2005.12.02 19:03 "[Tiff] Odd offsets in IFD entries", by Gary McGath
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?
Digital Library Software Engineer
Harvard University Library Office for Information Systems http://hul.harvard.edu/~gary/index.html