> I'm using Darktable for a while now, believing that it is indeed a
> great program with an amazing set of features.
>
> But lately it has come to my attention that the handling of highlights
> in not on par with other programs, like shown in this example:
>
> http://abload.de/img/dsc_1284dt_1a8ufz.jpg
>
> Result with Rawtherapee, same was possible with Lightroom:
>
> http://abload.de/img/dsc_1284rt_10xunr.jpg
>
> If you look at the sky there is clearly a difference in quality.
>
> The RAW file can be found here:
>
> https://anonfiles.com/file/48a69ae002f8466bb1ca9f19a29fc85d
>
> Is there some trick I am missing (I tried every curve I found) or just
> room for some further improvement of Darktable?
>
> I think that's quite an important topic because one of the main
> reasons to use RAW is the somewhat larger dynamic range that can be
> recovered - at least if the used tool supports that properly.
It's important to understand that there are two different sorts of
'highlight recovery' in RAW files versus JPEGs. The easy case is where
the data is present in the RAW file but not in the JPEG because the RAW
file has more dynamic range than the JPEG. The second case is where the
RAW file itself has partial or complete clipping of some of the colour
channels. The first case can be recovered simply by changing curves or
exposure or the like to pull down the highlights so they fit into the
dynamic range of the JPEG. The second case is much more difficult and
you can't really 'recover' detail and colour so much as guess what it
should be and try to recreate it.
Your RAW file here has two patches of the second highlight issue. You
can see this by opening it up, selecting a point in the history stack
below the base curve, and turning on highlight/shadow warning; areas
that show up have at least one clipped RAW colour channel[*]. At this
point we're outside the realm of 'highlight recovery' and into the realm
of 'highlight reconstruction'.
Darktable used to be somewhere between so-so and 'I'm not even going
to really try' for highlight reconstruction, giving the really ugly
results that your darktable version shows (look not at the totally
whited out area but at the colour artifacts in the center top). The
current development version of darktable has much better support for
this and can usually do better, but you have to do it manually. The
crucial step is to go to the 'highlight reconstruction' module and
switch it to 'reconstruct color' as the method.
With the highlight colours hopefully reconstructed decently you can
then switch to all of the usual tools to recover highlights. I tend
to switch the base curve to linear, apply some sort of tone curve to
recover as much contrast as doesn't blow the highlights again, often
knock down the sky with graduated density, and fiddle around with
exposure and maybe the zone system.
To show what's possible in the current development version with
some quick work, here's what I did from your RAW:
http://www.cs.toronto.edu/~cks/tmp/darktable/DSC_1284-dt.jpg
http://www.cs.toronto.edu/~cks/tmp/darktable/DSC_1284.NEF.xmp
(this is a version of Darktable built from git tip a couple of days
ago. Results may vary slightly since then.)
In general, because reconstructing highlights in the face of clipped
RAW channels is really a set of heuristics and guesses (since the actual
data simply isn't there) there's always going to be variations between
programs and room for improvement. Some program will probably always do
better than darktable on specific sorts of scenes simply because its
heuristics are a better match to what was there before the data was
clipped away.
- cks
[*: Technically if you want a completely accurate picture of channel
clipping in the RAW you need to neutralize white balance so that
all channels have a multiplier of 1 (what's sometimes called
'UniWB'). I usually don't bother, partly because it usually doesn't
really matter in practice unless the white balance is terribly off
to start with.
(I suppose I could make this easier by saving a 'UniWB' white balance
preset.)
]
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