Hi Stéphane, As far as I know the reason for the base curve is there to convert between the different way our eyes and digital sensors see light, the digital sensor being linear and the eye (kind of) logarithmic. That is, you have to convert from what the camera sees, being uniform in terms of the number of photons falling on a particular pixel, to what the eye would see, i.e. a "perceptually uniform" space. An example of this is that the human eye sees 18% grey as "middle grey", i.e. half-way between white and black, whereas the sensor sees 50% grey as half-way between white and black. This is also the reason for "expose to the right" as there is far more information in the highlights than the darks.
http://en.wikipedia.org/wiki/Lightness_(color) http://www.luminous-landscape.com/tutorials/expose-right.shtml The s-shaped curve for increasing contrast is distinct from that. I would normally do that in the tone curve as this has the ability to handle saturation in an intelligent way (increasing it in areas that are being brightened and decreasing it in areas that are being darkened). HTH, Rob On 26/06/14 10:30, Stéphane Gourichon wrote: > 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 > > > > ------------------------------------------------------------------------------ > Open source business process management suite built on Java and Eclipse > Turn processes into business applications with Bonita BPM Community Edition > Quickly connect people, data, and systems into organized workflows > Winner of BOSSIE, CODIE, OW2 and Gartner awards > http://p.sf.net/sfu/Bonitasoft > > > > _______________________________________________ > Darktable-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/darktable-users > ------------------------------------------------------------------------------ Open source business process management suite built on Java and Eclipse Turn processes into business applications with Bonita BPM Community Edition Quickly connect people, data, and systems into organized workflows Winner of BOSSIE, CODIE, OW2 and Gartner awards http://p.sf.net/sfu/Bonitasoft _______________________________________________ Darktable-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/darktable-users
