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January 2007
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2007.01.31 22:24 "Re: Interpreting SubIFDs tag", by Ed Grissom
TIFFers Here is the document that invented the SubIFD concept -- this is the "Tiff Trees" approach. (http://partners.adobe.com/public/developer/en/tiff/TIFFPM6.pdf) It clearly states in the final paragraph that each SubIFD value is to be considered a pointer to the first IFD in a list of IFDs. Consider that instead of a single thumbnail, you might have a full set of reduced resolution versions of the main image. This is typically used for LARGE raster files to provide fast zooming capabilities. -- ed grissom ed.grissom@intergraph.com -----Original Message----- From: tiff-bounces@lists.maptools.org [mailto:tiff-bounces@lists.maptools.org] On Behalf Of mikk Sent: Wednesday, January 31, 2007 4:54 AM To: tiff@lists.maptools.org Subject: [Tiff] Interpreting SubIFDs tag Hello all, I'm a bit confused with interpreting SubIFDs tag value. Each value of N values in SubIFDs tag is an offset (from the beginning of the TIFF file, as always) to a child IFD. But is each value of N values an offset to a single child IFD, or to the first IFD from a linked list of IFDs? When I read each of these N child IFDs, am I supposed to interpret NextIFDOffset field and read next directory from this position for each of child IFDs? Could anyone put some light on this, please? Thanks in advance, kind regards, mikk