#!/usr/bin/env python
# -*- coding: utf-8 -*-
"""
ARIB STD-B67 (Hybrid Log-Gamma)
===============================
Defines *ARIB STD-B67 (Hybrid Log-Gamma)* opto-electrical transfer function
(OETF / OECF) and electro-optical transfer function (EOTF / EOCF):
- :func:`oetf_ARIBSTDB67`
- :func:`eotf_ARIBSTDB67`
See Also
--------
`RGB Colourspaces Jupyter Notebook
<http://nbviewer.jupyter.org/github/colour-science/colour-notebooks/\
blob/master/notebooks/models/rgb.ipynb>`_
References
----------
.. [1] Association of Radio Industries and Businesses. (2015). Essential
Parameter Values for the Extended Image Dynamic Range Television
(EIDRTV) System for Programme Production. Arib Std-B67. Retrieved from
http://www.arib.or.jp/english/html/overview/std-b67.html
"""
from __future__ import division, unicode_literals
import numpy as np
from colour.utilities import Structure, as_numeric, warning
__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__ = ['ARIBSTDB67_CONSTANTS', 'oetf_ARIBSTDB67', 'eotf_ARIBSTDB67']
ARIBSTDB67_CONSTANTS = Structure(a=0.17883277, b=0.28466892, c=0.55991073)
"""
*ARIB STD-B67 (Hybrid Log-Gamma)* constants.
ARIBSTDB67_CONSTANTS : Structure
"""
[docs]def oetf_ARIBSTDB67(E, r=0.5):
"""
Defines *ARIB STD-B67 (Hybrid Log-Gamma)* opto-electrical transfer
function (OETF / OECF).
Parameters
----------
E : numeric or array_like
Voltage normalized by the reference white level and proportional to
the implicit light intensity that would be detected with a reference
camera color channel R, G, B.
r : numeric, optional
Video level corresponding to reference white level.
Returns
-------
numeric or ndarray
Resulting non-linear signal :math:`E'`.
Examples
--------
>>> oetf_ARIBSTDB67(0.18) # doctest: +ELLIPSIS
0.2121320...
"""
E = np.asarray(E)
a = ARIBSTDB67_CONSTANTS.a
b = ARIBSTDB67_CONSTANTS.b
c = ARIBSTDB67_CONSTANTS.c
E_p = np.where(E <= 1, r * np.sqrt(E), a * np.log(E - b) + c)
return as_numeric(E_p)
[docs]def eotf_ARIBSTDB67(E_p, r=0.5):
"""
Defines *ARIB STD-B67 (Hybrid Log-Gamma)* electro-optical transfer
function (EOTF / EOCF).
Parameters
----------
E_p : numeric or array_like
Non-linear signal :math:`E'`.
r : numeric, optional
Video level corresponding to reference white level.
Returns
-------
numeric or ndarray
Voltage :math:`E` normalized by the reference white level and
proportional to the implicit light intensity that would be detected
with a reference camera color channel R, G, B.
Warning
-------
*ARIB STD-B67 (Hybrid Log-Gamma)* doesn't specify an electro-optical
transfer function. This definition is used for symmetry in unit tests and
other computations but should not be used as an *EOTF*.
Examples
--------
>>> eotf_ARIBSTDB67(0.212132034355964) # doctest: +ELLIPSIS
0.1799999...
"""
warning(('*ARIB STD-B67 (Hybrid Log-Gamma)* doesn\'t specify an '
'electro-optical transfer function. This definition is used '
'for symmetry in unit tests and others computations but should '
'not be used as an *EOTF*!'))
E_p = np.asarray(E_p)
a = ARIBSTDB67_CONSTANTS.a
b = ARIBSTDB67_CONSTANTS.b
c = ARIBSTDB67_CONSTANTS.c
E = np.where(E_p <= oetf_ARIBSTDB67(1), (E_p / r) ** 2,
np.exp((E_p - c) / a) + b)
return as_numeric(E)