#!/usr/bin/env python
# -*- coding: utf-8 -*-
"""
hdr-CIELAB Colourspace
======================
Defines the *hdr-CIELAB* colourspace transformations:
- :func:`XYZ_to_hdr_CIELab`
- :func:`hdr_CIELab_to_XYZ`
See Also
--------
`hdr-CIELAB Colourspace Jupyter Notebook
<http://nbviewer.jupyter.org/github/colour-science/colour-notebooks/\
blob/master/notebooks/models/hdr_cie_lab.ipynb>`_
References
----------
.. [1] Fairchild, M. D., & Wyble, D. R. (2010). hdr-CIELAB and hdr-IPT:
Simple Models for Describing the Color of High-Dynamic-Range and
Wide-Color-Gamut Images. In Proc. of Color and Imaging Conference
(pp. 322–326). ISBN:9781629932156
"""
from __future__ import division, unicode_literals
import numpy as np
from colour.colorimetry import (ILLUMINANTS, lightness_Fairchild2010,
luminance_Fairchild2010)
from colour.models import xy_to_xyY, xyY_to_XYZ
from colour.utilities import tsplit, tstack
__author__ = 'Colour Developers'
__copyright__ = 'Copyright (C) 2013-2017 - Colour Developers'
__license__ = 'New BSD License - http://opensource.org/licenses/BSD-3-Clause'
__maintainer__ = 'Colour Developers'
__email__ = 'colour-science@googlegroups.com'
__status__ = 'Production'
__all__ = ['XYZ_to_hdr_CIELab', 'hdr_CIELab_to_XYZ', 'exponent_hdr_CIELab']
[docs]def XYZ_to_hdr_CIELab(
XYZ,
illuminant=ILLUMINANTS['CIE 1931 2 Degree Standard Observer']['D50'],
Y_s=0.2,
Y_abs=100):
"""
Converts from *CIE XYZ* tristimulus values to *hdr-CIELAB* colourspace.
Parameters
----------
XYZ : array_like
*CIE XYZ* tristimulus values.
illuminant : array_like, optional
Reference *illuminant* *xy* chromaticity coordinates or *CIE xyY*
colourspace array.
Y_s : numeric or array_like
Relative luminance :math:`Y_s` of the surround in domain [0, 1].
Y_abs : numeric or array_like
Absolute luminance :math:`Y_{abs}` of the scene diffuse white in
:math:`cd/m^2`.
Returns
-------
ndarray
*hdr-CIELAB* colourspace array.
Notes
-----
- Conversion to polar coordinates to compute the *chroma* :math:`C_{hdr}`
and *hue* :math:`h_{hdr}` correlates can be safely performed with
:func:`colour.Lab_to_LCHab` definition.
- Conversion to cartesian coordinates from the *Lightness*
:math:`L_{hdr}`, *chroma* :math:`C_{hdr}` and *hue* :math:`h_{hdr}`
correlates can be safely performed with :func:`colour.LCHab_to_Lab`
definition.
- Input *CIE XYZ* tristimulus values are in domain [0, math:`\infty`].
- Input *illuminant* *xy* chromaticity coordinates or *CIE xyY*
colourspace array are in domain [0, :math:`\infty`].
Examples
--------
>>> XYZ = np.array([0.07049534, 0.10080000, 0.09558313])
>>> XYZ_to_hdr_CIELab(XYZ) # doctest: +ELLIPSIS
array([ 24.9020664..., -46.8312760..., -10.14274843])
"""
X, Y, Z = tsplit(XYZ)
X_n, Y_n, Z_n = tsplit(xyY_to_XYZ(xy_to_xyY(illuminant)))
e = exponent_hdr_CIELab(Y_s, Y_abs)
L_hdr = lightness_Fairchild2010(Y / Y_n, e)
a_hdr = 5 * (lightness_Fairchild2010(X / X_n, e) - L_hdr)
b_hdr = 2 * (L_hdr - lightness_Fairchild2010(Z / Z_n, e))
Lab_hdr = tstack((L_hdr, a_hdr, b_hdr))
return Lab_hdr
[docs]def hdr_CIELab_to_XYZ(
Lab_hdr,
illuminant=ILLUMINANTS['CIE 1931 2 Degree Standard Observer']['D50'],
Y_s=0.2,
Y_abs=100):
"""
Converts from *hdr-CIELAB* colourspace to *CIE XYZ* tristimulus values.
Parameters
----------
Lab_hdr : array_like
*hdr-CIELAB* colourspace array.
illuminant : array_like, optional
Reference *illuminant* *xy* chromaticity coordinates or *CIE xyY*
colourspace array.
Y_s : numeric or array_like
Relative luminance :math:`Y_s` of the surround in domain [0, 1].
Y_abs : numeric or array_like
Absolute luminance :math:`Y_{abs}` of the scene diffuse white in
:math:`cd/m^2`.
Returns
-------
ndarray
*CIE XYZ* tristimulus values.
Notes
-----
- Input *illuminant* *xy* chromaticity coordinates or *CIE xyY*
colourspace array are in domain [0, :math:`\infty`].
- Output *CIE XYZ* tristimulus values are in range [0, math:`\infty`].
Examples
--------
>>> Lab_hdr = np.array([24.90206646, -46.83127607, -10.14274843])
>>> hdr_CIELab_to_XYZ(Lab_hdr) # doctest: +ELLIPSIS
array([ 0.0704953..., 0.1008 , 0.0955831...])
"""
L_hdr, a_hdr, b_hdr = tsplit(Lab_hdr)
X_n, Y_n, Z_n = tsplit(xyY_to_XYZ(xy_to_xyY(illuminant)))
e = exponent_hdr_CIELab(Y_s, Y_abs)
Y = luminance_Fairchild2010(L_hdr, e) * Y_n
X = luminance_Fairchild2010((a_hdr + 5 * L_hdr) / 5, e) * X_n
Z = luminance_Fairchild2010((-b_hdr + 2 * L_hdr) / 2, e) * Z_n
XYZ = tstack((X, Y, Z))
return XYZ
[docs]def exponent_hdr_CIELab(Y_s, Y_abs):
"""
Computes *hdr-CIELAB* colourspace *Lightness* :math:`\epsilon` exponent.
Parameters
----------
Y_s : numeric or array_like
Relative luminance :math:`Y_s` of the surround in range [0, 1].
Y_abs : numeric or array_like
Absolute luminance :math:`Y_{abs}` of the scene diffuse white in
:math:`cd/m^2`.
Returns
-------
array_like
*hdr-CIELAB* colourspace *Lightness* :math:`\epsilon` exponent.
Examples
--------
>>> exponent_hdr_CIELab(0.2, 100) # doctest: +ELLIPSIS
1.8360198...
"""
Y_s = np.asarray(Y_s)
Y_abs = np.asarray(Y_abs)
lf = np.log(318) / np.log(Y_abs)
sf = 1.25 - 0.25 * (Y_s / 0.184)
epsilon = 1.50 * sf * lf
return epsilon