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 management2. 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.
> tools. Look at the things you're scanning with a magnifying glass and you
> should see many different colors and a halftone screen dot
> I haven't tried this, but maybe fiddling with indexed mode and3. 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.
> limiting the number of colors would help.
> 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.