Le 26/06/2014 10:54, Artur de Sousa Rocha a écrit :
it
would be an interesting project to have some software that would take
corresponding raw file and JPEG from camera and figure out a
color/curves/etc. profile and lens correction.
There is a tool that figures out a basecurve *and* a tone curve from a
bunch of JPEG+RAW (or from RAW only when it embeds a JPEG).
See about basecurves | darktable
<http://www.darktable.org/2013/10/about-basecurves/>
http://www.darktable.org/2013/10/about-basecurves/
Regarding what is written there, I don't understand well why the
basecurve is applied so early in darktable pipeline. (And since it is
mentioned in places, yes, I read 3.4. Modules | user manual | darktable
<http://www.darktable.org/usermanual/ch03s04.html.php> 3.4.2. Tone group
| user manual | darktable
<http://www.darktable.org/usermanual/ch03s04s02.html.php> Mastering
color with Lab tone curves | darktable
<http://www.darktable.org/2012/02/mastering-color-with-lab-tone-curves/>
and did run ./tools/iop_dependencies.py in a source tree, see the
generated PDF and, oh, something is complicated in there.)
3.4. Modules | user manual | darktable
<http://www.darktable.org/usermanual/ch03s04.html.php> says:
Camera sensors provide data in linear RGB format, the original image
appears flat and dull.
To me, the first reason why it appears flat and dull is that paper (and
computer monitor with usual luminosity set so that full white is not
aggressive to the eye) actually have low dynamic range, lower than most
photographed scenes (and virtually any scene under a visible sun).
So, the ""S-shaped" view transform." mentioned in about basecurves |
darktable <http://www.darktable.org/2013/10/about-basecurves/> is a way
to /compensate/ for the poor dynamic range of the final medium/display
device (computer screen, projector, or even photographic paper).
To put it short and incomplete : the S shape curve enhances contrast on
the "medium luminance" areas for a pleasing apparent contrast, which
mathematically "eats" most the the available dynamic range. This forces
to compress further the shadows and highlights, and it looks better to
have compressed highlights than clipping anyway.
Indeed, you can experience that a linear scene (no base curve, no tone
curve) displayed on a monitor set to high luminosity looks better
(natural rendering, more details in shadow and highlights) than the same
scene with S-shaped curve on a limited dynamic medium (paper, or the
same monitor with regular luminosity settings).
Does that make sense to you ?
Back to my question, why is base curve applied so early in darktable ?
Am I misguided about the S-shaped curve, the base curve, the tone curve
? The documents referenced above do not explain well why there are
already two S-shaped curves (base curve, tone curve) offered.
Anyone had experience with other similar tools and how they do that ?
Thank you for your attention.
--
Stéphane Gourichon
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