"""
Optical Society of America Uniform Colour Scales (OSA UCS)
==========================================================
Define the *OSA UCS* colourspace transformations.
- :func:`colour.XYZ_to_OSA_UCS`
- :func:`colour.OSA_UCS_to_XYZ`
References
----------
- :cite:`Cao2013` : Cao, R., Trussell, H. J., & Shamey, R. (2013). Comparison
of the performance of inverse transformation methods from OSA-UCS to
CIEXYZ. Journal of the Optical Society of America A, 30(8), 1508.
doi:10.1364/JOSAA.30.001508
- :cite:`Moroney2003` : Moroney, N. (2003). A Radial Sampling of the OSA
Uniform Color Scales. Color and Imaging Conference, 2003(1), 175-180.
ISSN:2166-9635
- :cite:`Schlomer2019` : Schlömer, N. (2019). On the conversion from OSA-UCS
to CIEXYZ (Version 2). arXiv. doi:10.48550/ARXIV.1911.08323
"""
from __future__ import annotations
import typing
import numpy as np
from colour.algebra import sdiv, sdiv_mode, spow, vecmul
if typing.TYPE_CHECKING:
from colour.hints import NDArrayFloat
from colour.hints import ( # noqa: TC001
Domain100,
NDArrayFloat,
Range100,
)
from colour.models import XYZ_to_xyY
from colour.utilities import (
from_range_100,
to_domain_100,
tsplit,
tstack,
)
__author__ = "Colour Developers"
__copyright__ = "Copyright 2013 Colour Developers"
__license__ = "BSD-3-Clause - https://opensource.org/licenses/BSD-3-Clause"
__maintainer__ = "Colour Developers"
__email__ = "colour-developers@colour-science.org"
__status__ = "Production"
__all__ = [
"XYZ_to_OSA_UCS",
"OSA_UCS_to_XYZ",
]
MATRIX_XYZ_TO_RGB_OSA_UCS: NDArrayFloat = np.array(
[
[0.799, 0.4194, -0.1648],
[-0.4493, 1.3265, 0.0927],
[-0.1149, 0.3394, 0.717],
]
)
"""
*OSA UCS* matrix converting from *CIE XYZ* tristimulus values to *RGB*
colourspace.
"""
MATRIX_RGB_TO_XYZ_OSA_UCS: NDArrayFloat = np.linalg.inv(MATRIX_XYZ_TO_RGB_OSA_UCS)
"""
*OSA UCS* matrix converting from *RGB* colourspace to *CIE XYZ* tristimulus
values (inverse of MATRIX_XYZ_TO_RGB_OSA_UCS).
"""
[docs]
def XYZ_to_OSA_UCS(XYZ: Domain100) -> Range100:
"""
Convert from *CIE XYZ* tristimulus values under the
*CIE 1964 10 Degree Standard Observer* to *OSA UCS* colourspace.
The lightness axis, *L*, is typically in range [-9, 5] and centered
around middle gray (Munsell N/6). The yellow-blue axis, *j*, is
typically in range [-15, 15]. The red-green axis, *g*, is typically in
range [-20, 15].
Parameters
----------
XYZ
*CIE XYZ* tristimulus values under the
*CIE 1964 10 Degree Standard Observer*.
Returns
-------
:class:`numpy.ndarray`
*OSA UCS* :math:`Ljg` lightness, jaune (yellowness), and greenness.
Notes
-----
+------------+-----------------------+--------------------+
| **Domain** | **Scale - Reference** | **Scale - 1** |
+============+=======================+====================+
| ``XYZ`` | 100 | 1 |
+------------+-----------------------+--------------------+
+------------+-----------------------+--------------------+
| **Range** | **Scale - Reference** | **Scale - 1** |
+============+=======================+====================+
| ``Ljg`` | 100 | 1 |
+------------+-----------------------+--------------------+
- *OSA UCS* uses the *CIE 1964 10 Degree Standard Observer*.
References
----------
:cite:`Cao2013`, :cite:`Moroney2003`
Examples
--------
>>> import numpy as np
>>> XYZ = np.array([0.20654008, 0.12197225, 0.05136952]) * 100
>>> XYZ_to_OSA_UCS(XYZ) # doctest: +ELLIPSIS
array([-3.0049979..., 2.9971369..., -9.6678423...])
