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Re: Masking



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On Wed, 27 Oct 1999, Fred. Vellieux wrote:
> David J. Schuller wrote:
> > there is no such thing as maskless averaging.
> 
> I do not agree with this statement. A properly conceived density averaging
> program will offer this possibility.
[followed by a lengthy discussion of auto-masking and proper vs. improper
NCS]

allow me to repeat my statement, in context:
> there is no such thing as maskless averaging. averaging always uses a
> mask, whether you specify it yourself or let some program calculate it
> for you.

i believe what you describe is the generation of an averaging mask within
the averaging program.  to me, that would be a perfect example of "letting
some program calculate a mask for you".  it is only by masking out the
context that you were able to isolate something with which you could
disagree.

as for the separate question of whether a "properly conceived density
averaging program" will do auto-masking, i happen to disagree with you.
this is a result of my experience both with structures having
improper NCS and with structures requiring multi-domain averaging,
another situation for which auto-masking is inappropriate.
auto-masking may be a convenience for simple cases of NCS, but its absence
would in no way make an averaging program "improper" or incorrect.

cheers,

=======================================================================
     Crystals are pretty, but they just kind of sit there,
         not doing much of anything.  - Larry Wall
=======================================================================
                        David J. Schuller
                        modern man in a post-modern world
                        University of California-Irvine
                        schuller@uci.edu