Sometimes there are problems with a particular channel.
I have had this kind of problem also with the blue channel (not the sky= in
some specific conditions where there are few data on the blue channel and
the white balance / colour balance tends to lower it even more.
In this case, you can ends up with not enough data to overides the noise.

I have had to use a TC correction filter to get more data on this channel
and then to be able to process it.

2018-03-10 16:56 GMT+01:00 Robert Krawitz <r...@alum.mit.edu>:

> On Sat, 10 Mar 2018 16:44:24 +0100, David Vincent-Jones wrote:
> > Your examples interest me since they are shown on a blue subject. My
> > experience with a fully profiled sensor is that the 'basic' Denoise
> > (profiled) works quite well by itself EXCEPT for blue sky areas.
> >
> > My sky areas tend to form into rosette clumps whenever some reasonable
> > degree of processing is applied. It has led me to believe that it is the
> > blue color that is causing the greatest problem.
>
> That's not my experience -- I'm seeing clumps like this in plenty of
> other places.
> --
> Robert Krawitz                                     <r...@alum.mit.edu>
>
> ***  MIT Engineers   A Proud Tradition   http://mitathletics.com  ***
> Member of the League for Programming Freedom  --  http://ProgFree.org
> Project lead for Gutenprint   --    http://gimp-print.sourceforge.net
>
> "Linux doesn't dictate how I work, I dictate how Linux works."
> --Eric Crampton
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