"""
XYZ = to_domain_100(XYZ)
x, y, Y = tsplit(XYZ_to_xyY(XYZ))
Y_0 = Y * (
4.4934 * x**2 + 4.3034 * y**2 - 4.276 * x * y - 1.3744 * x - 2.5643 * y + 1.8103
)
o_3 = 1 / 3
Y_0_es = spow(Y_0, o_3) - 2 / 3
# Gracefully handles Y_0 < 30.
Y_0_s = Y_0 - 30
Lambda = 5.9 * (Y_0_es + 0.042 * spow(Y_0_s, o_3))
RGB = vecmul(MATRIX_XYZ_TO_RGB_OSA_UCS, XYZ)
RGB_3 = spow(RGB, 1 / 3)
with sdiv_mode():
C = sdiv(Lambda, 5.9 * Y_0_es)
L = (Lambda - 14.4) / spow(2, 1 / 2)
j = C * np.dot(RGB_3, np.array([1.7, 8, -9.7]))
g = C * np.dot(RGB_3, np.array([-13.7, 17.7, -4]))
Ljg = tstack([L, j, g])
return from_range_100(Ljg)
[docs]
def OSA_UCS_to_XYZ(Ljg: Domain100, optimisation_kwargs: dict | None = None) -> Range100:
"""
Convert from *OSA UCS* colourspace to *CIE XYZ* tristimulus values under
the *CIE 1964 10 Degree Standard Observer*.
The lightness axis, *L*, is typically in range [-9, 5] and centered
around middle gray (Munsell N/6). The yellow-blue axis, *j*, is
typically in range [-15, 15]. The red-green axis, *g*, is typically in
range [-20, 15].
Parameters
----------
Ljg
*OSA UCS* :math:`Ljg` lightness, jaune (yellowness), and greenness.
optimisation_kwargs
Parameters for Newton iteration. Supported parameters:
- *iterations_maximum*: Maximum number of iterations (default: 20).
- *tolerance*: Convergence tolerance (default: 1e-10).
- *epsilon*: Step size for numerical derivative (default: 1e-8).
Returns
-------
:class:`numpy.ndarray`
*CIE XYZ* tristimulus values under the
*CIE 1964 10 Degree Standard Observer*.
Notes
-----
+------------+-----------------------+--------------------+
| **Domain** | **Scale - Reference** | **Scale - 1** |
+============+=======================+====================+
| ``Ljg`` | 100 | 1 |
+------------+-----------------------+--------------------+
+------------+-----------------------+--------------------+
| **Range** | **Scale - Reference** | **Scale - 1** |
+============+=======================+====================+
| ``XYZ`` | 100 | 1 |
+------------+-----------------------+--------------------+
- *OSA UCS* uses the *CIE 1964 10 Degree Standard Observer*.
- This implementation uses the improved algorithm from :cite:`Schlomer2019`
which employs Cardano's formula for solving the cubic equation and
Newton's method for the remaining nonlinear system.
References
----------
:cite:`Cao2013`, :cite:`Moroney2003`, :cite:`Schlomer2019`
Examples
--------
>>> import numpy as np
>>> Ljg = np.array([-3.00499790, 2.99713697, -9.66784231])
>>> OSA_UCS_to_XYZ(Ljg) # doctest: +ELLIPSIS
array([ 20.654008..., 12.197225..., 5.1369520...])
