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To: tiff@sgi.com
Subject: Re: Photoshop Thumbnails 
In-Reply-To: Your message of Mon, 23 Nov 1998 19:34:23 -0300 
             <01BE1718.46EF1480@lacripta> 
Date: Tue, 24 Nov 1998 10:45:52 -0500
Message-Id: <26182.911922352@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
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Precedence: bulk

fernando <wau@satlink.com> writes:
>> - Does PhotoShop use a private tag for storing thumbnails?
>> - Does anyone have any information about the tag(i.e., format)?

> Look for IPTC - NAA (Information Interchange Model/Digital Newsphoto
> Parameter Record Version 4)

I believe Photoshop only writes the IPTC record if the user has
explicitly added IPTC annotations to the file.  I don't think
that's where they keep the thumbnail, either.

Where they do keep the thumbnail (and a whole lot of other stuff)
is a private Adobe tag that holds a block-structured list of items.
They use this same block-structured add-on data in a whole bunch of
different file formats --- for example, in a JFIF JPEG file they
stick it into APP13 marker(s).  Thus, they don't bother to support
the file format's own native provision for thumbnails, if any.

You can find out a good deal about this topic in the Photoshop SDK
documentation, available free from Adobe's website.  The SDK fails
to specify the contents of all the block types, however, and I think
the thumbnail block is one of the things that could've been better
documented.  Expect to have to do some experimentation.

BTW I have a vague recollection that their JPEG-compressed thumbnails
are broken --- IIRC the red and blue channels are swapped, at least
in PS 4.

			regards, tom lane
			organizer, Independent JPEG Group
