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Adds Perceptual Quantizer (PQ) transfer functions #218

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Merged
merged 1 commit into from
Jan 7, 2024
Merged

Adds Perceptual Quantizer (PQ) transfer functions #218

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30 changes: 30 additions & 0 deletions src/hdr/transfer.js
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Original file line number Diff line number Diff line change
@@ -0,0 +1,30 @@
/*
https://en.wikipedia.org/wiki/Transfer_functions_in_imaging
*/

export const M1 = 0.1593017578125;
export const M2 = 78.84375;
export const C1 = 0.8359375;
export const C2 = 18.8515625;
export const C3 = 18.6875;

/*
Perceptual Quantizer, as defined in Rec. BT 2100-2 (2018)

* https://www.itu.int/rec/R-REC-BT.2100-2-201807-I/en
* https://en.wikipedia.org/wiki/Perceptual_quantizer
*/

/* PQ EOTF, defined for `v` in [0,1]. */
export function transferPqDecode(v) {
if (v < 0) return 0;
const c = Math.pow(v, 1 / M2);
return 1e4 * Math.pow(Math.max(0, c - C1) / (C2 - C3 * c), 1 / M1);
}

/* PQ EOTF^-1, defined for `v` in [0, 1e4]. */
export function transferPqEncode(v) {
if (v < 0) return 0;
const c = Math.pow(v / 1e4, M1);
return Math.pow((C1 + C2 * c) / (1 + C3 * c), M2);
}
22 changes: 12 additions & 10 deletions src/jab/convertJabToXyz65.js
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Original file line number Diff line number Diff line change
@@ -1,24 +1,26 @@
const n = 0.1593017578125; // = 2610 / Math.pow(2, 14);
import { M1 as n, C1, C2, C3 } from '../hdr/transfer.js';
const p = 134.03437499999998; // = 1.7 * 2523 / Math.pow(2, 5);
const c1 = 0.8359375; // = 3424 / Math.pow(2, 12);
const c2 = 18.8515625; // = 2413 / Math.pow(2, 7);
const c3 = 18.6875; // = 2392 / Math.pow(2, 7);
const d0 = 1.6295499532821566e-11;

/* `v` may be negative, in which case return 0 instead of NaN */
const pq_inv = v => {
const npow = (v, exp) => Math.sign(v) * Math.pow(Math.abs(v), exp);

/*
The encoding function is derived from Perceptual Quantizer.
*/
const jabPqDecode = v => {
if (v < 0) return 0;
let vp = Math.pow(v, 1 / p);
return 10000 * Math.pow((c1 - vp) / (c3 * vp - c2), 1 / n) || 0;
return 10000 * Math.pow((C1 - vp) / (C3 * vp - C2), 1 / n);
};

const rel = v => v / 203;

const convertJabToXyz65 = ({ j, a, b, alpha }) => {
let i = (j + d0) / (0.44 + 0.56 * (j + d0));

let l = pq_inv(i + 0.13860504 * a + 0.058047316 * b);
let m = pq_inv(i - 0.13860504 * a - 0.058047316 * b);
let s = pq_inv(i - 0.096019242 * a - 0.8118919 * b);
let l = jabPqDecode(i + 0.13860504 * a + 0.058047316 * b);
let m = jabPqDecode(i - 0.13860504 * a - 0.058047316 * b);
let s = jabPqDecode(i - 0.096019242 * a - 0.8118919 * b);

let res = {
mode: 'xyz65',
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20 changes: 10 additions & 10 deletions src/jab/convertXyz65ToJab.js
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Original file line number Diff line number Diff line change
@@ -1,14 +1,14 @@
const n = 0.1593017578125; // = 2610 / Math.pow(2, 14);
import { M1 as n, C1, C2, C3 } from '../hdr/transfer.js';
const p = 134.03437499999998; // = 1.7 * 2523 / Math.pow(2, 5);
const c1 = 0.8359375; // = 3424 / Math.pow(2, 12);
const c2 = 18.8515625; // = 2413 / Math.pow(2, 7);
const c3 = 18.6875; // = 2392 / Math.pow(2, 7);
const d0 = 1.6295499532821566e-11;

/* `v` may be negative, in which case return 0 instead of NaN */
const pq = v => {
/*
The encoding function is derived from Perceptual Quantizer.
*/
const jabPqEncode = v => {
if (v < 0) return 0;
let vn = Math.pow(v / 10000, n);
return Math.pow((c1 + c2 * vn) / (1 + c3 * vn), p) || 0;
return Math.pow((C1 + C2 * vn) / (1 + C3 * vn), p);
};

// Convert to Absolute XYZ
Expand All @@ -22,9 +22,9 @@ const convertXyz65ToJab = ({ x, y, z, alpha }) => {
let xp = 1.15 * x - 0.15 * z;
let yp = 0.66 * y + 0.34 * x;

let l = pq(0.41478972 * xp + 0.579999 * yp + 0.014648 * z);
let m = pq(-0.20151 * xp + 1.120649 * yp + 0.0531008 * z);
let s = pq(-0.0166008 * xp + 0.2648 * yp + 0.6684799 * z);
let l = jabPqEncode(0.41478972 * xp + 0.579999 * yp + 0.014648 * z);
let m = jabPqEncode(-0.20151 * xp + 1.120649 * yp + 0.0531008 * z);
let s = jabPqEncode(-0.0166008 * xp + 0.2648 * yp + 0.6684799 * z);

let i = (l + m) / 2;

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