Gamma transfer and correction functions for sRGB.

srgb_gamma.py

#
# Copyright (C) 2017 Pietro Saccardi <lizardm4@gmail.com>
#
# This work is free. You can redistribute it and/or modify it under the
# terms of the Do What The Fuck You Want To Public License, Version 2,
# as published by Sam Hocevar. See http://www.wtfpl.net/ for more details.
#
# ----
#
# Blend your RGB colors correctly! Convert them to a linear space first.
# https://www.youtube.com/watch?v=LKnqECcg6Gw (thanks MinutePhysics!)
#
from math import pow

_SRGB_A = 0.055
_SRGB_G = 2.4
_SRGB_S = 12.92
_SRGB_DIR_THR = 0.0031308
_SRGB_INV_THR = 0.04045


def srgb_gamma_dir(c):
    """
    The gamma transfer function for sRGB. This takes a linear RGB value and returns its storage representation (i.e.
    performs gamma correction of the value).
    :param c: A floating point value in the range [0., 1.] representing one of the R, G, B channels.
    :return: A floating point value in the range [0., 1.], representing the gamma-corrected channel.
    """
    return (1. + _SRGB_A) * pow(c, 1. / _SRGB_G) - _SRGB_A if c > _SRGB_DIR_THR else c * _SRGB_S


def srgb_gamma_inv(c):
    """
    The inverse gamma transfer function for sRGB. This takes a stored RGB value and returns its linear representation
    (i.e. not gamma-corrected).
    :param c: A floating point value in the range [0., 1.] representing the gamma-corrected channel.
    :return: A floating point value in the range [0., 1.], representing one of the R, G, B channels.
    """
    return pow((c + _SRGB_A) / (1. + _SRGB_A), _SRGB_G) if c > _SRGB_INV_THR else c / _SRGB_S


# A lookup table for the function srgb_gamma_dir, evaluated on all the integers from 0 to 255 (included).
SRGB_GAMMA_DIR_TABLE = [int(0.5 + 255 * srgb_gamma_dir(i / 255)) for i in range(256)]

# A lookup table for the function srgb_gamma_inv, evaluated on all the integers from 0 to 255 (included).
SRGB_GAMMA_INV_TABLE = [int(0.5 + 255 * srgb_gamma_inv(i / 255)) for i in range(256)]


def srgb_to_linear_rgb(rgb):
    """
    Convert sRGB values to linear RGB. Linear RGB values are suitable to be interpolated and blended linearly, because
    they are not gamma-corrected. For more information, see https://en.wikipedia.org/wiki/SRGB.
    :param rgb: A 3-tuple of R, G, B integers in the range 0..255. No check is performed on the input.
    :return: A 3-tuple of linear R, G, B integers in the range 0..255.
    """
    return SRGB_GAMMA_INV_TABLE[rgb[0]], SRGB_GAMMA_INV_TABLE[rgb[1]], SRGB_GAMMA_INV_TABLE[rgb[2]]


def linear_rgb_to_srgb(rgb):
    """
    Convert linear RGB values to sRGB. sRGB values are gamma-corrected and are suitable for storage. This is the format
    in which you get data from e.g. PIL. For more information, see https://en.wikipedia.org/wiki/SRGB.
    :param rgb: A 3-tuple of linear R, G, B integers in the range 0..255. No check is performed on the input.
    :return: A 3-tuple of sRGB R, G, B integers in the range 0..255.
    """
    return SRGB_GAMMA_DIR_TABLE[rgb[0]], SRGB_GAMMA_DIR_TABLE[rgb[1]], SRGB_GAMMA_DIR_TABLE[rgb[2]]