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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] rgb The RGB 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(const RGB<double> &rgb)
{
static const Vector<double, 3> rgb2y{{
0.299, 0.587, 0.114
}};
return rgb.dot(rgb2y);
}
/**
* \brief Estimate correlated colour temperature from RGB color space input
* \param[in] rgb The RGB 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 black body 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(const RGB<double> &rgb)
{
/*
* Convert the RGB values to CIE tristimulus values (XYZ) and divide by
* the sum of X, Y and Z to calculate the CIE xy chromaticity.
*/
static const Matrix<double, 3, 3> rgb2xyz({
-0.14282, 1.54924, -0.95641,
-0.32466, 1.57837, -0.73191,
-0.68202, 0.77073, 0.56332
});
Vector<double, 3> xyz = rgb2xyz * rgb;
xyz /= xyz.sum();
/* Calculate CCT */
double n = (xyz.x() - 0.3320) / (0.1858 - xyz.y());
return 449 * n * n * n + 3525 * n * n + 6823.3 * n + 5520.33;
}
} /* namespace ipa */
} /* namespace libcamera */
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