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<TITLE>sane-devel: Re: Which scanners REALLY provide 36 bit output? HP?</TITLE>
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<META NAME="Author" CONTENT="Marko Cebokli (s57uuu@hamradio.si)">
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<H1>Re: Which scanners REALLY provide 36 bit output? HP?</H1>
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<!-- received="Wed Dec 13 09:21:59 2000" -->
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<!-- name="Marko Cebokli" -->
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<!-- subject="Re: Which scanners REALLY provide 36 bit output? HP?" -->
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<STRONG>From:</STRONG> Marko Cebokli (<A HREF="mailto:s57uuu@hamradio.si?Subject=Re:%20Which%20scanners%20REALLY%20provide%2036%20bit%20output?%20HP?&In-Reply-To=<3A37B2AD.2722420E@hamradio.si>"><EM>s57uuu@hamradio.si</EM></A>)<BR>
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<STRONG>Date:</STRONG> Wed Dec 13 2000 - 09:32:29 PST
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<P>
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<LI><STRONG>Next message:</STRONG> <A HREF="0146.html">Oliver Rauch: "Re: Comments on sane-frontend"</A>
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<LI><STRONG>Previous message:</STRONG> <A HREF="0144.html">Oliver Rauch: "Re: download of sane-backends-pre1-1.0.4"</A>
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<LI><STRONG>In reply to:</STRONG> <A HREF="0131.html">Stephen Williams: "Re: Which scanners REALLY provide 36 bit output? HP?"</A>
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<LI><STRONG>Next in thread:</STRONG> <A HREF="0153.html">Stephen Williams: "Re: Which scanners REALLY provide 36 bit output? HP?"</A>
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<LI><STRONG>Next in thread:</STRONG> <A HREF="0071.html">Steve Underwood: "Re: Which scanners REALLY provide 36 bit output? HP?"</A>
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<LI><STRONG>Reply:</STRONG> <A HREF="0153.html">Stephen Williams: "Re: Which scanners REALLY provide 36 bit output? HP?"</A>
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<P>
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Stephen Williams wrote:
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<BR>
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<EM>>
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</EM><BR>
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<EM>> <A HREF="mailto:s57uuu@hamradio.si?Subject=Re:%20Which%20scanners%20REALLY%20provide%2036%20bit%20output?%20HP?&In-Reply-To=<3A37B2AD.2722420E@hamradio.si>">s57uuu@hamradio.si</A> said:
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</EM><BR>
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<EM>> > (Yes, it's 3 - the luminance sensors (the 'rods') have the same
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</EM><BR>
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<EM>> > sensitivity as the 'green' channel 'cones')
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</EM><BR>
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<EM>>
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</EM><BR>
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<EM>> Not quite, they respond to a wider spectrum then the green sensors.
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</EM><BR>
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<EM>> Though they are highly sensitive to green, they respond to most all
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</EM><BR>
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<EM>> of the visible color range. I think.
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</EM><BR>
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<EM>>
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</EM><BR>
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<P>They do have the same curve (they also use the same sensitive compuond).
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<BR>
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None of the eye (or film/CCD) sensors is really a 'rectangular' band
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<BR>
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pass. They all have relatively wide responses, with the skirts extending
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<BR>
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all over the visual range.
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<BR>
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<P>The eye curves are well measured and documented, you can find them
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<BR>
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tabulated and graphed in any serious book about imaging.
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<BR>
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<P>The film curves can be found in manufacturer's brochures. (the same
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<BR>
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goes for CCD's)
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<BR>
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<P>The linear approximation for the output of such a channel sensor is
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<BR>
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(let the S represent the integral sign and l lambda)
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<BR>
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<P>R = S r(l)i(l) dl
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<BR>
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<P>one such equation for each channel,
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<BR>
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where r(l) is the sensitivity curve of the sensor, and i(l) is the
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<BR>
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spectral intensity of the scene. Integration limits are over the
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<BR>
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full range where r(l) is nonzero.
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<BR>
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<P>The eye and film can be highly non-linear, but CCD's are for practical
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<BR>
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purposes perfectly linear (= ideal image sensors, they caused a
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<BR>
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revolution in astronomical and other scientific imaging)
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<BR>
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<P><EM>> But do the spectra of those three channels cause the RGB sensors in
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</EM><BR>
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<EM>> your scanner to respond similarly? That's a question for a photography
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</EM><BR>
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<P>The consequence of the above equation is that you have crosstalk
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<BR>
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between the channels, which depends on the shapes of all the curves
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<BR>
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involved, and these can differ between various films, photo-papers,
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<BR>
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printer inks, CRT phosphors and whatewer. Trouble guaranted! (and this
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<BR>
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was only the linear model ;-) There is a 3x3 matrix linking two such
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<BR>
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linear tricolor systems. Ideally, it would be a identity matrix, but as
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<BR>
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Murphy designed it, it never is.
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<BR>
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<P>Even if the artificial sensor's curves would match the eye curves
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<BR>
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perfectly, you wuold still have crosstalk. That's why you can't really
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<BR>
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reproduce ALL the colors using a tricolor system - you're limited to
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<BR>
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the inside area of the triangle defined by the primarie's places in
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<BR>
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the chromacity diagram. Most notably, pure spectral colors are always
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<BR>
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outside. But most (virtually all) colors in nature DO fall within
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<BR>
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this triangle, and with some (a lot of--) tweaking, one can get very
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<BR>
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good results.
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<BR>
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<P>Colorimetrists represent the 'outside' colors by assigning negative
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<BR>
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values to the channels. But that's only a mathematical trik, useful
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<BR>
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for computation, because in practice, you can't squeeze 'minus red'
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<BR>
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out of your monitor...
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<BR>
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<P>When you try color metrics (measuring differences between colors),
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<BR>
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things get even more scary. Colorimetrisis found out that the human
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<BR>
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chromacity space IS 3D, but its metrics are non-euclidean....
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<BR>
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<P>Do you still dare to scan? :-)
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<BR>
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<P>Marko Cebokli
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<BR>
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<P><PRE>
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--
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Source code, list archive, and docs: <A HREF="http://www.mostang.com/sane/">http://www.mostang.com/sane/</A>
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</PRE>
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<LI><STRONG>Next message:</STRONG> <A HREF="0146.html">Oliver Rauch: "Re: Comments on sane-frontend"</A>
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<LI><STRONG>Previous message:</STRONG> <A HREF="0144.html">Oliver Rauch: "Re: download of sane-backends-pre1-1.0.4"</A>
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<LI><STRONG>In reply to:</STRONG> <A HREF="0131.html">Stephen Williams: "Re: Which scanners REALLY provide 36 bit output? HP?"</A>
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<!-- nextthread="start" -->
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<LI><STRONG>Next in thread:</STRONG> <A HREF="0153.html">Stephen Williams: "Re: Which scanners REALLY provide 36 bit output? HP?"</A>
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<LI><STRONG>Next in thread:</STRONG> <A HREF="0071.html">Steve Underwood: "Re: Which scanners REALLY provide 36 bit output? HP?"</A>
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<LI><STRONG>Reply:</STRONG> <A HREF="0153.html">Stephen Williams: "Re: Which scanners REALLY provide 36 bit output? HP?"</A>
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<EM>
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This archive was generated by <A HREF="http://www.hypermail.org/">hypermail 2b29</A>
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: <EM>Wed Dec 13 2000 - 09:22:24 PST</EM>
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