2004.10.04 18:59 "Re: [Tiff] Re: BigTIFF extension issue", by Andy Cave

Hi All.

Here's my crunch argument for calling it .tiff.

As a good programmer, I should write a good library. My library should write a dummy header that could be either little tiff or big tiff. It should then write the possibly unknown sized compressed data. If the file size ends up being too big, then it should patch it up as a big tiff file. If the file size ends up being small enough, it should patch it up as a small tiff file. In the case of patching it up as small tiff, there would be a small amount of space wasted - so what.

The big tiff file format should also introduce a new header / dict tag which is EXPLICITLY designed to allow for space that can be used later on. If I open a small tiff file, and append some data which then makes it a big tiff file, IF the file is only small tiff and the library is small tiff lib (2.5.7,...), it will fail. If the library is big tiff compatible, and the image file has this extra header / dict tag, then this extra header / dict tag can be used to 'expand/convert' the existing date from small tiff to big tiff.

Now think what this means. All files produced that are less than 2/4G called .tif can be processed by existing applications. They can be expanded and remain as small tiff. They can be expanded and converted to big tiff. Nice and backwards compatible. Any files produced that are (end up being) more than 2/4G probably could not be processed by existing applications anyway, as these apps probably have in them inherently the 2/4G limit that TIFF files have anyway.

So, as part of the big tiff spec, we REQUIRE that this extra header info and extra dict tags be there that allows a big tiff conforming file to expand.

Customers keep their existing extension. Bigtif files that are written that are small NEVER have a problem with existing libraries, including 2.5.7, 3.0... of tiff. A bigtif library that reads a small tiff file, that does NOT contain these additions, is NOT allowed to append to that file and make it a bigtif file.

Everything works.

QED. End of discussion.

_andy.