Re: [hylafax-users] 1D MH v 2D MR/MMR

[Lee Howard <faxguy@xxxxxxxxxxxxxxxx>]

rortgiesen@xxxxxxxxxxxxxxxxxxxxxxx wrote:

> A client of ours is complaining that the reception of our fax is too
> "light", the log shows an excellent protocol exchange (no repeats or
> indications of noise), but their machine asks for 1D MH.  It returns no
> make and/or model ( nsf:Equipment:unknown :Station: )
>
> Would someone briefly explain the difference between these encryption
> methods?
>  

MH, MR, and MMR are all different *compression* methods for 
monochromatic image data.  All of them are lossless.

MH is a 1-D compression meaning that image data is only compressed 
horizontally by "codes" that represent various lengths of black or white 
pixels.  Each scan line is separated from another with a special 
end-of-line (EOL) code.  Any error in one line of data will cause the 
corruption of only that one scanline.

MR is a limited 2-D compression where lines are compressed horizontally 
similar to how MH is, but a small number (usually 4) of scanlines are 
also compressed vertically.  So MR is a bit more complex than is MH.  
Each scan line is also terminated with an EOL code.  Any error in one 
line of data can cause image corruption in the other referenced vertical 
lines, so usually it's limited to 4.

MMR is very similar to MR with the main difference being that the number 
of referenced vertical scanlines is unlimited... in other words, every 
horizontal scanline is compressed horizontally but is also compressed in 
reference to the line above it... all the way down the page, *AND* there 
are no EOL codes used in MMR.  Any error in the data will cause the 
image to be truncated at the point of the error.

So the image data can be converted losslessly between MH, MR, and MMR, 
however, usually MH will require more data for the same image than MR, 
and MR will require more data for the same image than MMR.  Again, there 
will be no visible difference between them.

If your receivers are complaining that the image is too light then it 
must have to do with the imaging process that is done *before* the fax 
is sent... for example the PostScript-to-TIFF conversion.

Lee.


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