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<!-- received="Fri Nov 26 12:27:48 1999 PST" -->
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<!-- sent="Fri, 26 Nov 1999 21:31:55 +0100" -->
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<!-- name="Ewald R. de Wit" -->
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<!-- email="ewald@pobox.com" -->
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<!-- subject="Re: How do I get a negative orange mask *back*?" -->
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<!-- inreplyto="Pine.SGI.4.02.9911251001560.345662-100000@spinat.rzg.mpg.de" -->
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<title>sane-devel: Re: How do I get a negative orange mask *back*?</title>
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<h1>Re: How do I get a negative orange mask *back*?</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>Fri, 26 Nov 1999 21:31:55 +0100</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#264">[ date ]</a><a href="index.html#264">[ thread ]</a><a href="subject.html#264">[ subject ]</a><a href="author.html#264">[ author ]</a>
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<li> <b>Next message:</b> <a href="0265.html">genkin@canada.com: "Are Mustek "Paragon A3 SP" and "Paragon 1200SP" supported (repost)"</a>
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<li> <b>Previous message:</b> <a href="0263.html">Manfred Muench: "Problems running an HP-scanner..."</a>
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Manuel Panea (<a href="mailto:mpd@rzg.mpg.de">mpd@rzg.mpg.de</a>) wrote:<br>
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<i>> Hi. Here is a question to the experts.</i><br>
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<i>> </i><br>
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<i>> I'm developing the Canon backend. The backend works with some Canon</i><br>
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<i>> flatbed scanner models and with my 35mm-film scanner model 2700F. This</i><br>
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<i>> model can scan negatives and slides.</i><br>
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<i>> </i><br>
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<i>> When scanning negatives, there is a SCSI command to set the scanning</i><br>
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<i>> speed. In this way, dark photos can be scanned at a lower speed, exposing</i><br>
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<i>> the film to more light so that the scanned image looks good. Alas, this</i><br>
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<i>> speed setting command works only for negatives, not for slides (that's</i><br>
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<i>> what the specs say and it's true, I've tried it). Unfortunately, the only</i><br>
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<i>> available and fixed scanning speed for slides is invariably too fast.</i><br>
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<i>> Almost all slides get scanned too dark. What to do?</i><br>
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<p>
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You can multiply the data by a constant and then gamma correct it<br>
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(with gamma ~2.2). This will of course amplify the already abundant<br>
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noise of the scanner too. What it boils down to is that this Canon<br>
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is just not very good for dark slides.<br>
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<p>
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Btw, I remember that the earlier 2700f models had a problem with being<br>
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too dark, if this is the problem perhaps you can return it. If you put<br>
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in an empty slide holder you should get 100% pure white, if not it is<br>
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just plain faulty. <br>
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<p>
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<i>> I thought I could try to outsmart the scanner and tell it to scan a</i><br>
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<i>> negative at a low speed when in fact I am feeding it a slide. Inverting</i><br>
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<i>> the colors is then an easy problem. The difficult problem is that, when</i><br>
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<i>> scanning negatives, the scanner automatically removes the orange color of</i><br>
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<i>> negative films. Slide films do not have this orange color, so if I try</i><br>
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<i>> this trick, the scanner gives me an image where all information about</i><br>
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<i>> orange-like colors is missing.</i><br>
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<p>
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You could get better definition of the blue channel this way but then<br>
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the R and G channel will suffer. The orange mask of negatives absorbes<br>
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a lot of blue and to get a good blue definition your scanner uses a<br>
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longer exposure time. But the green and moreso the red is absorbed<br>
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much less and to avoid overflow of the R and G channel your scanner<br>
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will diminish the R and G signal (for example by dividing). This way<br>
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the full R, G and B bit ranges are utilized without getting overflows.<br>
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<p>
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Now, if you put in a slide, you will get (for example) the full<br>
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range of the B channel, halve the range of the G channel and 1/3 the<br>
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range of the R channel.<br>
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<p>
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<i>> Does anyone know of a good way to get this orange back? Just "adding" it</i><br>
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<i>> with e.g. the Gimp doesn't help, the image still doesn't look good. With</i><br>
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<i>> some other operations, specially "equalize" I can get the image to look</i><br>
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<i>> almost good, but not quite, and worst of all, the histogram is then "full</i><br>
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<i>> of holes".</i><br>
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<p>
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It is not difficult to uncover the mapping that the scanner does. The<br>
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HP Photosmart scanner for example, when scanning negatives, uses a<br>
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1:2:3 ratio for the R:G:B exposure.<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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<!-- 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="0265.html">genkin@canada.com: "Are Mustek "Paragon A3 SP" and "Paragon 1200SP" supported (repost)"</a>
|
||
<li> <b>Previous message:</b> <a href="0263.html">Manfred Muench: "Problems running an HP-scanner..."</a>
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<!-- nextthread="start" -->
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