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/* SPDX-License-Identifier: BSD-2-Clause */
/*
* Copyright (C) 2024, Ideas on Board Oy
*
* libipa miscellaneous colour helpers
*/
#include "colours.h"
#include <algorithm>
#include <cmath>
namespace libcamera {
namespace ipa {
/**
* \file colours.h
* \brief Functions to reduce code duplication between IPA modules
*/
/**
* \brief Estimate luminance from RGB values following ITU-R BT.601
* \param[in] r The red value
* \param[in] g The green value
* \param[in] b The blue value
*
* This function estimates a luminance value from a triplet of Red, Green and
* Blue values, following the formula defined by ITU-R Recommendation BT.601-7
* which can be found at https://www.itu.int/rec/R-REC-BT.601
*
* \return The estimated luminance value
*/
double rec601LuminanceFromRGB(double r, double g, double b)
{
return (r * .299) + (g * .587) + (b * .114);
}
/**
* \brief Estimate correlated colour temperature from RGB color space input
* \param[in] red The input red value
* \param[in] green The input green value
* \param[in] blue The input blue value
*
* This function estimates the correlated color temperature RGB color space
* input. In physics and color science, the Planckian locus or black body locus
* is the path or locus that the color of an incandescent black body would take
* in a particular chromaticity space as the blackbody temperature changes.
*
* If a narrow range of color temperatures is considered (those encapsulating
* daylight being the most practical case) one can approximate the Planckian
* locus in order to calculate the CCT in terms of chromaticity coordinates.
*
* More detailed information can be found in:
* https://en.wikipedia.org/wiki/Color_temperature#Approximation
*
* \return The estimated color temperature
*/
uint32_t estimateCCT(double red, double green, double blue)
{
/* Convert the RGB values to CIE tristimulus values (XYZ) */
double X = (-0.14282) * (red) + (1.54924) * (green) + (-0.95641) * (blue);
double Y = (-0.32466) * (red) + (1.57837) * (green) + (-0.73191) * (blue);
double Z = (-0.68202) * (red) + (0.77073) * (green) + (0.56332) * (blue);
/* Calculate the normalized chromaticity values */
double x = X / (X + Y + Z);
double y = Y / (X + Y + Z);
/* Calculate CCT */
double n = (x - 0.3320) / (0.1858 - y);
return 449 * n * n * n + 3525 * n * n + 6823.3 * n + 5520.33;
}
} /* namespace ipa */
} /* namespace libcamera */
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