2008.04.17 06:02 "Re: [Tiff] efficient thumbnailing", by
Is there an efficient way to generate thumbnails from tiff files?
Stating the obvious but much neglected, make sure you use all and any thumbnails present in the file for starters.
One source of thumbnails or downsampled copies is the SubIFD child IFDs. A less known and older (and regrettable and discouraged) way to encode thumbnails, that is still in use today, is linking additional IFDs in the same main linked list, marking them as downsample with the SubfileType or NewSubfileType tags (see http://www.awaresystems.be/imaging/tiff/tifftags/search.html?q=subfile&Submit=Find+Tags).
There exist other common encodings of thumbnails, as some vendors seem to think embedding thumbnails is only usefull to themselves and therefore does not need to comply to a public standard. Regrettably, Adobe is amongst them. It's not uncommon to find the only source of thumbnail available is inside the Photoshop tag (see http://www.awaresystems.be/imaging/tiff/tifftags/docs/photoshopthumbnail.html).
The jpeg loader uses some fast-scaling trickery to scale down by a factor of 2, 4, or 8 first (http://sylvana.net/jpegcrop/djpeg) - I was wondering if there is a tiff equivalent.
LibJpeg is able to do this because of specific properties of the JPEG compression format. Few other compression formats can serve equally naturally in decoding to a scaled down version (without actually doing a full decoding and subsequent rescaling cycle, that is).
Of course, at least for TIFF IFDs with JPEG compression, we could theoretically support exactly the same...
Joris Van Damme
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