2021.09.30 09:06 "Re: [Tiff] Proposal to create a OGC BigTIFF standard", by Roger Leigh

Hi Kemp,

While not strictly related to “BigTIFF”, but certainly related to “Big” images, there is one big gap which would be useful to close to better meet the needs of medical and scientific imaging, and that is standardising full support for storing 3D image data. Libtiff does have some non-standard tags for this purpose, but for reasons of compatibility with other tools, I have never used them. Were they to be more formally recognised and adopted, storing volumetric data would become both practical and accessible. This is one of the main reasons for the adoption of other file formats for this type of data, despite TIFF being a perfectly suitable container. But TIFF is perfectly capable of this.

I’m thinking here of the addition of a Z dimension at both the image and tile level, with:

TIFFTAG_IMAGEDEPTH
TIFFTAG_TILEDEPTH
TIFFTAG_ZRESOLUTION
TIFFTAG_ZPOSITION

This would be the minimum required. And it’s already fully implemented as far as I can tell, unless other tags need extending for an optional third value which haven’t been done yet. If we could get this standardised that would be huge.

And others, e.g.

TIFFTAG_ZCLIPPATHUNITS?? Probably a can of worms to extend to 3D!
EXIFTAG_PIXELZDIMENSION?? Probably not needed if EXIF is restricted to 2D?

Aside: I saw TIFFTAG_XCLIPPATHUNITS duplicated in tif_dirinfo.c. With different types (SLONG and SBYTE). Is that a bug? Or intentional? TIFFTAG_YCLIPPATHUNITS is not duplicated.

Kind regards,

Roger