"""
Exposure Value Computation
==========================
Defines the exposure value computation objects:
- :func:`colour_hdri.average_luminance`
- :func:`colour_hdri.average_illuminance`
- :func:`colour_hdri.luminance_to_exposure_value`
- :func:`colour_hdri.illuminance_to_exposure_value`
- :func:`colour_hdri.adjust_exposure`
References
----------
- :cite:`Wikipediabj` : Wikipedia. (n.d.). EV as a measure of luminance and
illuminance. Retrieved November 14, 2015, from
https://en.wikipedia.org/wiki/Exposure_value#\
EV_as_a_measure_of_luminance_and_illuminance
"""
from __future__ import annotations
import numpy as np
from colour.hints import ArrayLike, NDArrayFloat
from colour.utilities import as_float, as_float_array
__author__ = "Colour Developers"
__copyright__ = "Copyright 2015 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__ = [
"average_luminance",
"average_illuminance",
"luminance_to_exposure_value",
"illuminance_to_exposure_value",
"adjust_exposure",
]
[docs]
def average_luminance(
N: ArrayLike,
t: ArrayLike,
S: ArrayLike,
k: ArrayLike = 12.5,
) -> NDArrayFloat:
"""
Compute the average luminance :math:`L` in :math:`cd\\cdot m^{-2}` from
given relative aperture *F-Number* :math:`N`, *Exposure Time* :math:`t`,
*ISO* arithmetic speed :math:`S` and *reflected light calibration constant*
:math:`k`.
Parameters
----------
N
Relative aperture *F-Number* :math:`N`.
t
*Exposure Time* :math:`t`.
S
*ISO* arithmetic speed :math:`S`.
k
*Reflected light calibration constant* :math:`k`.
*ISO 2720:1974* recommends a range for :math:`k` of 10.6 to 13.4 with
luminance in :math:`cd\\cdot m^{-2}`. Two values for :math:`k` are in
common use: 12.5 (Canon, Nikon, and Sekonic) and 14 (Minolta, Kenko,
and Pentax).
Returns
-------
:class:`numpy.ndarray`
Average luminance :math:`L` in :math:`cd\\cdot m^{-2}`.
References
----------
:cite:`Wikipediabj`
Examples
--------
>>> average_luminance(8, 1, 100)
8.0
"""
N = as_float_array(N)
t = as_float_array(t)
S = as_float_array(S)
k = as_float_array(k)
L = N**2 / t / S * k
return as_float(L)
[docs]
def average_illuminance(
N: ArrayLike,
t: ArrayLike,
S: ArrayLike,
c: ArrayLike = 250,
) -> NDArrayFloat:
"""
Compute the average illuminance :math:`E` in :math:`Lux` from given
relative aperture *F-Number* :math:`N`, *Exposure Time* :math:`t`, *ISO*
arithmetic speed :math:`S` and *incident light calibration constant*
:math:`c`.
Parameters
----------
N
Relative aperture *F-Number* :math:`N`.
t
*Exposure Time* :math:`t`.
S
*ISO* arithmetic speed :math:`S`.
c
*Incident light calibration constant* :math:`c`.
With a flat receptor, *ISO 2720:1974* recommends a range for
:math:`c`. of 240 to 400 with illuminance in :math:`Lux`; a value of
250 is commonly used. With a hemispherical receptor, *ISO 2720:1974*
recommends a range for :math:`c` of 320 to 540 with illuminance in
:math:`Lux`; in practice, values typically are between 320 (Minolta)
and 340 (Sekonic).
Returns
-------
:class:`numpy.ndarray`
Average illuminance :math:`E` in :math:`Lux`.
References
----------
:cite:`Wikipediabj`
Examples
--------
>>> average_illuminance(8, 1, 100)
160.0
"""
N = as_float_array(N)
t = as_float_array(t)
S = as_float_array(S)
c = as_float_array(c)
E = N**2 / t / S * c
return as_float(E)
[docs]
def luminance_to_exposure_value(
L: ArrayLike,
S: ArrayLike,
k: ArrayLike = 12.5,
) -> NDArrayFloat:
"""
Compute the exposure value :math:`EV` from given scene luminance
:math:`L` in :math:`cd\\cdot m^{-2}`, *ISO* arithmetic speed :math:`S` and
*reflected light calibration constant* :math:`k`.
Parameters
----------
L
Scene luminance :math:`L` in :math:`cd\\cdot m^{-2}`.
S
*ISO* arithmetic speed :math:`S`.
k
*Reflected light calibration constant* :math:`k`.
*ISO 2720:1974* recommends a range for :math:`k` of 10.6 to 13.4 with
luminance in :math:`cd\\cdot m^{-2}`. Two values for :math:`k` are in
common use: 12.5 (Canon, Nikon, and Sekonic) and 14 (Minolta, Kenko,
and Pentax).
Returns
-------
:class:`numpy.ndarray`
Exposure value :math:`EV`.
Notes
-----
- The exposure value :math:`EV` indicates a combination of camera
settings rather than the focal plane exposure, i.e. luminous exposure,
photometric exposure, :math:`H`. The focal plane exposure is
time-integrated illuminance.
References
----------
:cite:`Wikipediabj`
Examples
--------
>>> luminance_to_exposure_value(0.125, 100)
0.0
"""
L = as_float_array(L)
S = as_float_array(S)
k = as_float_array(k)
EV = np.log2(L * S / k)
return as_float(EV)
[docs]
def illuminance_to_exposure_value(
E: ArrayLike,
S: ArrayLike,
c: ArrayLike = 250,
) -> NDArrayFloat:
"""
Compute the exposure value :math:`EV` from given scene illuminance
:math:`E` in :math:`Lux`, *ISO* arithmetic speed :math:`S` and
*incident light calibration constant* :math:`c`.
Parameters
----------
E
Scene illuminance :math:`E` in :math:`Lux`.
S
*ISO* arithmetic speed :math:`S`.
c
*Incident light calibration constant* :math:`c`.
With a flat receptor, *ISO 2720:1974* recommends a range for
:math:`c`. of 240 to 400 with illuminance in :math:`Lux`; a value of
250 is commonly used. With a hemispherical receptor, *ISO 2720:1974*
recommends a range for :math:`c` of 320 to 540 with illuminance in
:math:`Lux`; in practice, values typically are between 320 (Minolta)
and 340 (Sekonic).
Returns
-------
:class:`numpy.ndarray`
Exposure value :math:`EV`.
Notes
-----
- The exposure value :math:`EV` indicates a combination of camera
settings rather than the focal plane exposure, i.e. luminous exposure,
photometric exposure, :math:`H`. The focal plane exposure is
time-integrated illuminance.
References
----------
:cite:`Wikipediabj`
Examples
--------
>>> illuminance_to_exposure_value(2.5, 100)
0.0
"""
E = as_float_array(E)
S = as_float_array(S)
c = as_float_array(c)
EV = np.log2(E * S / c)
return as_float(EV)
[docs]
def adjust_exposure(a: ArrayLike, EV: ArrayLike) -> NDArrayFloat:
"""
Adjust given array exposure using given :math:`EV` exposure value.
Parameters
----------
a
Array to adjust the exposure.
EV
Exposure adjustment value.
Returns
-------
:class:`numpy.ndarray`
Exposure adjusted array.
Examples
--------
>>> adjust_exposure(np.array([0.25, 0.5, 0.75, 1]), 1)
array([ 0.5, 1. , 1.5, 2. ])
"""
a = as_float_array(a)
EV = as_float_array(EV)
return as_float(a * pow(2, EV))