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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="Steve Underwood (steveu@coppice.org)">
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<META NAME="Subject" CONTENT="Re: Which scanners REALLY provide 36 bit output? HP?">
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<H1>Re: Which scanners REALLY provide 36 bit output? HP?</H1>
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<!-- received="Tue Dec 12 17:24:49 2000" -->
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<STRONG>From:</STRONG> Steve Underwood (<A HREF="mailto:steveu@coppice.org?Subject=Re:%20Which%20scanners%20REALLY%20provide%2036%20bit%20output?%20HP?&In-Reply-To=<3A36D172.D5B862BB@coppice.org>"><EM>steveu@coppice.org</EM></A>)<BR>
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<STRONG>Date:</STRONG> Tue Dec 12 2000 - 17:31:30 PST
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<P>
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<LI><STRONG>Next message:</STRONG> <A HREF="0136.html">Tom Martone: "Re: testing BH on copiscan 8080d"</A>
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<LI><STRONG>Previous message:</STRONG> <A HREF="0134.html">Mark Temple: "re: testing BH on copiscan 8080d"</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="0145.html">Marko Cebokli: "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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<HR NOSHADE><P>
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<P>
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Stephen Williams wrote:
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<BR>
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<P><EM>> <A HREF="mailto:s57uuu@hamradio.si?Subject=Re:%20Which%20scanners%20REALLY%20provide%2036%20bit%20output?%20HP?&In-Reply-To=<3A36D172.D5B862BB@coppice.org>">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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<P><EM>> (Actually, after saying that I'm not really sure how wide the spectral
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</EM><BR>
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<EM>> response is for luminance, but I do know that luminance calculations
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</EM><BR>
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<EM>> certainly all include some red and blue, and I presume there is a
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</EM><BR>
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<EM>> psychological/physiological reason for that.)
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</EM><BR>
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<P>Its not just about spectrum, but about resolution and sensitivity.
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<BR>
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<P>The luminence sensors are much denser than the chroma sensors. They are
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<BR>
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most sensitive at green, drop off badly below red, and drop off much worse
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<BR>
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in the blue.
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<BR>
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<P>The green/red sensing is at a much lower density than the luminence, but
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<BR>
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has fairly good sensitivity. NTSC and PAL colour TV take advantage of this,
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<BR>
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and use only about 1/4 of the bandwidth for the chroma info, as they use
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<BR>
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for the luminence info. The density and sensitivity of the blue sensors are
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<BR>
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very very poor. A pure red or green TV image looks bright and clear. A pure
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<BR>
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blue TV image looks dark and noisy. That is mostly due to the limitations
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<BR>
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of the eye, although the blue emitter in a TV tube is also poorer than the
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<BR>
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red or green..
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<BR>
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<P>The bottom line is we cannot see colour in detail. Try grouping a lot of
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<BR>
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tiny varously coloured images together. You can still the fine detail of
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<BR>
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their shapes, but they all appear white.
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<BR>
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<P>When you try to scan a colour photo or magazine print you are using three
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<BR>
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sensors which don't accurately match the spectral response of the eye, but
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<BR>
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have peak responses in roughly the same places. You are scanning images
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<BR>
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made of three dyes which strange spectral responses that only *very*
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<BR>
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roughly approxiamate the response of the eye's sensors. You will display
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<BR>
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the result on a CRT, LCD, or printer, with their own set of very rough
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<BR>
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approxiamations to the right spectral response. In addition, some of these
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<BR>
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steps are working in cyan, magenta, and yellow, rather than red, green and
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<BR>
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blue. There are further limitations in colour accuracy caused by this
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<BR>
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swapping of primary/secondary colour mode.
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<BR>
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<P>If the eye's colour sensing worked in an absolute way the result would
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<BR>
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probably always look awful. However, the eye senses colour only in a
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<BR>
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relative way. Basically, an averaging process takes place across the entire
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<BR>
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field of vision, and the eye assumes this average to be white. All colours
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<BR>
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are rendered within the brain's signal processing according to this
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<BR>
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averaging. Now, if the sun changes to flourescent light as night comes we
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<BR>
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don't see the world turn the awful blue which most flourescent tubes
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<BR>
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produce. We see just a minor tinting of the colouration of the scene. More
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<BR>
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puzzling, and not fully explained, is why we appear to see a somewhat
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<BR>
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colourful image under low pressure sodium lamps. These produce extremely
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<BR>
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narrow band monochromatic light. We see zero colour variation in the
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<BR>
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relected light from any part of the scene, and yet we don't see the world
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<BR>
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in pure orange. The eye is clearly cooking up some fake, but realistic,
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<BR>
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colourfulness. How that realism works is a mystery, as we seem to detect
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<BR>
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the approxiamately correct colour of objects we have never seen in white
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<BR>
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light!
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<BR>
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<P>Unless you understand these things, imaging will always be disappointing.
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<BR>
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Without a feel for the limitations of the chain of factors resulting in a
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<BR>
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final image, the nature of that result tends to be an unpleasant suprise.
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<BR>
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Often, the only way to prove to people just how bad something will look is
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<BR>
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to produce a sample! A sad, but necessary, waste of effort.
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<BR>
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<P>Regards,
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<BR>
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Steve
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<BR>
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<P><P><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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<!-- next="start" -->
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<LI><STRONG>Next message:</STRONG> <A HREF="0136.html">Tom Martone: "Re: testing BH on copiscan 8080d"</A>
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<LI><STRONG>Previous message:</STRONG> <A HREF="0134.html">Mark Temple: "re: testing BH on copiscan 8080d"</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="0145.html">Marko Cebokli: "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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<EM>
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This archive was generated by <A HREF="http://www.hypermail.org/">hypermail 2b29</A>
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: <EM>Tue Dec 12 2000 - 18:52:31 PST</EM>
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