Challenge: Reverse Engineering Standard Filters

["MacBeth, Bill" <bill.macbeth@compaq.com>]

All,
If any of you who wish to develop code for "standard" filters that ship with
Photoshop, PSP, etc. want a real challenge, try to duplicate the Photoshop
Despeckle filter.  It must be simple, because it executes extremely fast.
It does not appear to be a variant of "blur" (there are many pixels that do
not get changed at all), nor does it appear to be a median cut since it
executes too fast.  I have programmed about forty variants (really!) over
the years, and never hit it yet.  Several of you have been so kind as to
forward suggestions in the past, but none of those withstood the test
either.  There must be a technique in there that we are all missing .....

Regards, Bill