TELECOM Digest OnLine - Sorted: Re: Format For Image to be Faxed?

Re: Format For Image to be Faxed?

Robert Bonomi (
Tue, 01 Feb 2005 18:11:36 -0000

In article <>, AES
<> wrote:

> Pardon me if this is a bit off topic, but I suspect someone on one or
> the other of these groups may have the answer immediately at hand:

> If I want to fax a B&W or, more likely, a gray-scale image from within
> my computer (Apple iBook, using the internal modem), into what format
> (JPEG? GIF? ???) should I convert the image and at what resolution (72
> dpi?) before I fax it, to get the best balance of image sharpness and
> transmission time?

Standard FAX B&W transmission is 192 DPI _across_ the scan-lines, and
either 96 scan-lines/inch ('standard' resolution), or 192
scan-lines/inch ("fine" resolution)

Your "initial" file format doesn't matter a whole lot -- the fax
transmission software is going to _effectively_ 'render' that format
to a bitmap image in memory and construct the needed data strings in
accordance with the FAX G3 standards.

The fax encoding specification uses 'RLE' compression, so minimizing
the number of intensity changes on each scan-line results in
significantly decreased transmission time.

For the source file format, "what works best" is going to be _very_
dependant on the characteristics of the specific image involved. A
'lossy' compression scheme, like JPEG, may introduce 'artificats' that
end up _increasing_ the size of the _transmitted_ data-stream, even
though the 'source *file*' size is smaller. Considerable
experimentation -- with the type of images you plan to send -- is

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