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May 2005
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2005.05.26 13:03 "Re: Jpeg and YCbCr", by Joris Van Damme
> > with bitcount=24 image, i did like this.. > > but with grayscale image(bitcount= 8) > > If you want the widest standard for your jpeg stream, > compress as RGB/YCBCR 24 bits is better. > Tiff allows a one component photometric minisblack > ( with a unknown colorspace for jpeg stream ). 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. Using MinIsBlack or MinIsWhite photometrics, and JPEG-compressed single-channel data, may indeed also be an option... > Jpeg compression of palette color indexes would be > as strange idea as jpeg compression of CIELAB. > And it is out of spec. As to the jpeg compression of palette color data, I totally agree. JPEG is a compression method that is designed for photographic images, 'continuous-tone images', where, statistically speaking, neighbouring pixels have very similar colors. There should be some criminal punishment for people using JPEG compression on screenshots and the like... 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). 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 (http://www.awaresystems.be/imaging/tiff/tml.html), which I feel is a very necessary condition for easy reading (and for future planned additional automatic post-processing). Jean-Yves, I find myself consistently having to remove umpty XX markers before the '>' quote marker and such, and consistently having manually fix subject lines to remove the meaningless [2] and [3] markers and such... That is very tiresome. Please all consider making my job a little easier, by - using plain text format instead of bloat garbadge mail formats - using the minimal top-quoting that is needed for direct subscribers to make sense of it all (if that is more then what is needed for archive users, I'm fully prepared to remove the bulk part of the quoting in the archive, but that is the only job I should have to do) - using uniform quote markers ('>', followed by a single space) - try matching topic and subject line (don't start umpty threads for the same topic, don't raise umpty topics in a single thread) - try keeping subjects lines short and to the point (don't use 'help' or 'question about...' or 'TIFF question', but please also don't use subject lines that contain dozens of words since line breaking spoils index views) I've condemned myself to doing this job, long ago. I'm not complaining, and I'll continue no matter what. But please, Jean-Yves and 'Pattern', if you can, please consider making my job a little less bloody tiresome and time-consuming. 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