge, that's a 'no' on both questions. I've never
seen any file come my way that uses the Indexed tag. I can write it with
my proprietary codec, but I'm careful to not let those experimental
files escape into the wild and would not want to ship a version of my
codec that writes them to anyone.

I believe the design of the 'Indexed' tag is not without serious flaws.
For one thing, it changes the meaning of the other most crucial tags.
Photometric CIELab no longer means Photometric CIELab when the Indexed
tag is present, rather it means the Colormap has Photometric CIELab and
the image itself has Photometric Palette. That means all software prior
to update for support of Indexed tag, gets into serious trouble on all
such images.

So writer vendors rightly regard it dangerous to write the Indexed tag
because no readers support it (if writer vendors know of the tag at
all - MS apparently is still in OJPEG ages in their TIFF support in
Vista according to one report I've seen so we might want to be sceptical
as to the abilities of some writer vendors), so there's no such images
in the wild, so readers don't support it. Chicken and egg.

Another thing that is unclear to me (though I must admit it's been a
while since I looked at the spec of the Indexed tag), is what happens
with ExtraSamples and the like. Do these change meaning similar to the
Photometric tag? In other words, if ExtraSamples and Indexed tags are
both present, does the palette have extra samples, or does the image
data? As far as I remember, that's not very clear.


Best regards,

Joris Van Damme
info@awaresystems.be
http://www.awaresystems.be/
Download your free TIFF tag viewer for windows here:
http://www.awaresystems.be/imaging/tiff/astifftagviewer.html

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