[sane-devel] Scanning for fax - resolution problem

[Frank Millman]

Frank Millman wrote: 
> 
> Hi all
> 
> I have got a scanner and fax modem working perfectly on Windows. I am 
> trying to move them over to a new Fedora 10 server, using sane 
> (scanimage version
> 1.0.19) and hylafax+ (version 5.2.8).
> 
> I am battling with getting a suitable tiff file for faxing.
> 

Thanks to all for the replies. I will save some bandwidth and respond to
them all here ...

[allan]

> I dont think 'resolution' is the word you are looking for here, perhaps
quality? can you describe the problem a bit more?

A block of solid blue comes out as a patterned block of black and white
dots. A solid line comes out as a dotted line.

> i think your best bet is to scan at 200 dpi (or 204 if your scanner will
do it), and then pass the file thru something like convert to crop or pad it
to the right width instead of scaling it.

This agrees with the advice I got from the hylafax list. Unfortunately I
found myself out of my depth with all the options in ImageMagick. However, I
did get some other suggestions which seem promising - see below.

[Giuseppe]

> Is your fax really able to transmit grey images?

No, hylafax converts the image to monochrome first. Apparently it uses
Ghostscript to do this, and this is where the loss in quality comes from.

> I suggest you to scan in black and white and/or to increase dpi. 
> (Increased dpi will probably help when hylafax will convert your image 
> to black and white.)

I tried scanning in black and white (--mode=LineArt), but it gives a poor
result. Most of the light-blue lines disappear altogether. Maybe a better
scanner would give a higher quality, I don't know.

Increasing dpi did not help, I am afraid.

[Jeremy]

> Have you tried scanning your document using my "bscan" bash script?

Thanks very much for this, Jeremy, it looks interesting. Unfortunately I
could not get it to run - it complained about not finding 'gamma4scanimage'.
As I got some promising suggestions from the hylafax list I did not pursue
it, but I do appreciate your assistance.


I got this suggestion from Lee Howard on hylafax -

> Because you're using HylaFAX+ you have certain script features 
> available already.  You can improve the dithering behaviors greatly by 
> putting this into
> /var/spool/hylafax/etc/FaxModify:
> 
> DITHERING=libtiff-fs

I don't understand it, but it works. The downside is that it produces an
output file that is more than double the size, and therefore take more than
twice as long to transmit. I am following up one other possibility, but if
that does not prove worthwhile, I will stick with this suggestion.

I must say that there is a lot more to scanning and faxing than I had
realised. I am used to using Windows, where you install the driver and it
just works. Nevertheless, it is good to know that there are so many people
out there willing to help. I shall persevere until I get it working
properly.

Many thanks to all.

Frank