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Add LCH(ab) color space to bevy_render::color::Color (#7483)
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# Objective

- Fixes #766

## Solution

- Add a new `Lcha` member to `bevy_render::color::Color` enum

---

## Changelog

- Add a new `Lcha` member to `bevy_render::color::Color` enum
- Add `bevy_render::color::LchRepresentation` struct
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Ludwig DUBOS committed Feb 6, 2023
1 parent 5b930c8 commit 7b7b34f
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217 changes: 217 additions & 0 deletions crates/bevy_render/src/color/colorspace.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -102,6 +102,137 @@ impl HslRepresentation {
}
}

pub struct LchRepresentation;
impl LchRepresentation {
// References available at http://brucelindbloom.com/ in the "Math" section

// CIE Constants
// http://brucelindbloom.com/index.html?LContinuity.html (16) (17)
const CIE_EPSILON: f32 = 216.0 / 24389.0;
const CIE_KAPPA: f32 = 24389.0 / 27.0;
// D65 White Reference:
// https://en.wikipedia.org/wiki/Illuminant_D65#Definition
const D65_WHITE_X: f32 = 0.95047;
const D65_WHITE_Y: f32 = 1.0;
const D65_WHITE_Z: f32 = 1.08883;

/// converts a color in LCH space to sRGB space
#[inline]
pub fn lch_to_nonlinear_srgb(lightness: f32, chroma: f32, hue: f32) -> [f32; 3] {
let lightness = lightness * 100.0;
let chroma = chroma * 100.0;

// convert LCH to Lab
// http://www.brucelindbloom.com/index.html?Eqn_LCH_to_Lab.html
let l = lightness;
let a = chroma * hue.to_radians().cos();
let b = chroma * hue.to_radians().sin();

// convert Lab to XYZ
// http://www.brucelindbloom.com/index.html?Eqn_Lab_to_XYZ.html
let fy = (l + 16.0) / 116.0;
let fx = a / 500.0 + fy;
let fz = fy - b / 200.0;
let xr = {
let fx3 = fx.powf(3.0);

if fx3 > Self::CIE_EPSILON {
fx3
} else {
(116.0 * fx - 16.0) / Self::CIE_KAPPA
}
};
let yr = if l > Self::CIE_EPSILON * Self::CIE_KAPPA {
((l + 16.0) / 116.0).powf(3.0)
} else {
l / Self::CIE_KAPPA
};
let zr = {
let fz3 = fz.powf(3.0);

if fz3 > Self::CIE_EPSILON {
fz3
} else {
(116.0 * fz - 16.0) / Self::CIE_KAPPA
}
};
let x = xr * Self::D65_WHITE_X;
let y = yr * Self::D65_WHITE_Y;
let z = zr * Self::D65_WHITE_Z;

// XYZ to sRGB
// http://www.brucelindbloom.com/index.html?Eqn_XYZ_to_RGB.html
// http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html (sRGB, XYZ to RGB [M]-1)
let red = x * 3.2404542 + y * -1.5371385 + z * -0.4985314;
let green = x * -0.969266 + y * 1.8760108 + z * 0.041556;
let blue = x * 0.0556434 + y * -0.2040259 + z * 1.0572252;

[
red.linear_to_nonlinear_srgb().max(0.0).min(1.0),
green.linear_to_nonlinear_srgb().max(0.0).min(1.0),
blue.linear_to_nonlinear_srgb().max(0.0).min(1.0),
]
}

/// converts a color in sRGB space to LCH space
#[inline]
pub fn nonlinear_srgb_to_lch([red, green, blue]: [f32; 3]) -> (f32, f32, f32) {
// RGB to XYZ
// http://www.brucelindbloom.com/index.html?Eqn_RGB_to_XYZ.html
let red = red.nonlinear_to_linear_srgb();
let green = green.nonlinear_to_linear_srgb();
let blue = blue.nonlinear_to_linear_srgb();

// http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html (sRGB, RGB to XYZ [M])
let x = red * 0.4124564 + green * 0.3575761 + blue * 0.1804375;
let y = red * 0.2126729 + green * 0.7151522 + blue * 0.072175;
let z = red * 0.0193339 + green * 0.119192 + blue * 0.9503041;

// XYZ to Lab
// http://www.brucelindbloom.com/index.html?Eqn_XYZ_to_Lab.html
let xr = x / Self::D65_WHITE_X;
let yr = y / Self::D65_WHITE_Y;
let zr = z / Self::D65_WHITE_Z;
let fx = if xr > Self::CIE_EPSILON {
xr.cbrt()
} else {
(Self::CIE_KAPPA * xr + 16.0) / 116.0
};
let fy = if yr > Self::CIE_EPSILON {
yr.cbrt()
} else {
(Self::CIE_KAPPA * yr + 16.0) / 116.0
};
let fz = if yr > Self::CIE_EPSILON {
zr.cbrt()
} else {
(Self::CIE_KAPPA * zr + 16.0) / 116.0
};
let l = 116.0 * fy - 16.0;
let a = 500.0 * (fx - fy);
let b = 200.0 * (fy - fz);

