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January 2007
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2007.01.31 17:31 "Re: 16bit grayscale with colormap", by Joris Van Damme
Reinhard, Reinhard Mayr aka Czerwinski wrote: > I deal with 16bit grayscale TIFFs. The files originate from a 12bit > CCD, so the linear conversion 16bit->8bit turns my image almost > black. You can write 12bit grayscale. It is perfectly legit TIFF. However, few readers will support it. You can also rescale to a more common bitdepth, like 8bit or 16bit, before writing. > To cope with that, I would like to add a ColorMap into the TIFF. For > testing, I altered an existing 16bit file: > o photometrics = 3 (ColorMap) > o added a colormap (tag 0140), length: 3*2^16= 196608 bytes You mean 3*2^16 words, not bytes. Palettes (Colormaps) in TIFF are 3 word values per palette entry. > Is this a proper interpretation of the TIFF specs (TIFF6 specs p.23 > allow only 4/8 bits per sample -- why?)? Yes it is a proper interpretation of the TIFF spec. The pages you refer to, are *baseline* TIFF. The whole concept of baseline TIFF is a bit dated, as many flavours that aren't originally regarded baseline are commonly supported these days. But in any case, even originally, there wasn't a restriction to 4 or 8 bits per sample in Colormap TIFFs as long as you know they're not baseline. In general, TIFF is extremely flexible this way. You can have a 21 bit Colormap TIFF, or a 36 bit grayscale TIFF, or an 11-13-15 bits per sample RGB TIFF. It's all unambigious, and all legit. It is, however, badly supported by most readers. Best regards, 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