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May 2005
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2005.05.26 14:10 "Re: Jpeg and YCbCr", by Jean-yves Le Ridant
Joris, > I've seen JPEG-in-TIFF, with Photometric indicating YCbCr, and > SamplesPerPixel=1. The JPEG data, in this case, had a single channel, as > expected. The results when interpreting this channel as the Y component of YCbCr > were fine. > > I've also seen TIFFs with Photometric indicating L*a*b*, and SamplesPerPixel=1. > The single channel, interpreted as the L* component of L*a*b*, made perfect > sense, visually. > > In my humble opinion, if a multichannel colorspace start with a brightness > channel, and the SamplesPerPixel value equals 1, that is a unambigious > situation, and may be preferable over storing grayscale as full YCbCr. Yes, but do this conform to TIFF specifications ? > However, I've seen jpeg compression of CIE L*a*b*, and to me that *does* make > perfect sense. L*a*b* is a colorspace with a brightness channel and two chroma > channels, just like YCbCr. But there's good reason to assume L*a*b* is more > perceptually uniform compared to YCbCr, so that actually makes it technically > *more* suitable to be used as input for the JPEG compression algorithm... There > is also ITULAB, a color-fax oriented standard for encoding L*a*b* in JPEG (and > in JPEG-in-TIFF). I agree that LABs colorspace could be seen as "the" colorspace for jpeg compression, and ICCLAB and/or ITULAB are perfect. The problem is with tif's PHOTOMETRIC_CIELAB continous tones *signed* representation which are, for now, without special case, traited as unsigned value by jpeg compressor/decompressor. A slight "2 or 3" difference around "128" unsigned values, can cause at the end, a differences like +/- 125 in signed vals. > As a sidenode, people, please, would you consider posting more uniformly? You > guys are forcing me to do a lot of post-editing on your mails to ensure uniform > appearance in the archive Is it ok like this ? -- Jean-Yves