arnaud@calvo-france.com said:
> Please have a look at www.calvo-france.com/cacharel.jpg and tell me :
> - why do I obtain these dark spots (inside the red ellipse) when
> scanning
That Moire is caused by the interaction of the half-tone pattern in the
document and the sampling of either the scanner or the scaler. The frequency
of the pattern is the difference between the frequency of the halftone
on the page and the sample frequency of the scaled video.
The 45deg slope of the pattern comes from the 45deg slope of the half-tone
on the printed page. Halftones are done at an angle because that makes
the pattern less visible to the eye--an accident of perception that is used
to advantage. (Actually, color book printing tends to use at least 4 colors,
and each color's halftone mask is at a different angle.)
I tend to first blame the scanner. Try asking for a different resolution.
Scalers in low- and mid- range scanners tend to be pretty horrid, as in
they just subsample/replicate and this is what causes the moire, typically.
If you know the natural resolution of the scanner, try to use that.
Also, when viewing the image in gimp, if you are suspicious, make sure you
zoom in to 1:1 or better to examine the moire to make sure the viewer is not
causing it. (Gimp is usually pretty good.)
Finally, if it persists, try various filters in gimp. A carefully constructed
convolution matrix might do the trick.
The best hope, though, is to find the natural resolution of the scanner,
and scan at that resolution. The CCD sensors naturally interpolate a bit,
so the moire won't be present if you can get the unscaled video. Then use
gimp to do your scaling. You can even do a blur then sharpen to maybe get
rid of some of the halftone. Experiment.
-- Steve Williams "The woods are lovely, dark and deep. steve@icarus.com But I have promises to keep, steve@picturel.com and lines to code before I sleep, http://www.picturel.com And lines to code before I sleep."-- Source code, list archive, and docs: http://www.mostang.com/sane/ To unsubscribe: echo unsubscribe sane-devel | mail majordomo@mostang.com
This archive was generated by hypermail 2b29 : Mon Sep 04 2000 - 13:08:01 PDT