2007.01.31 22:24 "RE: [Tiff] 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

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,