Hi,
The problem of a 2-passes denoising method involving 2 differents
algorithms, the later applied where the former failed, could be the
grain structure (the shape of the noise) would be different along the
picture, thus very unpleasing.
I thought maybe we could instead create some sort of total variation
threshold on other denoising modules :
1. compute the total variation of each channel of each pixel as the
divergence divided by the L1 norm of the gradient - we then obtain a
"heatmap" of the gradients over the picture (contours and noise)
2. let the user define a total variation threshold and form a mask
where the weights above the threshold are the total variation and
the weights below the threshold are zeros (sort of a highpass filter
actually)
3. apply the bilateral filter according to this mask.
This way, if the user wants to stack several denoising modules, he could
protect the already-cleaned areas from further denoising.
What do you think ?
Aurélien.
Le 13/06/2018 à 03:16, rawfiner a écrit :
> Hi,
>
> I don't have the feeling that increasing K is the best way to improve
> noise reduction anymore.
> I will upload the raw next week (if I don't forget to), as I am not at
> home this week.
> My feeling is that doing non local means on raw data gives much bigger
> improvement than that.
> I still have to work on it yet.
> I am currently testing some raw downsizing ideas to allow a fast
> execution of the algorithm.
>
> Apart of that, I also think that to improve noise reduction such as
> the denoise profile in nlm mode and the denoise non local means, we
> could do a 2 passes algorithm, with non local means applied first, and
> then a bilateral filter (or median filter or something else) applied
> only on pixels where non local means failed to find suitable patches
> (i.e. pixels where the sum of weights was close to 0).
> The user would have a slider to adjust this setting.
> I think that it would make easier to have a "uniform" output (i.e. an
> output where noise has been reduced quite uniformly)
> I have not tested this idea yet.
>
> Cheers,
> rawfiner
>
> Le lundi 11 juin 2018, johannes hanika <hana...@gmail.com
> <mailto:hana...@gmail.com>> a écrit :
>
> hi,
>
> i was playing with noise reduction presets again and tried the large
> neighbourhood search window. on my shots i could very rarely spot a
> difference at all increasing K above 7, and even less so going above
> 10. the image you posted earlier did show quite a substantial
> improvement however. i was wondering whether you'd be able to share
> the image so i can evaluate on it? maybe i just haven't found the
> right test image yet, or maybe it's camera dependent?
>
> (and yes, automatic and adaptive would be better but if we can ship a
> simple slider that can improve matters, maybe we should)
>
> cheers,
> jo
>
>
>
> On Mon, Jan 29, 2018 at 2:05 AM, rawfiner <rawfi...@gmail.com
> <mailto:rawfi...@gmail.com>> wrote:
> > Hi
> >
> > Yes, the patch size is set to 1 from the GUI, so it is not a
> bilateral
> > filter, and I guess it corresponds to a patch window size of 3x3
> in the
> > code.
> > The runtime difference is near the expected quadratic slowdown:
> > 1,460 secs (8,379 CPU) for 7 and 12,794 secs (85,972 CPU) for
> 25, which
> > means about 10.26x slowdown
> >
> > If you want to make your mind on it, I have pushed a branch here
> that
> > integrates the K parameter in the GUI:
> > https://github.com/rawfiner/darktable.git
> <https://github.com/rawfiner/darktable.git>
> > The branch is denoise-profile-GUI-K
> >
> > I think that it may be worth to see if an automated approach for
> the choice
> > of K may work, in order not to integrate the parameter in the GUI.
> > I may try to implement the approach of Kervann and Boulanger
> (the reference
> > from the darktable blog post) to see how it performs.
> >
> > cheers,
> > rawfiner
> >
> >
> > 2018-01-27 13:50 GMT+01:00 johannes hanika <hana...@gmail.com
> <mailto:hana...@gmail.com>>:
> >>
> >> heya,
> >>
> >> thanks for the reference! interesting interpretation how the
> blotches
> >> form. not sure i'm entirely convinced by that argument.
> >> your image does look convincing though. let me get this right.. you
> >> ran with radius 1 which means patch window size 3x3? not 1x1 which
> >> would be a bilateral filter effectively?
> >>
> >> also what was the run time difference? is it near the expected
> >> quadratic slowdown from 7 (i.e. 15x15) to 25 (51x51) so about
> 11.56x
> >> slower with the large window size? (test with darktable -d perf)
> >>
> >> since nlmeans isn't the fastest thing, even with this coalesced
> way of
> >> implementing it, we should certainly keep an eye on this.
> >>
> >> that being said if we can often times get much better results we
> >> should totally expose this in the gui, maybe with a big warning
> that
> >> it really severely impacts speed.
> >>
> >> cheers,
> >> jo
> >>
> >> On Sat, Jan 27, 2018 at 7:34 AM, rawfiner <rawfi...@gmail.com
> <mailto:rawfi...@gmail.com>> wrote:
> >> > Thank you for your answer
> >> > I perfectly agree with the fact that the GUI should not become
> >> > overcomplicated.
> >> >
> >> > As far as I understand, the pixels within a small zone may
> suffer from
> >> > correlated noise, and there is a risk of noise to noise matching.
