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Primaries Inset

Create a reshaped version of a colorspace's gamut and apply it on images using a 3x3 matrix. It is possible to visuallize the gamut transformation in a CIE xy graph plot.

The reshape transformation is called an "inset" as we are creating a smaller gamut than the original (but note we actually apply the inverse conversion making the source gamut smaller than the new gamut).

This is the main concept behind the AgX DRT ( personal fork) which was also ported to darktable.

demo recording of the node utilisation in Nuke

demo.mp4
enabled disabled
nuke screenshot with PrimariesInset enable nuke screenshot with PrimariesInset disabled

The above example is in the context of a traditional ACES workflow (note the ACES view-transform in the viewer) but could be used with any other "tonemapping".

The PrimaryInset operation works bests with the application of a 1D tonescale curve (usually reffered as tonemapping) after itself thanks to the increase of complimentary in the RGB values, which help achieve crosstalk during per-channel (produced by the tonescale).

Instructions

Install

Requirements

The tool use the following features :

  • python code for Presets but works on non-commercial versions.
    • The python code should be python 2 compatible but has only been tested on latest python3 versions of Nuke.
  • blink script but works on non-commercial versions >= 14.0

Basic-Usage

  • Expect an image to be transformed as input.
  • Select the preset corresponding to the input image's colorspace encoding.
  • Click the apply button: the primary X and whitepoint knobs are updated.
  • In the Option section, start playing with the global inset.
  • To see exactly how it affect the original colorspace you can click on show in the Plot section : your image disapear to leave a dark squared canvas with only a gamut visible.
  • Keep playing with the Options to see how it works.

default values

Here is some default values that are recommended when using BT.2020 as source gamut. They are based on the darktable implementation and Troy Sobotka work.

  • inset: 0.15 - 0.25
  • inset R: 0.06 : additional decrease of purity for red that can be still quite srong
  • inset B: 0.15 : additional decrease of purity for blue that tend to look deeper
  • rotate R: 5 : shift reds in favor of yellowness which work better for fire and sunsets.
  • rotate B: -6 : sift blues to compensate for Abney effect

Outset-workflow

If you find the effect of the inset too strong you can apply an outset after the tonescale has been applied to restore purity but preserve the "whiteness" in highlights:

nuke screenshot of the outset workflow

Unfortunately the workflow is complicated because you need to apply the tonescale before applying the outset. And the tonescale is usually part of the output-transform.

In the case of an ACES workflow :

  1. apply PrimaryInset node as usual
  2. apply ACES view-transform (OCIO Display node)
  3. revert the sRGB EOTF conversion to get back ACEScg value
  4. duplicate the first PrimaryInset node from step 1. but check the Invert option
  5. reapply the sRGB EOTF conversion for display.
  6. make sure the viewer view-transform is set to raw or similar.

Plotting

It is possible to preview the new inset colorspace as a plot in the CIE1931 xy space (note this is not a perceptual space).

Just check the show checkbox next to the Plot title.

nuke gif of the plot being edited interactively

While plotting you can check Invert Inset to see the actual destination colorspace being used for the math conversion (kind of unintuitive yes).

Developer

See the ./src/ folder for development instructions.

TODO

  • fix NO_HANDLE flag that is not preserved between sessions (woudl require onCreate callback)
  • add grid to plot
  • draw gamuts boundaries with lines