[sane-devel] Scanning for fax - resolution problem

[m. allan noah]

On Sun, Jan 11, 2009 at 9:19 AM, Frank Millman <frank at chagford.com> wrote:
> 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.

You should see if the scanner has brightness/contrast/threshold
options. If so, they can be used to adjust the quality of the gray to
lineart transition. But, scanning in gray, and letting libtiff dither
it might be better.

> 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.

It is apparently changing the dithering algo. If you dont dither, then
sections of gray will come out either solid white or solid black
(based on being above or below a static threshold). With good
dithering, you can output the right ratio of black/white pixels to
make a section of the image look like the correct gray from a
distance.

 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.

Uhm, you can't have it both ways- If you want more data in the file,
it is gonna get bigger. This is particularly true with G3/G4 fax,
which use a kind of run-length encoding. Dithering tends to counteract
the compression. But, fax was really designed to send B/W text, not
grayscale images.

In fact, that begs the question- why the heck are you still sending faxes :)

allan
-- 
"The truth is an offense, but not a sin"