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ictcp.js
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ictcp.js
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import ColorSpace from "../ColorSpace.js";
import {multiplyMatrices} from "../util.js";
import XYZ_Abs_D65 from "./xyz-abs-d65.js";
const c1 = 3424 / 4096;
const c2 = 2413 / 128;
const c3 = 2392 / 128;
const m1 = 2610 / 16384;
const m2 = 2523 / 32;
const im1 = 16384 / 2610;
const im2 = 32 / 2523;
// The matrix below includes the 4% crosstalk components
// and is from the Dolby "What is ICtCp" paper"
const XYZtoLMS_M = [
[ 0.3592832590121217, 0.6976051147779502, -0.0358915932320290 ],
[ -0.1920808463704993, 1.1004767970374321, 0.0753748658519118 ],
[ 0.0070797844607479, 0.0748396662186362, 0.8433265453898765 ],
];
// linear-light Rec.2020 to LMS, again with crosstalk
// rational terms from Jan Fröhlich,
// Encoding High Dynamic Range andWide Color Gamut Imagery, p.97
// and ITU-R BT.2124-0 p.2
/*
const Rec2020toLMS_M = [
[ 1688 / 4096, 2146 / 4096, 262 / 4096 ],
[ 683 / 4096, 2951 / 4096, 462 / 4096 ],
[ 99 / 4096, 309 / 4096, 3688 / 4096 ]
];
*/
// this includes the Ebner LMS coefficients,
// the rotation, and the scaling to [-0.5,0.5] range
// rational terms from Fröhlich p.97
// and ITU-R BT.2124-0 pp.2-3
const LMStoIPT_M = [
[ 2048 / 4096, 2048 / 4096, 0 ],
[ 6610 / 4096, -13613 / 4096, 7003 / 4096 ],
[ 17933 / 4096, -17390 / 4096, -543 / 4096 ],
];
// inverted matrices, calculated from the above
const IPTtoLMS_M = [
[ 0.9999999999999998, 0.0086090370379328, 0.1110296250030260 ],
[ 0.9999999999999998, -0.0086090370379328, -0.1110296250030259 ],
[ 0.9999999999999998, 0.5600313357106791, -0.3206271749873188 ],
];
/*
const LMStoRec2020_M = [
[ 3.4375568932814012112, -2.5072112125095058195, 0.069654319228104608382],
[-0.79142868665644156125, 1.9838372198740089874, -0.19240853321756742626 ],
[-0.025646662911506476363, -0.099240248643945566751, 1.1248869115554520431 ]
];
*/
const LMStoXYZ_M = [
[ 2.0701522183894223, -1.3263473389671563, 0.2066510476294053 ],
[ 0.3647385209748072, 0.6805660249472273, -0.0453045459220347 ],
[ -0.0497472075358123, -0.0492609666966131, 1.1880659249923042 ],
];
// Only the PQ form of ICtCp is implemented here. There is also an HLG form.
// from Dolby, "WHAT IS ICTCP?"
// https://professional.dolby.com/siteassets/pdfs/ictcp_dolbywhitepaper_v071.pdf
// and
// Dolby, "Perceptual Color Volume
// Measuring the Distinguishable Colors of HDR and WCG Displays"
// https://professional.dolby.com/siteassets/pdfs/dolby-vision-measuring-perceptual-color-volume-v7.1.pdf
export default new ColorSpace({
id: "ictcp",
name: "ICTCP",
// From BT.2100-2 page 7:
// During production, signal values are expected to exceed the
// range E′ = [0.0 : 1.0]. This provides processing headroom and avoids
// signal degradation during cascaded processing. Such values of E′,
// below 0.0 or exceeding 1.0, should not be clipped during production
// and exchange.
// Values below 0.0 should not be clipped in reference displays (even
// though they represent “negative” light) to allow the black level of
// the signal (LB) to be properly set using test signals known as “PLUGE”
coords: {
i: {
refRange: [0, 1], // Constant luminance,
name: "I",
},
ct: {
refRange: [-0.5, 0.5], // Full BT.2020 gamut in range [-0.5, 0.5]
name: "CT",
},
cp: {
refRange: [-0.5, 0.5],
name: "CP",
},
},
base: XYZ_Abs_D65,
fromBase (XYZ) {
// move to LMS cone domain
let LMS = multiplyMatrices(XYZtoLMS_M, XYZ);
return LMStoICtCp(LMS);
},
toBase (ICtCp) {
let LMS = ICtCptoLMS(ICtCp);
return multiplyMatrices(LMStoXYZ_M, LMS);
},
});
function LMStoICtCp (LMS) {
// apply the PQ EOTF
// we can't ever be dividing by zero because of the "1 +" in the denominator
let PQLMS = LMS.map (function (val) {
let num = c1 + (c2 * ((val / 10000) ** m1));
let denom = 1 + (c3 * ((val / 10000) ** m1));
return (num / denom) ** m2;
});
// LMS to IPT, with rotation for Y'C'bC'r compatibility
return multiplyMatrices(LMStoIPT_M, PQLMS);
}
function ICtCptoLMS (ICtCp) {
let PQLMS = multiplyMatrices(IPTtoLMS_M, ICtCp);
// From BT.2124-0 Annex 2 Conversion 3
let LMS = PQLMS.map (function (val) {
let num = Math.max((val ** im2) - c1, 0);
let denom = (c2 - (c3 * (val ** im2)));
return 10000 * ((num / denom) ** im1);
});
return LMS;
}