#!/usr/bin/env python
# -*- coding: utf-8 -*-
"""
Image Exposure Value Computation
================================
Defines image exposure value computation objects:
- :func:`exposure_value`
- :func:`adjust_exposure`
- :func:`average_luminance`
"""
from __future__ import division, unicode_literals
import numpy as np
__author__ = 'Colour Developers'
__copyright__ = 'Copyright (C) 2015-2017 - Colour Developers'
__license__ = 'New BSD License - http://opensource.org/licenses/BSD-3-Clause'
__maintainer__ = 'Colour Developers'
__email__ = 'colour-science@googlegroups.com'
__status__ = 'Production'
__all__ = ['exposure_value',
'adjust_exposure',
'average_luminance']
[docs]def exposure_value(f_number, exposure_time, iso):
"""
Computes the exposure value from given image *FNumber*, *Exposure Time* and
*ISO* values.
Parameters
----------
f_number : array_like
Image *FNumber*.
exposure_time : array_like
Image *Exposure Time*.
iso : array_like
Image *ISO*.
Returns
-------
ndarray
Image exposure value.
Examples
--------
>>> exposure_value(8, 1, 100)
6.0
"""
N = np.asarray(f_number)
t = np.asarray(exposure_time)
S = np.asarray(iso)
EV = np.log2(N ** 2) + np.log2(1 / t) - np.log2(100 / S)
return EV
[docs]def adjust_exposure(a, EV):
"""
Adjusts given array exposure using given :math:`EV` exposure value.
Parameters
----------
a : array_like
Array to adjust the exposure.
EV : numeric
Exposure adjustment value.
Returns
-------
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 = np.asarray(a)
return a * pow(2, EV)
[docs]def average_luminance(f_number, exposure_time, iso, k=12.5):
"""
Computes the average luminance from given image *FNumber*, *Exposure Time*
and *ISO* values.
Parameters
----------
f_number : array_like
Image *FNumber*.
exposure_time : array_like
Image *Exposure Time*.
iso : array_like
Image *ISO*.
k : numeric, optional
Reflected light calibration constant :math:`K`.
Returns
-------
ndarray
Image average luminance.
References
----------
.. [1] 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
Examples
--------
>>> average_luminance(8, 1, 100)
0.125
"""
N = np.asarray(f_number)
t = np.asarray(exposure_time)
S = np.asarray(iso)
L = (S * t) / (k * N ** 2)
return L