2006.06.13 16:04 "Re: [Tiff] Editing a tiff image using libtiff", by Joris Van Damme
An additional way to handle space is the introduction of an 'alloc-tag'. This tag indicates the size of the 'allocated' block in the file in contrast to the size tag which indicates the count of the bytes
> > used.
There are already the FreeByteOffsets and FreeByteCounts (288 and 289) tags for keeping track of unused space. Nothing is specified about in which IFD they belong, though (the first? the last? wherever you want, and use all of them together?). The spec also says: "not recommended for general interchange", probably partly because of this, and also because it's not very nice to give someone a space-wasting file, I guess :-).
Those tags were an attempt at memory-management, but a bad one, that gave more trouble then it was worth, and was subsequently abandoned.
One reason why they were trouble, is that, indeed, they violate the principle of tag scope. There should be no tag that has a logical meaning outside of the particular IFD it's in. Otherwise, unlinking IFDs and all that sort of thing becomes unfeasable.
Joris Van Damme
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