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// SPDX-License-Identifier: GPL-2.0-or-later
/** @file Support for the OKLab/OKLch perceptual color space.
*/
/*
* Authors:
* Rafał Siejakowski <rs@rs-math.net>
*
* Copyright (C) 2022 Authors
*
* Released under GNU GPL v2+, read the file 'COPYING' for more information.
*/
#ifndef SEEN_OKLAB_H
#define SEEN_OKLAB_H
#include <array>
#include "hsluv.h"
namespace Oklab {
using Triplet = Hsluv::Triplet;
using Hsluv::to_linear, Hsluv::from_linear;
/** @brief Convert an OKLab color to the linear RGB color space.
*
* @param oklab_color A triplet containing the L, a, b components.
* @return a triplet containing linear (de-gamma'd) RGB components, all in [0, 1].
*/
Triplet oklab_to_linear_rgb(Triplet const &oklab_color);
/** @brief Convert a linear RGB color to OKLab coordinates.
*
* @param linear_rgb_color A triplet containing degamma'd (linearized) R, G, B components.
* @return a triplet containing the (L, a, b) coordinates of the color in the OKLab space.
*/
Triplet linear_rgb_to_oklab(Triplet const &linear_rgb_color);
/** Convert an OKLab color to a gamma-compressed sRGB color. */
inline Triplet oklab_to_rgb(Triplet const &oklab_color)
{
auto rgb = oklab_to_linear_rgb(oklab_color);
for (auto &component : rgb) {
component = from_linear(component);
}
return rgb;
}
/** Convert a gamma-compressed sRGB color to an OKLab color. */
inline Triplet rgb_to_oklab(Triplet const &rgb_color)
{
Triplet linear;
for (size_t i = 0; i < 3; i++) {
linear[i] = to_linear(rgb_color[i]);
}
return linear_rgb_to_oklab(linear);
}
/** @brief Convert an OKLab color to the OKLch coordinates.
*
* The OKLch coordinates are more closely aligned with the perceptual properties of a color
* and therefore are more convenient for the end user. They consist of:
* L – luminance of the color, in the interval [0, 1] (this is the same as the L in (L, a, b)).
* c – chroma; how far the color is from grayscale. The range of c-values is [0, cmax] but
* cmax depends on L and h; @see Oklab::max_chroma().
* h – hue. A number in the interval [0, 360), interpreted as a hue angle on the color wheel.
*
* @param ok_lab_color A color in the OKLab color space.
* @return The same color expressed in the Lch coordinates.
*/
Triplet oklab_to_oklch(Triplet const &ok_lab_color);
/** @brief Convert an OKLch color to the OKLab coordinates.
*
* For the meaning of the Lch color coordinates, @see oklab_to_oklch().
*/
Triplet oklch_to_oklab(Triplet const &ok_lch_color);
Triplet oklch_radians_to_oklab(Triplet const &ok_lch_with_hue_in_radians);
/** @brief Convert an OKLab color to an OKHSL representation.
*
* As of late 2022, OK-HSL (hue, saturation, lightness) is not a fully standardized color
* space. The version used here stores colors as triples (h, s, L) of doubles, all in
* the interval [0, 1]. These coordinates have the following meaning:
*
* \li \c h is the hue angle, scaled to the interval [0, 1].
* \li \c s is the linear saturation. For a given OKLch color (L, c, h), linear saturation
* is the ratio of c to the maximum possible chroma c_max such that (L, c_max, h)
* fits in the sRGB gamut. Therefore, s=1 always corresponds to a maximally saturated
* color and s=0 is always a grayscale color.
* \li \c L is the lightness; it is the same coordinate as the L in OKLab or OKLch.
*
* Note that Björn Ottosson proposes a new, non-standard way of compressing the saturation
* (similar to gamma compression) which results in a different saturation scale. He further
* suggests varying this compression depending on the hue, although he does not standardize
* this proposed OKHSL space or the saturation transfer functions. Instead, he uses a somewhat
* ad-hoc piecewise-linear saturation transfer function in his own picker, which unfortunately
* breaks differentiability of the parametrization, a great advantage of the OKLch color space.
* If an OKHSL space is ever standardized, the behaviour of this function may change.
* See https://bottosson.github.io/posts/colorpicker/ for more details.
*
* @param ok_lab_color A color in the OKLab color space.
* @return The same color expressed in the (h, s, L) coordinates.
*/
Triplet oklab_to_okhsl(Triplet const &ok_lab_color);
/** @brief Convert an OKHSL color to the OKLab coordinates.
*
* For a description of the OKHSL color scale used here, see oklab_to_okhsl().
* If an OKHSL space is ever standardized, the behaviour of this function may change.
*
* @param ok_hsl_color A color in the OKHSL coordinates, all normalized to [0, 1].
* @return The same color expressed in the (L, a, b) coordinates.
*/
Triplet okhsl_to_oklab(Triplet const &ok_hsl_color);
/** @brief Find the maximum OKLch chroma for the given luminosity and hue.
*
* @param l OKLab/OKLch luminosity, in the interval [0, 1].
* @param h OKLch hue angle in degrees (interval [0, 360]).
* @return The maximum chroma c such that the color oklch(l, c, h) fits in the sRGB gamut.
*/
double max_chroma(double l, double h);
/** @brief Helper functions for rendering color strips used in color sliders.
*
* @param h Hue angle in degrees (range 0-360).
* @param s Saturation (percentage of allowed chroma) in the range 0-1.
* @param l OKLab/OKLch lightness in the range 0-1.
* @param[out] map Working store and RGB output showing the range of one of the color coordinates.
* @return A pointer to the raw data of the map.
*/
uint8_t const *render_hue_scale(double s, double l, std::array<uint8_t, 4 * 1024> *map);
uint8_t const *render_saturation_scale(double h, double l, std::array<uint8_t, 4 * 1024> *map);
uint8_t const *render_lightness_scale(double h, double s, std::array<uint8_t, 4 * 1024> *map);
} // namespace Oklab
#endif // SEEN_OKLAB_H
/*
Local Variables:
mode:c++
c-file-style:"stroustrup"
c-file-offsets:((innamespace . 0)(inline-open . 0)(case-label . +))
indent-tabs-mode:nil
fill-column:99
End:
*/
// vim: filetype=cpp:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:fileencoding=utf-8:textwidth=99 :
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