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<TITLE>sane-devel: Re: Calibration algorithms</TITLE>
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<META NAME="Author" CONTENT="Karsten Festag (karsten.festag@t-online.de)">
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<META NAME="Subject" CONTENT="Re: Calibration algorithms">
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<H1>Re: Calibration algorithms</H1>
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<!-- received="Mon May 21 13:54:04 2001" -->
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<!-- name="Karsten Festag" -->
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<!-- subject="Re: Calibration algorithms" -->
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<STRONG>From:</STRONG> Karsten Festag (<A HREF="mailto:karsten.festag@t-online.de?Subject=Re:%20Calibration%20algorithms&In-Reply-To=<3B097311.6CADC379@t-online.de>"><EM>karsten.festag@t-online.de</EM></A>)<BR>
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<STRONG>Date:</STRONG> Mon May 21 2001 - 12:57:05 PDT
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<LI><STRONG>Next message:</STRONG> <A HREF="0282.html">Andy Angrick: "RE: something very bad??"</A>
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<LI><STRONG>Previous message:</STRONG> <A HREF="0280.html">Karsten Festag: "Re: something very bad??"</A>
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<LI><STRONG>In reply to:</STRONG> <A HREF="0261.html">Glenn Ramsey: "Calibration algorithms"</A>
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<P>
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Hi,
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<BR>
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<P>The microtek2 scanners I dealt with in the last time (Phantom 330CX and
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<BR>
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636CX) also do backend calibration. The white stripe inside the cover is
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<BR>
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the white point reference. I'm not sure about the dots but I assume they
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<BR>
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are the reference point for the starting position of the scanhead.
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<BR>
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I made up the backend calibration as follows: (you can download my code
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<BR>
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at <A HREF="http://home.t-online.de/home/karsten.festag">http://home.t-online.de/home/karsten.festag</A>)
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<BR>
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<P>-first do a scan of the white stripe with full optical resolution and
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<BR>
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maximum bit depth whith the lamp switched on, then the same with the
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<BR>
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lamp switched off. Thus you get 2 images with the width of the CCD and
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<BR>
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the height of about 50...80% of the white stripe. Don't use the whole
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<BR>
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stripe because sometimes there are artefacts at the borders of the
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<BR>
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stripe.
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<BR>
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My Phantom 330CX provides a special command 'read shading' which reads
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<BR>
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the black and white shading data at once (function read_cx_shading in my
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<BR>
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backend). The 636CX provides information about the location and size of
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<BR>
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the white stripe via the 'scanner attribute' command and the backend has
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<BR>
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to scan the stripe like a normal image (function read_shading_image).
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<BR>
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<P>-now calculate the average over the height of these images (function
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<BR>
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calc_cx_shading_line for the 330CX and prepare_shading_data for the
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<BR>
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636CX). Now you have one line of white and one line of dark pixels, each
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<BR>
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pixel corresponding to a pixel of the CCD. This data can be stored so
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<BR>
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that the scanner has to read the data not for every scan. I do a new
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<BR>
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calibration at the first scan when the backend was started and when the
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<BR>
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mode was changed (gray/color).
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<BR>
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By the way, if your scanner needs backend calibration, I won't recommend
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<BR>
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using the lineart mode of the scanner. Do a calibration corrected
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<BR>
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grayscale scan and make lineart out of it in the backend. Otherwise you
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<BR>
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get a too white middle and too black sides in your scan.
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<BR>
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<P>-when you perform a scan you will have to know which pixels of the CCD
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<BR>
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are used for scanning because you will have to take the corresponding
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<BR>
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pixels of the shading data for calibration. This is especially essential
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<BR>
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when scanning with low resolution because only a few ccd pixels are used
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<BR>
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in this case.
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<BR>
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The Phantom scanners have a command 'read control bits'. You have to
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<BR>
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perform this before the 'read image' command. You recieve some bytes in
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<BR>
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which each set bit represents a used CCD pixel. For examlple: The 330CX
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<BR>
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is a 300dpi scanner with 2550 pixel. That means you need (2550 pixel)/(8
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<BR>
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bit per byte) = 319 control bytes. I recieve 320 control bytes and have
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<BR>
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some junk at the start and the end. If I scan an image in the middle of
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<BR>
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the scan area with 100dpi the control bits look like:
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<BR>
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...00000000000000000000001001001001001001001001001001001000000000000000000...
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<BR>
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<P>-now you have to create the shading data that suits the scanned image
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<BR>
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(condensed shading data): you have to pick the pixels out of your
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<BR>
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shading data that correspond to a set control bit. (function
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<BR>
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condense_shading)
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<BR>
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<P>-now you can scan the image and take the raw image data from the scanner
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<BR>
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and the condensed shading data and perform the following shading
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<BR>
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correction:
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<BR>
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<P>pixel_to_frontend = ( *from_effective - s_d ) * maxval / ( s_w - s_d );
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<BR>
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<P>(*from_effective contains the raw data, s_d is the dark shading value,
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<BR>
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s_w the white and maxval depends on the bit depth (e.g. 255 for 8bit)
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<BR>
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<P>It's a simple linear interpolation, maybe a nonlinear function suits
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<BR>
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better, but the results I get are pretty good.
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<BR>
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<P>This is done in gray_copy_pixels / segreg_copy_pixels /
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<BR>
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lplconcat_copy_pixels.
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<BR>
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<P>Note that some scanners seem to do only white calibration (no scan with
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<BR>
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dark lamp). In this case s_d is 0.
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<BR>
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<P>Happy coding!
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<BR>
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<P>Karsten
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<BR>
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<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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<LI><STRONG>Next message:</STRONG> <A HREF="0282.html">Andy Angrick: "RE: something very bad??"</A>
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<LI><STRONG>Previous message:</STRONG> <A HREF="0280.html">Karsten Festag: "Re: something very bad??"</A>
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<LI><STRONG>In reply to:</STRONG> <A HREF="0261.html">Glenn Ramsey: "Calibration algorithms"</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>Mon May 21 2001 - 13:57:50 PDT</EM>
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