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<!-- received="Thu Jul 29 06:41:23 1999 PDT" -->
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<!-- sent="Thu, 29 Jul 1999 15:58:46 +0200" -->
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<!-- name="Ewald R. de Wit" -->
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<!-- subject="Re: SANE & exposure times" -->
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<!-- inreplyto="199907290852.KAA24165@fritz.sema.be" -->
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<title>sane-devel: Re: SANE & exposure times</title>
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<h1>Re: SANE & exposure times</h1>
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<b>Ewald R. de Wit</b> (<a href="mailto:ewald@pobox.com"><i>ewald@pobox.com</i></a>)<br>
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<i>Thu, 29 Jul 1999 15:58:46 +0200</i>
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<p>
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<ul>
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<li> <b>Messages sorted by:</b> <a href="date.html#214">[ date ]</a><a href="index.html#214">[ thread ]</a><a href="subject.html#214">[ subject ]</a><a href="author.html#214">[ author ]</a>
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<li> <b>Next message:</b> <a href="0215.html">Jens Scheithauer: "scanimage and HP scanners"</a>
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<li> <b>Previous message:</b> <a href="0213.html">Ewald R. de Wit: "Re: SANE & exposure times"</a>
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<li> <b>In reply to:</b> <a href="0211.html">Didier Carlier: "Re: SANE & exposure times"</a>
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<li> <b>Next in thread:</b> <a href="0212.html">Andreas Rick: "Re: SANE & exposure times"</a>
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</ul>
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Didier Carlier (<a href="mailto:Didier.Carlier@sema.be">Didier.Carlier@sema.be</a>) wrote:<br>
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<p>
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<i>> What is the meaning of an RGB value divided by a time ? Nothing says </i><br>
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<i>> for instance that the sensitivity of the detectors is the same for all</i><br>
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<i>> colors so that even for a picture that looks perfectly grey, you would get </i><br>
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<i>> completely different values for the quotients.</i><br>
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<p>
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I was thinking of the 'normalized' exposure times that have been<br>
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corrected for these hardware issues (not the the physical exposure<br>
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times given in say milliseconds). So a R:G:B of 1:1:1 should leave<br>
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gray gray.<br>
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<p>
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<i>> I'm just curious to understand in what application that division makes sense.</i><br>
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<p>
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The main use is for scanning negatives. For negatives, density of blue<br>
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is ~3x that of red, and that of green is ~2x that of red (hence the<br>
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orange color of the negative mask). So if you scan with 1:1:1 exposure<br>
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time, the green channel will only use 1/2 of it's full range. In other<br>
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words, you have lost one bit of what is basically your luminance. The<br>
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blue channel is filled for only 1/3 and this poor blue definition can<br>
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also be very visible to the human eye.<br>
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<p>
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All this can be solved by scanning with a 1:2:3 exposure so that the<br>
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full range of all channels is utilized.<br>
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<p>
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<pre>
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--
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-- Ewald
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<p>
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<p>
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<p>
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<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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<p>
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<ul>
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<!-- next="start" -->
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<li> <b>Next message:</b> <a href="0215.html">Jens Scheithauer: "scanimage and HP scanners"</a>
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<li> <b>Previous message:</b> <a href="0213.html">Ewald R. de Wit: "Re: SANE & exposure times"</a>
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<li> <b>In reply to:</b> <a href="0211.html">Didier Carlier: "Re: SANE & exposure times"</a>
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<!-- nextthread="start" -->
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<li> <b>Next in thread:</b> <a href="0212.html">Andreas Rick: "Re: SANE & exposure times"</a>
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<!-- reply="end" -->
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</ul>
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