Gamma - Differential

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Translation in progress. Any help welcome

Generalities

The tone curves are one of the key points of all the image processing softwares. RawTherapee does not escape to this rule... and we can even say the user is fulfil by the number and the types of the available curves (may be they are too many?) in RGB mode and L*a*b*, CIECAM, ... The question is often to know - and for the "Input" (matrix, ICC, DCP...) as well - what are we looking for and what are we acting on? Usually we act on the luminance L (L* of L*a*b*), or Y (Y of XYZ), or L equivalent rebuilt from the RGB values, ... Of course, users and programmers are aware of a problem! it is enough to read the forums, everyone try to add its contributions suggesting some options - mixing or not with the colour component - as for instance the introduction of a logarithmic scale or a tainted appearance model.

Proposition

  • The colour management - LCMS for RawTherapee - put at programmer and user disposal some performing tools to handle the data.
  • These tools are used:
  1. A the process start by attributing an input profile of matrix type, ICC or DCP,
  2. During the conversion of the data stored on the sensor into data usable by the main process, in a colour space (sRGB, Adobe, ...Prophoto), immediately before the image processing,
  3. At last, to convert these data for various use (screen, Web, printing, ...) into an output space.

It is obvious that the points 1, 2 and 3 must be improved in order to make RawTherapee more efficient, but the goal of this proposition is new for reason of the introduction of colour management in the middle of the processing in order to allow:

  1. tone modifications that take colour management (primaries, gamma, TRC) into account and not only the luminance variations
  2. a differentiated action inside a part of the processing in order to boost or attenuate the actions in relation with luminance, without globally alter the processing.

Methods

Several methods are at your disposal:

Standard (Relative ICC)

  • Apply a ICC profile over the current parameters ("gamma sRGB" witch is the RawTherapee default parameter)
  • This profile use the current colour space primaries and apply a gamma and a TRC on the RGB data placed after the RGB conversion and the white balance for each canal Red, Green and Blue. That leads to the modification of the RGB values that is taken into account in the processes that use these data, and of the L*a*b* values that are deduced of it.
  • You can act - in a relative way - on the gamma from 0.5 to 3.0, and on the slope from 0.0 to 20.0
  • All the no-raw processes are concerned. So, are excluded: Demosaicing, Raw Black and White points, Preprocessing, Chromatic aberration, Dark Frame, Flat field, Retinex, Noise Reduction, White Balance, Crop, Resize, Lens/Geometry, Distorsion, Vignetting,...

Standard (Absolute ICC)

  • Apply a ICC profile on data from which the original gamma ("gamma sRGB") has been subtracted
  • We could thing that this method is like a "softproofing"; it's almost true... but there are some differences because the transformation is done at the start of the process not at the end.
  • This profile use the current colour space primaries (except if "use output profile instead of working" is enabled), and apply a gamma and a TRC on the RGB data placed after the RGB conversion and the white balance for each canal Red, Green and Blue. That leads to the modification of the RGB values that is taken into account in the processes that use these data, and of the L*a*b* values that are deduced of it.
  • You can act - in an absolute way - on the gamma from 0.5 to 3.0, and on the slope from 0.0 to 20.0 and thus handle specific renderings.
    • Linear : gamma=1.0
    • BT-709 : gamma=2.22 slope=4.5
    • sRGB: gamma=2.4 slope=12.92
    • Adobe : gamma=2.2 slope=0.0
    • Prophoto : gamma=1.8 slope=0.0
    • and so on.
  • All the no-raw processes are concerned (dito Relative ICC).
  • If you enable "use output profile instead of working", the output profile will be applied - The gamma and slope sliders are disabled - (keep in mind that only the option "Output profile" with Output gamma = default and Free gamma disabled is taken into account).

Standard (Absolute ICC g=1 + end)

  • At the very beginning of the RGB process, we subtract the original gamma ("gamma sRGB") from the data.
  • then we apply a ICC profile, at the end of the RawTherapee process, immediately before the conversion towards TIFF/JPG.
  • if you choose a gamma=2.40 and slope=12.92, this leads to have the whole process RGB + L*a*b* treated in linear mode without gamma). Note: We can achieve the same result with "RGB + L*a*b* differential (ICC)" and gamma=0.417 slope=12.92.
  • all the no-raw processes are concerned (dito Relative ICC)
  • same options and remarks as "Absolute ICC".

RGB differential (Only gamma)

  • Apply a gamma, over the current parameters for one part of the RGB process, then at the end of the process, apply an inverse gamma
  • creates a differentiated action inside a part of the process (RGB here) in order to boost or attenuate the actions in relation with luminance, without globally alter the processing.
  • I set up this mode partially for pedagogical reasons to show out the difference between applying a gamma and its inverse, with the following method (RGB + L*a*b* differential ICC). The method "RGB differential Only gamma" can introduce artefacts when acting a lot on the sliders.
  • If all the RGB process settings are at null (or neutral) or the curves disabled, this dual conversion has no effect. On the other hand, if a setting is enabled, it will be boosted or attenuated according to the chosen settings for gamma and slope
  • You can act - in an differential way - on the gamma from 0.5 to 3.0, (reduced values about 0.7 et 2.5 are advised in order to limit the artefacts) and on the slope.
  • following processes are concerned: Exposure, Channel Mixer, HSV equalizer, RGB Curves, Color Toning

RGB + L*a*b* differential (ICC)

  • Apply a ICC profile, over current parameters and apply an inverse ICC profile at the end of the RawTherapee main precess, immediately before the conversion towards the output profile (output TIFF, JPG,...)
  • creates a differentiated action inside a part of the process (RGB + L*a*b* here) in order to boost or attenuate the actions in relation with luminance, without globally alter the processing.
  • This profile uses the primaries of the current colour space and apply a gamma and a TRC on the RGB data placed after the RGB conversion and the white balance, for each canal Red, Green and Blue and an inverse system at the end of the main process. That leads to an alteration of the RGB and L*a*b* values that is taken into account in the processes that use these data.
  • This process has the drawback, compared to "RGB differential", to increase a lot the processing time, but in the extremes cases it will produce less artefacts.
  • If all the RGB and L*a*b processes settings are at null (or neutral) or the curves disabled, this dual conversion has no effect. On the other hand, if a setting is enabled, it will be boosted or attenuated according to the chosen settings for gamma and slope
  • So, we will be able to heavily differentiate the image processing according to the chosen gamma, to boost a processing for the shadows or the highlights, and so on.
  • all the no-raw processes are concerned (dito Relative ICC)