> >> > That's why this paper suggest not to take pixels that are too
> close to
> >> > the
> >> > zone we are correcting, but to take them a little farther
> (see the
> >> > caption
> >> > of Figure 2 for a quick explaination):
> >> >
> >> >
> >> >
>
> https://pdfs.semanticscholar.org/c458/71830cf535ebe6c2b7656f6a205033761fc0.pdf
>
> <https://pdfs.semanticscholar.org/c458/71830cf535ebe6c2b7656f6a205033761fc0.pdf>
> >> > (in case you ask, unfortunately there is a patent associated
> with this
> >> > approach, so we cannot implement it)
> >> >
> >> > Increasing the neighborhood parameter results in having
> proportionally
> >> > less
> >> > problem of correlation between surrounding pixels, and
> decreases the
> >> > size of
> >> > the visible spots.
> >> > See for example the two attached pictures: one with size 1,
> force 1, and
> >> > K 7
> >> > and the other with size 1, force 1, and K 25.
> >> >
> >> > I think that the best would probably be to adapt K
> automatically, in
> >> > order
> >> > not to affect the GUI, and as we may have different levels of
> noise in
> >> > different parts of an image.
> >> > In this post
> >> >
> (https://www.darktable.org/2012/12/profiling-sensor-and-photon-noise/
> <https://www.darktable.org/2012/12/profiling-sensor-and-photon-noise/>),
> >> > this
> >> > paper is cited:
> >> >
> >> > [4] charles kervrann and jerome boulanger: optimal spatial
> adaptation
> >> > for
> >> > patch-based image denoising. ieee trans. image process. vol.
> 15, no. 10,
> >> > 2006
> >> >
> >> > As far as I understand, it gives a way to choose an adaptated
> window
> >> > size
> >> > for each pixel, but I don't see in the code anything related
> to that
> >> >
> >> > Maybe is this paper related to the TODOs in the code ?
> >> >
> >> > Was it planned to implement such a variable window approach ?
> >> >
> >> > Or if it is already implemented, could you point me where ?
> >> >
> >> > Thank you
> >> >
> >> > rawfiner
> >> >
> >> >
> >> >
> >> >
> >> > 2018-01-26 9:05 GMT+01:00 johannes hanika <hana...@gmail.com
> <mailto:hana...@gmail.com>>:
> >> >>
> >> >> hi,
> >> >>
> >> >> if you want, absolutely do play around with K. in my tests
> it did not
> >> >> lead to any better denoising. to my surprise a larger K
> often led to
> >> >> worse results (for some reason often the relevance of discovered
> >> >> patches decreases with distance from the current point).
> that's why K
> >> >> is not exposed in the gui, no need for another irrelevant
> and cryptic
> >> >> parameter. if you find a compelling case where this indeed
> leads to
> >> >> better denoising we could rethink that.
> >> >>
> >> >> in general NLM is a 0-th order denoising scheme, meaning the
> prior is
> >> >> piecewise constant (you claim the pixels you find are trying to
> >> >> express /the same/ mean, so you average them). if you let that
> >> >> algorithm do what it would really like to, it'll create
> unpleasant
> >> >> blotches of constant areas. so for best results we need to
> tone it
> >> >> down one way or another.
> >> >>
> >> >> cheers,
> >> >> jo
> >> >>
> >> >>
> >> >>
> >> >> On Fri, Jan 26, 2018 at 7:36 AM, rawfiner
> <rawfi...@gmail.com <mailto:rawfi...@gmail.com>> wrote:
> >> >> > Hi
> >> >> >
> >> >> > I am surprised to see that we cannot control the neighborhood
> >> >> > parameter
> >> >> > for
> >> >> > the NLM algorithm (neither for the denoise non local mean,
> nor for
> >> >> > the
> >> >> > denoise profiled) from the GUI.
> >> >> > I see in the code (denoiseprofile.c) this TODO that I don't
> >> >> > understand:
> >> >> > "//
> >> >> > TODO: fixed K to use adaptive size trading variance and bias!"
> >> >> > And just some lines after that: "// TODO: adaptive K tests
> here!"
> >> >> > (K is the neighborhood parameter of the NLM algorithm).
> >> >> >
> >> >> > In practice, I think that being able to change the
> neighborhood
> >> >> > parameter
> >> >> > allows to have a better noise reduction for one image.
> >> >> > For example, choosing a bigger K allows to reduce the
> spotted aspect
> >> >> > that
> >> >> > one can get on high ISO images.
> >> >> >
> >> >> > Of course, increasing K increase computational time, but I
> think we
> >> >> > could
> >> >> > find an acceptable range that would still be useful.
> >> >> >
> >> >> >
> >> >> > Is there any reason for not letting the user control the
> neighborhood
> >> >> > parameter in the GUI ?
> >> >> > Also, do you understand the TODOs ?
> >> >> > I feel that we would probably get better denoising by
> fixing these,
> >> >> > but
> >> >> > I
> >> >> > don't understand them.
> >> >> >
> >> >> > I can spend some time on these TODOs, or to add the K
> parameter to
> >> >> > the
> >> >> > interface if you think it is worth it (I think so but it
> is only my
> >> >> > personal
> >> >> > opinion), but I have to understand what the TODOs mean before
> >> >> >
> >> >> > Thank you for your help
> >> >> >
> >> >> > rawfiner
> >> >> >
> >> >> >
> >> >> >
> >> >> >
>
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