// Lab to LCH
// http://www.brucelindbloom.com/index.html?Eqn_Lab_to_LCH.html
let c = (a.powf(2.0) + b.powf(2.0)).sqrt();
let h = {
let h = b.to_radians().atan2(a.to_radians()).to_degrees();

if h < 0.0 {
h + 360.0
} else {
h
}
};

(
(l / 100.0).max(0.0).min(1.5),
(c / 100.0).max(0.0).min(1.5),
h,
)
}
}

#[cfg(test)]
mod test {
use super::*;
Expand Down Expand Up @@ -214,4 +345,90 @@ mod test {
assert_eq!((saturation * 100.0).round() as u32, 83);
assert_eq!((lightness * 100.0).round() as u32, 51);
}

#[test]
fn lch_to_srgb() {
// "truth" from http://www.brucelindbloom.com/ColorCalculator.html

// black
let (lightness, chroma, hue) = (0.0, 0.0, 0.0);
let [r, g, b] = LchRepresentation::lch_to_nonlinear_srgb(lightness, chroma, hue);
assert_eq!((r * 100.0).round() as u32, 0);
assert_eq!((g * 100.0).round() as u32, 0);
assert_eq!((b * 100.0).round() as u32, 0);

// white
let (lightness, chroma, hue) = (1.0, 0.0, 0.0);
let [r, g, b] = LchRepresentation::lch_to_nonlinear_srgb(lightness, chroma, hue);
assert_eq!((r * 100.0).round() as u32, 100);
assert_eq!((g * 100.0).round() as u32, 100);
assert_eq!((b * 100.0).round() as u32, 100);

let (lightness, chroma, hue) = (0.501236, 0.777514, 327.6608);
let [r, g, b] = LchRepresentation::lch_to_nonlinear_srgb(lightness, chroma, hue);
assert_eq!((r * 100.0).round() as u32, 75);
assert_eq!((g * 100.0).round() as u32, 25);
assert_eq!((b * 100.0).round() as u32, 75);

// a red
let (lightness, chroma, hue) = (0.487122, 0.999531, 318.7684);
let [r, g, b] = LchRepresentation::lch_to_nonlinear_srgb(lightness, chroma, hue);
assert_eq!((r * 100.0).round() as u32, 70);
assert_eq!((g * 100.0).round() as u32, 19);
assert_eq!((b * 100.0).round() as u32, 90);

// a green
let (lightness, chroma, hue) = (0.732929, 0.560925, 164.3216);
let [r, g, b] = LchRepresentation::lch_to_nonlinear_srgb(lightness, chroma, hue);
assert_eq!((r * 100.0).round() as u32, 10);
assert_eq!((g * 100.0).round() as u32, 80);
assert_eq!((b * 100.0).round() as u32, 59);

// a blue
let (lightness, chroma, hue) = (0.335030, 1.176923, 306.7828);
let [r, g, b] = LchRepresentation::lch_to_nonlinear_srgb(lightness, chroma, hue);
assert_eq!((r * 100.0).round() as u32, 25);
assert_eq!((g * 100.0).round() as u32, 10);
assert_eq!((b * 100.0).round() as u32, 92);
}

#[test]
fn srgb_to_lch() {
// "truth" from http://www.brucelindbloom.com/ColorCalculator.html

// black
let (lightness, chroma, hue) = LchRepresentation::nonlinear_srgb_to_lch([0.0, 0.0, 0.0]);
assert_eq!((lightness * 100.0).round() as u32, 0);
assert_eq!((chroma * 100.0).round() as u32, 0);
assert_eq!(hue.round() as u32, 0);

// white
let (lightness, chroma, hue) = LchRepresentation::nonlinear_srgb_to_lch([1.0, 1.0, 1.0]);
assert_eq!((lightness * 100.0).round() as u32, 100);
assert_eq!((chroma * 100.0).round() as u32, 0);
assert_eq!(hue.round() as u32, 0);

let (lightness, chroma, hue) = LchRepresentation::nonlinear_srgb_to_lch([0.75, 0.25, 0.75]);
assert_eq!((lightness * 100.0).round() as u32, 50);
assert_eq!((chroma * 100.0).round() as u32, 78);
assert_eq!(hue.round() as u32, 328);

// a red
let (lightness, chroma, hue) = LchRepresentation::nonlinear_srgb_to_lch([0.70, 0.19, 0.90]);
assert_eq!((lightness * 100.0).round() as u32, 49);
assert_eq!((chroma * 100.0).round() as u32, 100);
assert_eq!(hue.round() as u32, 319);

// a green
let (lightness, chroma, hue) = LchRepresentation::nonlinear_srgb_to_lch([0.10, 0.80, 0.59]);
assert_eq!((lightness * 100.0).round() as u32, 73);
assert_eq!((chroma * 100.0).round() as u32, 56);
assert_eq!(hue.round() as u32, 164);

// a blue
let (lightness, chroma, hue) = LchRepresentation::nonlinear_srgb_to_lch([0.25, 0.10, 0.92]);
assert_eq!((lightness * 100.0).round() as u32, 34);
assert_eq!((chroma * 100.0).round() as u32, 118);
assert_eq!(hue.round() as u32, 307);
}
}
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