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August 2005
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2005.08.15 02:19 "Re: TIFF Thumbnail", by Joris Van Damme
Katrina, katrina maramba wrote: > I have observed that not all TIFF images have multiple directories, > which means it has no thumbnail. And not all that have multiple > directories have thumbnails in their secondary (or whatever) > directory. I assume this is actually a question, even though your quoting style confuses the hell out of me. Please try and read http://web.presby.edu/~nnqadmin/nnq/nquote.html. In particular, please don't post an entire new message incorrectly marking it completely as quote. It is very important you get that right. Not just for you, and your chances of working towards a good definitive answer here, but also for us, as we need to read and make sense of the lot. In fact, you're other message posted at the same time, I find too ugly and too much trouble to make sense of. If my answer might have been a good one, you missed it, because the question message wasn't nearly clean enough. The definitive document on thumbnails in TIFF, and related, is the specification supplement that describes the SubIFD tag, which can be found here: http://partners.adobe.com/public/developer/en/tiff/TIFFPM6.pdf In short, it says that the SubIFD tag can contain the offset of a thumbnail image IFD. That solves a lot of trouble. For instance, you don't have to scan subsequent IFDs in order to determine if it has a thumbnail relationship with current IFD. Also, you can easily do IFD list level operations, like deleting (unlinking) a page, or resorting pages, without badly interfering with main/thumbnail relationships. Etc, really, the number of examples of why this and only this scheme is a good thing is as big as you want it be, it simply makes sense. This being said, there are a number of other schemes 'out there'. Here are two of the most common: - Some software writes thumbnails the 'old-style' pre-SubIFD way. This usually results in a two-page TIFF, the second page being a thumbnail of the first bigger image. You can detect this situation by checking - number of pages (must be 2) - NewSubfileType or SubfileType tags (must be present and reflect this relationship) - sizes (2nd must be smallest) - Some software writes thumbnails in proprietary ways. Usually, this kind of thumbnail is not even an IFD. An example of this that is often encountered 'in the wild' is http://www.awaresystems.be/imaging/tiff/tifftags/docs/photoshopthumbnail.html. > How can the reader determine if a particular image has a thumbnail > or not? I recommend your code to do this, in this particular order: - Check existence of SubIFD tag. If it's there, make additional sanity checks. If all works out, you've got the thumbnail in a child IFD. - If not found yet: If you want to go one step further, check for the presense of the old-style pre-SubIFD thumbnail scheme, detailed above - If not found yet: If you want to go one step further, check for existence of Photoshop tag, scan it for thumbnail presence. > And how can the reader determine the directory number(?) in which > the thumbnail belongs (so it doesnt have to read the other > directories)? That's one of the problems with the old-style pre-SubIFD way of writing thumbnails. From IFD 0, there is no clear indication that IFD 1 contains or does not contain a thumbnail. And if next a software along that inserts a page between these IFDs, by rewriting 'next IFD offset' members, all is lost. If you're looking for a tag that contains an index that is the directory number of the thumbnail, there is none. It wouldn't fit in TIFF, either, as software can unlink and relinking pages, and shouldn't have to scan through all tags in all directories in order to 'restore' their value to new situation or something. However, there is the SubIFD tag, which is almost what you're looking for. But that one isn't included in pre-SubIFD-style TIFFs, of course. Joris Van Damme info@awaresystems.be http://www.awaresystems.be/ Download your free TIFF tag viewer for windows here: http://www.awaresystems.be/imaging/tiff/astifftagviewer.html