Add descreening mode

[Robert Ancell]

Many, many scans are done from already printed material (regardless if text or picture).
In this case you'll get some ugly rasterized output. Even more ugly: whenever you try to print such a scan you get bad looking moiree patterns.

Therefore every serious scanning programm (naturally not available for linux) has a function called 'descreening'.

Unfortunately there is no easy way for linux which offers 'descreening' (Exactimage - http://www.exactcode.de/site/open_source/exactimage/ - has such a thing on his to-do list).

But there is a resonably good workaround:

1. Scan in double the size you need (e.g. 600DPI instead of 300 DPI, using more than 300DPI for already printed material is absolut overkill and degrates quality instead of making it better).

2. Apply 'gaussian blur'

3. Downscale to 300DPI

4. Apply 'unsharp mask'

Ideally simple-scan (for beeing really simple) would have a profile 'print', which does all these steps in the background without any users interaction needed.