hey,
profiled denoise is purely driven by numbers.. the input is measured noise
variances and the output is adjusted to minimise some mean square error if
you don't do any other processing. it does that by minimising the risk of
destroying data that was actually present in the input, while trying to
remove as much noise as it dares to. that might not be what you want or
what looks most pleasing to your individual taste. but i would argue it's
still the best starting point for further processing, even if modules you
apply on top will change the perception you have of the noise in the image.
also there are quite a couple of cameras for which the noise model doesn't
really match reality (some of it due to secret in-camera processing, some
inherent to the poissonian/gaussian prior) so there is some sub-optimal
noise removal in darker regions. if you brighten that it'll look even
worse, of course.
doing it the other way around (remove noise at the end, do processing in
between) makes very little sense to me. first off, modules will be driven
by noisy input and produce weird artifacts because of that. secondly, you
don't only scale the values, but you will end up with unpredictable noise
characteristics with interdependencies between pixels and non-trivial
distributions (probably both the natural image prior and the noise model
will be off).
-j.
On Mon, Dec 16, 2013 at 6:30 PM, Rob Z. Smith <[email protected]> wrote:
> Thanks Jo, I wonder if I have misunderstood.
>
>
>
> Say I take an image at 800 ISO and profiled denoise handles it just fine,
> but then I come along and hit it with shadows and highlights or otherwise
> boost shadows by two stops to pull out detail. My eyes now tell me that I
> can see noise in those lighter shadows they couldn’t see before and the
> denoise settings needed to deal with those pushed shadows are sort of
> equivalent to two stops higher at 3200 ISO not 800. Is this my imagination
> or is that how it actually works?
>
>
>
> Rgds,
>
> Rob.
>
>
>
> *From:* johannes hanika [mailto:[email protected]]
> *Sent:* 16 December 2013 15:39
> *To:* Rob Z. Smith
> *Cc:* Halgeir Kjønås Rennehvammen; darktable-devel
>
> *Subject:* Re: [darktable-devel] white dots hight ISO
>
>
>
>
>
>
>
> On Mon, Dec 16, 2013 at 2:03 PM, Rob Z. Smith <[email protected]> wrote:
>
>
>
> While we are on the subject of denoise - if I could choose something for
> my Xmas list it would be for dt to keep track of how much each pixel (or
> area) of the screen has been boosted in brightness by the various modules
> and then offer denoise (or a mask) based on the effective ISO we have
> raised those pixels to.
>
>
> that's why noise reduction is done before all the modules mess with your
> data.
>
> -jo
>
>
>
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