This implements a von-Kries-style chromatic adaption using the Bradford matrix.
The adaption is performed in scene linear space in the OCIO GLSL shader, with
the matrix being computed on the host.
The parameters specify the white point of the input, which is to be mapped to
the white point of the scene linear space. The main parameter is temperature,
specified in Kelvin, which defines the blackbody spectrum that is used as the
input white point. Additionally, a tint parameter can be used to shift the
white point away from pure blackbody spectra (e.g. to match a D illuminant).
The defaults are set to match D65 so there is no immediate color shift when
enabling the option. Tint = 10 is needed since the D-series illuminants aren't
perfect blackbody emitters.
As an alternative to manually specifying the values, there's also a color
picker. When a color is selected, temperature and tint are set such that this
color ends up being balanced to white.
This only works if the color is close enough to a blackbody emitter -
specifically, for tint values within +-150. Beyond this, there can be ambiguity
in the representation.
Currently, in this case, the input is just ignored and temperature/tint aren't
changed. Ideally, we'd eventually give UI feedback for this.
Presets are supported, and all the CIE standard illuminants are included.
One part that I'm not quite happy with is that the tint parameter starts to
give weird results at moderate values when the temperature is low.
The reason for this can be seen here:
https://commons.wikimedia.org/wiki/File:Planckian-locus.png
Tint is moving along the isotherm lines (with the plot corresponding to +-150),
but below 4000K some of that range is outside of the gamut. Not much can
be done there, other than possibly clipping those values...
Adding support for this to the compositor should be quite easy and is planned
as a next step.
Pull Request: https://projects.blender.org/blender/blender/pulls/123278
6967255906
