Adams,
Thank you for your detailed explanation. I had basically two batches
of logos, one scanned from a very old scanner and the other from a newer
one. The driver of the old scanner did not have (or maybe I did not notice)
the descreening function so I have to stand that if possible. But the newer
one, I have tried to use a few descreening parameters but it doesn't seem
useful. In fact, I do not even understand the meaning of the parameter
of this function.
For resolution, I have tried 72dpi but seems it do not work. What resolution,
in your experience, should be used?
I am interested in the theory of CYMK. I think if I know the theory
more then I may be able to do something about it. Do you mean that each
perceptual pixel is made of a 2x2 pixels composed of the four components?
Is there any way to tell which 4 pixels should be grouped as one?
Kent.
cate adams wrote:
I don't know if this will help, I am a photoshop
user sharing this address, I've used gimp a bit and find it wonderful.
it's amazing to get free software that seems to do everything photoshop
does and more. i haven't scanned in gimp.
I do what you are trying to do regularly in photoshop (so maybe
in gimp or other scanning software) when you scan you can choose to descreen
the image as you scan. you choose between newspaper, magazine and art magazine
etc. depending on how course the printed screen is.
You are getting this "dithering, anti-aliasing and "texture"
of publication paper" effect because when the image is printed in a
magazine the colour is made up of a fine dot screen of cyan, magenta, yellow
and black, (cmyk) to make up all the colours you see, so even if they appear
flat they generally aren't. for example fire engine red is 100%magenta
and 100%yellow, green is percentages of cyan and yellow and so on.
Your scanner is picking up each little dot so you get a pixel of
cyan and a pixel of yellow and it dithers it to try to get the inbetween
colours.
I have found descreening the best way, i used to try it with filters
like you are, getting pretty bad results as you are. If you can't do that
you could try scanning on a much lower resolution so the scanner can't
see the dots. It might sound funny but it works. You might sharpen the
edges later if you need to though as you've discovered you have to be careful
of getting a halo.
cheers cate
From: "Cheung Koon Tung, Kent" [EMAIL PROTECTED]>
Organization: City University of Hong Kong
Date: Sat, 03 Feb 2001 11:56:00 +0800
To: GIMP [EMAIL PROTECTED]>
Subject: Re: Scanned Image Processing
Hi all,
Thank you for all the nice guys who have given me input. Here are my
experiences about your suggestions:
> Perhaps doing Posterization (Image/Color/posterize)
1. It is hard to determine the paramter of posterization. Setting it too
high is not useful. Unfortunately, setting is low will cause some pixels
to vanish. Besides, some noise pixels with very different colours from
the perceived one will be produced.
> Check out Image Magick, it has some nifty color management
> tools. Look at the things you're scanning with a magnifying
glass and you
> should see many different colors and a halftone screen dot
> pattern.
2. ImageMagick is a very good program that allows infinite programability
that provides more or less the same as GIMP or other image processing applications.
If I had found the way to process my images in general, I will be able
to write a script for GIMP or ImageMagick. Unfortnately, I am still searching
for the method.
> I haven't tried this, but maybe fiddling with indexed mode
and
> limiting the number of colors would help.
3. If I index the colors with the standard palette, then some of the original
colors will be lost. However, creating palettes for all images I am going
to process is a nightmare.
> You might want to try the non-linear filter (nlfilt).
4. I think it doesn't help much. I don't even notice the changes to
the image! Of course, I have changed the parameters but not any one set
of parameters are satsifactory.
5. Is there any standard routine to remove the three artifacts I mentioned,
namely, dithering, anti-aliasing and "texture" of publication paper? Since
these are produced by standard routines, I expect that there might be standard
methods to undo the effects.
6. I have another thought: is it possible to let the user to specify
a small number of colors that the image should have. Then, program GIMP
to change the pixels which don't have these colors to its correct colour.
The interface is certainly possible using Gimp-Perl. However, the way to
determine the correct color is difficult to determine. Any idea?
Thank you for all your kind help.
Kent.
