1 | import { epsilon, kappa, wd50X, wd50Y, wd50Z, matrix } from './util';
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2 |
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3 | /* Convert between Lab and XYZ
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4 | /* ========================================================================== */
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5 |
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6 | export function lab2xyz(labL, labA, labB) {
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7 | // compute f, starting with the luminance-related term
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8 | const f2 = (labL + 16) / 116;
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9 | const f1 = labA / 500 + f2;
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10 | const f3 = f2 - labB / 200;
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11 |
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12 | // compute pre-scaled XYZ
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13 | const [ initX, initY, initZ ] = [
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14 | Math.pow(f1, 3) > epsilon ? Math.pow(f1, 3) : (116 * f1 - 16) / kappa,
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15 | labL > kappa * epsilon ? Math.pow((labL + 16) / 116, 3) : labL / kappa,
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16 | Math.pow(f3, 3) > epsilon ? Math.pow(f3, 3) : (116 * f3 - 16) / kappa
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17 | ];
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18 |
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19 | const [ xyzX, xyzY, xyzZ ] = matrix(
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20 | // compute XYZ by scaling pre-scaled XYZ by reference white
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21 | [ initX * wd50X, initY * wd50Y, initZ * wd50Z ],
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22 | // calculate D65 XYZ from D50 XYZ
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23 | [
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24 | [ 0.9555766, -0.0230393, 0.0631636],
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25 | [-0.0282895, 1.0099416, 0.0210077],
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26 | [ 0.0122982, -0.0204830, 1.3299098]
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27 | ]
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28 | );
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29 |
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30 | return [ xyzX, xyzY, xyzZ ];
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31 | }
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32 |
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33 | export function xyz2lab(xyzX, xyzY, xyzZ) {
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34 | // calculate D50 XYZ from D65 XYZ
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35 | const [ d50X, d50Y, d50Z ] = matrix([ xyzX, xyzY, xyzZ ], [
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36 | [ 1.0478112, 0.0228866, -0.0501270],
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37 | [ 0.0295424, 0.9904844, -0.0170491],
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38 | [-0.0092345, 0.0150436, 0.7521316]
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39 | ]);
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40 |
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41 | // calculate f
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42 | const [ f1, f2, f3 ] = [
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43 | d50X / wd50X,
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44 | d50Y / wd50Y,
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45 | d50Z / wd50Z
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46 | ].map(
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47 | value => value > epsilon ? Math.cbrt(value) : (kappa * value + 16) / 116
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48 | );
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49 |
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50 | const [ labL, labA, labB ] = [
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51 | 116 * f2 - 16,
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52 | 500 * (f1 - f2),
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53 | 200 * (f2 - f3)
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54 | ];
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55 |
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56 | return [ labL, labA, labB ];
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57 | }
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58 |
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59 | /*
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60 |
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61 | References
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62 | ----------
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63 |
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64 | - https://www.w3.org/TR/css-color-4/#rgb-to-lab
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65 | - https://www.w3.org/TR/css-color-4/#color-conversion-code
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66 | - https://www.easyrgb.com/en/math.php
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67 |
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68 | /* ========================================================================== */
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