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Message-ID: <43909A98.8030303@hulmail.harvard.edu>
Date: Fri, 02 Dec 2005 14:03:52 -0500
From: Gary McGath <gary@rockhop.harvard.edu>
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Subject: [Tiff] Odd offsets in IFD entries
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Status:   

In TIFF, are all offsets specified by an IFD entry required to be on an 
even byte boundary?  Odd byte boundaries work with practically all 
software, but I'm asking the question from the standpoint of validating 
a TIFF file for archival purposes.

The TIFF 6.0 spec says on page 15: "Bytes 8-11 // The Value Offset, the 
file offset (in bytes) of the Value for the field. The Value is expected 
to begin on a word boundary; the corresponding Value Offset will thus be 
an even number. This file offset may point anywhere in the file, even 
after the image data."

"Will be an even number" sounds like a requirement to me, and we've been 
assuming in the implementation of JHOVE that it is.  But we've 
encountered large numbers of files that violate this requirement and 
still work with existing software.

We don't subscribe to the philosophy that the software defines the spec, 
but we do have to take practice into account.  Can anyone give a more or 
less authoritative answer on whether odd byte offsets in an IFD entry 
are legitimate?



-- 
Gary McGath
Digital Library Software Engineer
Harvard University Library Office for Information Systems
http://hul.harvard.edu/~gary/index.html

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