2006.10.16 21:24 "Re: [Tiff] Tag 346", by Joris Van Damme

Ross,

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/
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