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129 wiersze
6.8 KiB
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<!-- received="Sat Jun 20 11:45:56 1998 PDT" -->
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<!-- sent="Sat, 20 Jun 1998 11:45:08 -0700 (PDT)" -->
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<!-- name="Zach Fine" -->
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<!-- email="czyz@u.washington.edu" -->
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<!-- subject="Re: UMax 1220S" -->
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<!-- id="199806201845.LAA32094@saul9.u.washington.edu" -->
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<!-- inreplyto="UMax 1220S" -->
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<title>sane-devel: Re: UMax 1220S</title>
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<h1>Re: UMax 1220S</h1>
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<b>Zach Fine</b> (<a href="mailto:czyz@u.washington.edu"><i>czyz@u.washington.edu</i></a>)<br>
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<i>Sat, 20 Jun 1998 11:45:08 -0700 (PDT)</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#173">[ date ]</a><a href="index.html#173">[ thread ]</a><a href="subject.html#173">[ subject ]</a><a href="author.html#173">[ author ]</a>
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<!-- next="start" -->
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<li> <b>Next message:</b> <a href="0174.html">Nicolas Lucas de Peslouan: "Re: UMax 1220S"</a>
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<li> <b>Previous message:</b> <a href="0172.html">Oliver Rauch: "Re: UMax 1220S"</a>
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<li> <b>Maybe in reply to:</b> <a href="0171.html">Jim Ford: "UMax 1220S"</a>
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<!-- nextthread="start" -->
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<li> <b>Next in thread:</b> <a href="0176.html">becka@rz.uni-duesseldorf.de: "Re: UMax 1220S"</a>
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<li> <b>Next in thread:</b> <a href="0174.html">Nicolas Lucas de Peslouan: "Re: UMax 1220S"</a>
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<li> <b>Reply:</b> <a href="0176.html">becka@rz.uni-duesseldorf.de: "Re: UMax 1220S"</a>
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<!-- reply="end" -->
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</ul>
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<!-- body="start" -->
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<i>>>>>> "Jim" == Jim Ford <<a href="mailto:jf001@watford.net">jf001@watford.net</a>> writes:</i><br>
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<p>
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Jim> This is to report that the Umax Astra 1220S - the latest in<br>
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Jim> the 1200 series - appears to work fine with SANE, with no<br>
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Jim> nasty noises from the mechanism! The resolution on xscanimage<br>
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Jim> only goes up to 600dpi though (but see below)<br>
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...snip...<br>
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Jim> Why does my scanner provide resolutions up to 9600 dpi when<br>
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Jim> the default resolution for photos seems to be 100 and OCR<br>
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Jim> 300? What use is 9600 - scanning postage stamps?<br>
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<p>
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FWIW, most consumer level scanners on the market scan with a maximum<br>
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optical resolution of 300-600dpi. Many scanners are marketed with<br>
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such claims as "scans at up to 9600dpi (interpolated)". This is a<br>
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joke, false advertising, and ought to be illegal. <br>
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<p>
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Interpolation is a process of creating a new pixel based on the pixels<br>
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surrounding it. Interpolation can look ok at low levels, but it is<br>
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not adding any real, captured information to your scanned image. I'm<br>
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always surprised that the marketing folk stop at 9600dpi, they might<br>
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as well advertise "scans at 1billion dpi (interpolated)", since<br>
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besides disk and memory space, there's no limit to how many<br>
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interpolated pixels can be added to a scan.<br>
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<p>
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As for the proper resolution for scanning, the probably reason for<br>
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those defaults is that when printing with a laser printer, one must<br>
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use halftones to produce levels of gray. A halftone consists of a<br>
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bunch of dots of varying size, placed along lines which are often at a<br>
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45 degree angle. The fineness of a halftone screen is measured in<br>
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lines-per-inch (lpi). A newspaper might print at 85lpi, a magazine at<br>
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above 105lpi. Printing in color gets a bit more complicated, let's<br>
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not go into it.<br>
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<p>
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The lines-per-inch of a halftone and the dots-per-inch of the printer<br>
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that produce it are not directly related. A 1200dpi printer could<br>
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print at 15lpi, 50lpi, 80lpi, 105lpi, or any other number under<br>
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600lpi. As you increase the halftone screen, the resolution and<br>
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fineness of the printed photo increases, but at the same time the<br>
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number of levels of gray goes down, leading to photo reproduction that<br>
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looks rather xerox like. <br>
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<p>
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Theoretically, a 300dpi laser printer could print at a resolution of<br>
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150lpi, as you could print that many distinct lines of single pixel<br>
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dots. However, the dots along these lines would not be able to vary in<br>
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size without running into one another and becoming black, or<br>
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disappearing altogether and becoming white. If you were to print at a<br>
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lower lpi, say 55, the halftone screen would be clearly visible, yet<br>
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many levels of gray could be reproduced. I used to have a formula for<br>
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calculating this lying around. Suffice it to say that 55lpi is a<br>
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pretty optimal halftone screen for a 300dpi laser printer, although<br>
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it's a bit low for many folks' tastes.<br>
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<p>
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And if you know what halftone screen you're printing at, you can then<br>
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calculate the resolution at which you should scan your photos. I<br>
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don't remember the exact formula for this either, but if you print at<br>
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65lpi like my college paper used to, your photos should be scanned to<br>
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be at least 135dpi at the size at which they're finally printed<br>
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(i.e. if you scan at 135dpi and then enlarge the photo to fill a whole<br>
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page, it's no longer 135dpi). We used to scan at 150dpi so that we'd<br>
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have a little room to play with.<br>
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<p>
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But the basic idea is that the halftone screen limits the amount of<br>
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detail that will be visible, so scanning at higher resolutions is<br>
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unnecessary.<br>
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<p>
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I'd bet that the 100dpi default is with laser printed halftoning in<br>
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mind, probably at 40-55lpi. If you're scanning photos to print on one<br>
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of those super-duper epson stylus printers or some other high-res<br>
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inkjet, you might want to scan quite a bit higher. 300dpi ought to be<br>
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high enough for just about anything, unless you're printing a<br>
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magazine.<br>
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<p>
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72dpi is the standard computer monitor resolution, should you be<br>
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scanning for the web.<br>
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<p>
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Suffice it to say that you'll probably never need to scan at higher<br>
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than 300dpi, unless you're trying to magnify a detail. And scanning at<br>
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resolutions beyond 600dpi will probably not capture any more detail<br>
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than a 600dpi scan would, it'll probably just create a whole bunch<br>
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through interpolation and make your filesizes larger. Unless of<br>
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course the Astra 1220S has 1200dpi optical resolution.<br>
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<p>
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-Zach Fine<br>
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<a href="mailto:czyz@u.washington.edu">czyz@u.washington.edu</a><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="0174.html">Nicolas Lucas de Peslouan: "Re: UMax 1220S"</a>
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<li> <b>Previous message:</b> <a href="0172.html">Oliver Rauch: "Re: UMax 1220S"</a>
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<li> <b>Maybe in reply to:</b> <a href="0171.html">Jim Ford: "UMax 1220S"</a>
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<!-- nextthread="start" -->
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<li> <b>Next in thread:</b> <a href="0176.html">becka@rz.uni-duesseldorf.de: "Re: UMax 1220S"</a>
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<li> <b>Next in thread:</b> <a href="0174.html">Nicolas Lucas de Peslouan: "Re: UMax 1220S"</a>
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<li> <b>Reply:</b> <a href="0176.html">becka@rz.uni-duesseldorf.de: "Re: UMax 1220S"</a>
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<!-- reply="end" -->
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</ul>
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