/* 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 */