"""
Ljg = to_domain_100(Ljg)
shape = Ljg.shape
Ljg = np.atleast_1d(np.reshape(Ljg, (-1, 3)))
# Default optimization settings
settings: dict[str, typing.Any] = {
"iterations_maximum": 20,
"tolerance": 1e-10,
"epsilon": 1e-8,
}
if optimisation_kwargs is not None:
settings.update(optimisation_kwargs)
L, j, g = tsplit(Ljg)
# Step 1: Compute L' from L
# Forward: L = (Lambda - 14.4) / sqrt(2)
# Backward: Lambda = L * sqrt(2) + 14.4
# But L' = Lambda in the intermediate calculation
sqrt_2 = np.sqrt(2)
L_prime = L * sqrt_2 + 14.4
# Step 2: Solve for Y0 using Cardano's formula
# Equation: 0 = f(t) = (L'/5.9 + 2/3 - t)³ - 0.042^3(t^3 - 30)
# where t = Y0^(1/3)
u = L_prime / 5.9 + 2.0 / 3.0
v = 0.042**3
# Cubic equation: at^3 + bt^2 + ct + d = 0
a = -(v + 1)
b = 3 * u
c = -3 * u**2
d = u**3 + 30 * v
# Convert to depressed cubic: x³ + px + q = 0
p = (3 * a * c - b**2) / (3 * a**2)
q = (2 * b**3 - 9 * a * b * c + 27 * a**2 * d) / (27 * a**3)
# Cardano's formula
discriminant = (q / 2) ** 2 + (p / 3) ** 3
with sdiv_mode():
t = (
-b / (3 * a)
+ spow(-q / 2 + np.sqrt(discriminant), 1.0 / 3.0)
+ spow(-q / 2 - np.sqrt(discriminant), 1.0 / 3.0)
)
Y0 = t**3
# Step 3: Compute C, a, b
with sdiv_mode():
C = sdiv(L_prime, 5.9 * (t - 2.0 / 3.0))
a_coef = sdiv(g, C)
b_coef = sdiv(j, C)
# Step 4: Solve for RGB using Newton iteration
# Matrix A from equation (4)
A = np.array([[-13.7, 17.7, -4.0], [1.7, 8.0, -9.7]])
# Augment A with [1, 0, 0] to make it non-singular (set w = cbrt(R))
A_augmented = np.vstack([A, [1.0, 0.0, 0.0]])
A_inv = np.linalg.inv(A_augmented)
# Initial guess for w (corresponds to cbrt(R))
# w0 = cbrt(79.9 + 41.94) from paper
w = np.full_like(L, (79.9 + 41.94) ** (1.0 / 3.0))
# Newton iteration
for _iteration in range(settings["iterations_maximum"]):
# Solve for [cbrt(R), cbrt(G), cbrt(B)] given current w
ab_w = np.array([a_coef, b_coef, w]).T
RGB_cbrt = np.dot(ab_w, A_inv.T)
RGB = RGB_cbrt**3
XYZ = vecmul(MATRIX_RGB_TO_XYZ_OSA_UCS, RGB)
X, Y, Z = tsplit(XYZ)
with sdiv_mode():
sum_XYZ = X + Y + Z
x = sdiv(X, sum_XYZ)
y = sdiv(Y, sum_XYZ)
K = (
4.4934 * x**2
+ 4.3034 * y**2
- 4.276 * x * y
- 1.3744 * x
- 2.5643 * y
+ 1.8103
)
Y0_computed = Y * K
error = Y0_computed - Y0
if np.all(np.abs(error) < settings["tolerance"]):
break
# Newton step: compute derivative and update w
# Derivative is computed numerically for robustness
epsilon = settings["epsilon"]
w_plus = w + epsilon
ab_w_plus = np.array([a_coef, b_coef, w_plus]).T
RGB_cbrt_plus = np.dot(ab_w_plus, A_inv.T)
RGB_plus = RGB_cbrt_plus**3
XYZ_plus = vecmul(MATRIX_RGB_TO_XYZ_OSA_UCS, RGB_plus)
X_plus, Y_plus, Z_plus = tsplit(XYZ_plus)
with sdiv_mode():
sum_XYZ_plus = X_plus + Y_plus + Z_plus
x_plus = sdiv(X_plus, sum_XYZ_plus)
y_plus = sdiv(Y_plus, sum_XYZ_plus)
K_plus = (
4.4934 * x_plus**2
+ 4.3034 * y_plus**2
- 4.276 * x_plus * y_plus
- 1.3744 * x_plus
- 2.5643 * y_plus
+ 1.8103
)
Y0_computed_plus = Y_plus * K_plus
with sdiv_mode():
derivative = sdiv(Y0_computed_plus - Y0_computed, epsilon)
w = w - sdiv(error, derivative)
return from_range_100(np.reshape(XYZ, shape))
