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100 wiersze
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<!-- received="Fri Jul 30 03:03:28 1999 PDT" -->
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<!-- sent="Fri, 30 Jul 1999 12:18:01 +0200" -->
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<!-- name="Andreas Rick" -->
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<!-- email="rickand@gemse.fr" -->
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<!-- subject="Re: SANE & exposure times" -->
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<!-- inreplyto="SANE & exposure times" -->
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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>Andreas Rick</b> (<a href="mailto:rickand@gemse.fr"><i>rickand@gemse.fr</i></a>)<br>
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<i>Fri, 30 Jul 1999 12:18:01 +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#221">[ date ]</a><a href="index.html#221">[ thread ]</a><a href="subject.html#221">[ subject ]</a><a href="author.html#221">[ author ]</a>
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<li> <b>Next message:</b> <a href="0222.html">Stephen Williams: "Re: SANE & exposure times"</a>
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<li> <b>Previous message:</b> <a href="0220.html">Didier Carlier: "Re: SANE & exposure times"</a>
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<li> <b>Maybe in reply to:</b> <a href="0208.html">Ewald R. de Wit: "SANE & exposure times"</a>
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<!-- nextthread="start" -->
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<li> <b>Next in thread:</b> <a href="0222.html">Stephen Williams: "Re: SANE & exposure times"</a>
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<li> <b>Reply:</b> <a href="0222.html">Stephen Williams: "Re: SANE & exposure times"</a>
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<li> <b>Reply:</b> <a href="0223.html">Ewald R. de Wit: "Re: SANE & exposure times"</a>
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</ul>
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<!-- body="start" -->
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Didier Carlier wrote:<br>
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<i>> </i><br>
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<i>> <a href="mailto:ewald@pobox.com">ewald@pobox.com</a> said:</i><br>
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<i>> > The main use is for scanning negatives. For negatives, density of blue</i><br>
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<i>> > is ~3x that of red, and that of green is ~2x that of red (hence the</i><br>
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<i>> > orange color of the negative mask). So if you scan with 1:1:1 exposure</i><br>
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<i>> > time, the green channel will only use 1/2 of it's full range. In other</i><br>
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<i>> > words, you have lost one bit of what is basically your luminance. The</i><br>
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<i>> > blue channel is filled for only 1/3 and this poor blue definition can</i><br>
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<i>> > also be very visible to the human eye.</i><br>
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<i>> ></i><br>
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<i>> > All this can be solved by scanning with a 1:2:3 exposure so that the</i><br>
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<i>> > full range of all channels is utilized.</i><br>
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<i>> </i><br>
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<i>> If I look at the RGB values after a scan of a negative, the values of all</i><br>
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<i>> channels go from 0 to 255, so I'm not quite sure of what you are trying to do.</i><br>
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<i>> </i><br>
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<i>> Putting the multiple scan issue aside you can't get more than 8 bits of</i><br>
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<i>> resolution per channel so that you don't loose definition on the blue channel,</i><br>
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<i>> you loose dynamic range.</i><br>
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<i>> I.e. you do not extract the clearer blue details, but you do keep the full 8 bit</i><br>
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<i>> color resolution for the dynamic range that was scanned.</i><br>
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<i>> Am I missing something here ?</i><br>
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<p>
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I think you are right: When scanning negatives with the Coolscan scanners<br>
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the backend tells the scanner it is doing a negatives and the<br>
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scanner adapts its exposure range to the right levels for negatives.<br>
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That's why you don't get the same image when scanning a negative<br>
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using "positive" as a backend-option and inverting the image yourself.<br>
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I havn't tested the quality of this exposure adaptation throughout<br>
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a significant image base of negatives yet, but I didn't have any problems with<br>
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it either.<br>
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Perhabs this is different for the HP-Photosmart?<br>
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<p>
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So without doing multiple scanning there is no way of getting more<br>
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than 8-Bit out of the LS-20, 10 bit out of the LS-30 or 12 bit out of the<br>
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LS-2000 and there doesn't seem to be much neccessity to change<br>
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exposure values to get these 8/10/12 bit.<br>
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Can the other users of these Scanners confirm this hypothesis?<br>
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<p>
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Apart from that I still think it is a neat idea to have a <br>
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calibrated system where you can figure out the optical density<br>
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from the RGB values (but I know it is difficult to achieve).<br>
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When scanning color images, optical density allone is not enough,<br>
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we need to know what color (wavelength) was used to do the<br>
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scanning and what is the detector efficiency as a function of<br>
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wavelength.<br>
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We may need to say what is "standard Red" what is Green and what is Blue<br>
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and try to do a color space conversion from the scanners<br>
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color to the standard space.<br>
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I don't know too much about the subject, but I think there must be<br>
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some standards allready defined that we can use.<br>
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Who does know more about this.<br>
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<p>
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Andreas<br>
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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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<!-- body="end" -->
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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="0222.html">Stephen Williams: "Re: SANE & exposure times"</a>
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<li> <b>Previous message:</b> <a href="0220.html">Didier Carlier: "Re: SANE & exposure times"</a>
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<li> <b>Maybe in reply to:</b> <a href="0208.html">Ewald R. de Wit: "SANE & exposure times"</a>
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<!-- nextthread="start" -->
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<li> <b>Next in thread:</b> <a href="0222.html">Stephen Williams: "Re: SANE & exposure times"</a>
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<li> <b>Reply:</b> <a href="0222.html">Stephen Williams: "Re: SANE & exposure times"</a>
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<li> <b>Reply:</b> <a href="0223.html">Ewald R. de Wit: "Re: SANE & exposure times"</a>
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<!-- reply="end" -->
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</ul>
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