Are the Conversion: header lines enough for you?
No. The binary (with/without) granularity isn't fine enough. And
what I think I learned back when I took may last course in
communications and information theory over a quarter century ago,
"without loss" is a really interesting philosophical concept, but
impractical in practice: something about the assurance of shared
semantics... :-)
Suppose you had a
bitmap of some Known and Defined format in your message and the
destination was delivering messages to a fax machine.
...
This is actually a perfect example. In the general case, I don't know
about the delivery device. If I start with a character stream message,
one can talk about probable percentage distortion loss models which are,
in general, pretty complicated--in part because, if the delivery
translator prints VERY LARGE on the fax machine, 100x200 may be plenty,
but, if it decides to print in 6 or 8 point, there may be big trouble.
Moreover, in deciding the minimum acceptable size to print, a system
that "knows" it is transmitting only upper-case ASCII can print lots
smaller than the same system printing from a repertoire of a few
thousand Kanji with equivalent levels of information loss (discuss this
problem with almost any producer of OCR software).
Even if I start with a bitmapped image at, say, 400x400 dpi and a
simple graphic case, I may want/need to say "sampling and smoothing this
down to 200x200 is ok, but don't go below that, and if you are going to
sample and not smooth, the limit is 300x300".
Those types of statements are reasonable, but a bit more complex than
"allowed/prohibited".
That said, this is, again, an argument for keeping the MTAs out of the
business, not an argument for giving up entirely. If the dialogue
between the final delivery MTA and the receiver UA can logically contain
"hey, I've got this 400x400 dpi image for you, what would you like to do
with it?", then answers like "wait until I find a high-resolution
workstation", "go ahead and display it on this fuzzy fax and I'd see if
I can read it, but save the 400x400 form in case I need to make a better
plan", or "print it on the fax machine, but do every page as four so
none of the dots get lost" all become options. If the sending UA (or
the user driving it) wants to include headers that say "hey recipient,
don't even think about reading this until you can do so at 400x400 or
above", I think it is a dandy idea to have a good mechanism for passing
that along, but I don't want any intermediate MTA making decisions on
that basis.
--john
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