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/** @import { LabColor } from './color-types.d.js' */
const { sqrt, pow, cos, atan2, sin, abs, exp, PI, hypot } = Math
/**
* Returns diff between c1 and c2 using the CIEDE2000 algorithm
*
* Implemented as in "The CIEDE2000 Color-Difference Formula:
* Implementation Notes, Supplementary Test Data, and Mathematical Observations"
* by Gaurav Sharma, Wencheng Wu and Edul N. Dalal.
*
* @param {LabColor} c1 CIE L*a*b* color object, should have fields L,a,b
* @param {LabColor} c2 CIE L*a*b* color object, should have fields L,a,b
* @returns {number} Difference between c1 and c2
*/
export function ciede2000 (c1, c2) {
// Get L,a,b values for color 1
const L1 = c1.L
const a1 = c1.a
const b1 = c1.b
// Get L,a,b values for color 2
const L2 = c2.L
const a2 = c2.a
const b2 = c2.b
// Weight factors
const kL = 1
const kC = 1
const kH = 1
/**
* Step 1: Calculate C₁', C₂', h₁', h₂'
*/
const C1 = hypot(a1, b1) // C₁ (2)
const C2 = hypot(a2, b2) // C₂ (2)
const a_C1_C2 = (C1 + C2) / 2.0 // (3)
const G = 0.5 * (1 - sqrt(pow(a_C1_C2, 7.0) /
(pow(a_C1_C2, 7.0) + 25.0 ** 7.0))) // (4)
const a1p = (1.0 + G) * a1 // (5)
const a2p = (1.0 + G) * a2 // (5)
const C1p = hypot(a1p, b1) // C₁' (6)
const C2p = hypot(a2p, b2) // C₂' (6)
const h1p = hp_f(b1, a1p) // h₁' (7)
const h2p = hp_f(b2, a2p) // h₂' (7)
/**
* Step 2: Calculate 𝚫L', 𝚫C', 𝚫H'
*/
const dLp = L2 - L1 // 𝚫L' (8)
const dCp = C2p - C1p // 𝚫C' (9)
const dhp = dhp_f(C1, C2, h1p, h2p) // (10)
const dHp = 2 * sqrt(C1p * C2p) * sin(radians(dhp) / 2.0) // 𝚫H' (11)
/**
* Step 3: Calculate CIEDE2000 Color-Difference
*/
const a_L = (L1 + L2) / 2.0 // (12)
const a_Cp = (C1p + C2p) / 2.0 // (13)
const a_hp = a_hp_f(C1, C2, h1p, h2p) // (14)
const T = 1 - 0.17 * cos(radians(a_hp - 30)) + 0.24 * cos(radians(2 * a_hp)) +
0.32 * cos(radians(3 * a_hp + 6)) - 0.20 * cos(radians(4 * a_hp - 63)) // (15)
const d_ro = 30 * exp(-(pow((a_hp - 275) / 25, 2))) // (16)
const RC = sqrt((pow(a_Cp, 7.0)) / (pow(a_Cp, 7.0) + pow(25.0, 7.0)))// (17)
const SL = 1 + ((0.015 * pow(a_L - 50, 2)) /
sqrt(20 + pow(a_L - 50, 2.0)))// (18)
const SC = 1 + 0.045 * a_Cp// (19)
const SH = 1 + 0.015 * a_Cp * T// (20)
const RT = -2 * RC * sin(radians(2 * d_ro))// (21)
// (22)
return sqrt(pow(dLp / (SL * kL), 2) + pow(dCp / (SC * kC), 2) +
pow(dHp / (SH * kH), 2) + RT * (dCp / (SC * kC)) *
(dHp / (SH * kH)))
}
/**
* Calculates h' value, follows the next formula:
*
* ⎧ 0 b*=a'=0
* h' = ⎨
* ⎩ tan⁻¹(b*, a') otherwise
*
* @param {number} x - b* value
* @param {number} y - a' value
* @returns {number} h' value
*/
function hp_f (x, y) { // (7)
if (x === 0 && y === 0) return 0
else {
const tmp_hp = degrees(atan2(x, y))
if (tmp_hp >= 0) return tmp_hp
else return tmp_hp + 360
}
}
/**
* Calculates 𝚫h' value, follows the next formula:
*
* ⎧ 0 C₁'C₂'=0
* ⎪
* ⎪ h₂'-h₁' C₁'C₂'≠0; |h₂'-h₁'|≤180°
* 𝚫h' = ⎨
* ⎪ h₂'-h₁'-360 C₁'C₂'≠0; (h₂'-h₁')>180°
* ⎪
* ⎩ h₂'-h₁'+360 C₁'C₂'≠0; (h₂'-h₁')<-180°
*
* @param {number} C1 - C₁' value
* @param {number} C2 - C₂' value
* @param {number} h1p - h₁' value
* @param {number} h2p - h₂' value
* @returns {number} 𝚫h' value
*/
function dhp_f (C1, C2, h1p, h2p) { // (10)
if (C1 * C2 === 0) return 0
else if (abs(h2p - h1p) <= 180) return h2p - h1p
else if ((h2p - h1p) > 180) return (h2p - h1p) - 360
else if ((h2p - h1p) < -180) return (h2p - h1p) + 360
else throw (new Error())
}
/**
* Calculates h͞' value, follows the next formula:
*
* ⎧ h₁'+h₂' C₁'C₂'=0
* ⎪
* ⎪ h₁'+h₂' C₁'C₂'≠0; |h₁'-h₂'|≤180°
* ⎪ ───────
* ⎪ 2
* ⎪
* h͞' = ⎨ h₁'+h₂'+360 C₁'C₂'≠0; |h₁'-h₂'|>180°; (h₁'+h₂')<360°
* ⎪ ───────────
* ⎪ 2
* ⎪
* ⎪ h₁'+h₂'-360 C₁'C₂'≠0; |h₁'-h₂'|>180°; (h₁'+h₂')≥360°
* ⎪ ───────────
* ⎩ 2
*
* @param {number} C1 - C₁' value
* @param {number} C2 - C₂' value
* @param {number} h1p - h₁' value
* @param {number} h2p - h₂' value
* @returns {number} 𝚫h' value
*/
function a_hp_f (C1, C2, h1p, h2p) { // (14)
if (C1 * C2 === 0) return h1p + h2p
else if (abs(h1p - h2p) <= 180) return (h1p + h2p) / 2.0
else if ((abs(h1p - h2p) > 180) && ((h1p + h2p) < 360)) return (h1p + h2p + 360) / 2.0
else if ((abs(h1p - h2p) > 180) && ((h1p + h2p) >= 360)) return (h1p + h2p - 360) / 2.0
else throw (new Error())
}
/**
* Converts radians to degrees
* @param {number} radians - number in radians
*/
function degrees (radians) { return radians * (180 / PI) }
/**
* Converts degrees to radians
* @param {number} degrees - number in degrees
*/
function radians (degrees) { return degrees * (PI / 180) }
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