Adjustments to itp color space:

* Add references in Color Spaces docs
* Avoid creating intermediary objects in conversion functions
* Handle missing (`none`) components in the conversion functions
* Clean up HDR constants

Author: danburzo

docs/color-spaces.md

@@ -442,15 +442,17 @@ It has the default _Chroma from Luma_ adjustment applied (effectively Y is subtr
 
 Does not have gamut limits.
 
-#### `itp`
+### <abbr>IC<sub>t</sub>C<sub>p</sub></abbr>
+
+<abbr>IC<sub>t</sub>C<sub>p</sub></abbr> (or <abbr>ITP</abbr>) color space developed by [Dolby Laboratories](https://professional.dolby.com/siteassets/pdfs/ictcp_dolbywhitepaper_v071.pdf), as defined in [<abbr>ITU-R</abbr> Recommendation BT.2100](https://www.itu.int/rec/R-REC-BT.2100). It is included in the <abbr>CSS</abbr> [<abbr>HDR</abbr> Color Module Level 1](https://drafts.csswg.org/css-color-hdr/#ICtCp) specification.
 
-IC<sub>t</sub>C<sub>p</sub> (or ITP) color space, as defined in ITU-R Recommendation BT.2100.
+#### `itp`
 
 | Channel | Range             | Description           |
 |---------|-------------------|-----------------------|
 | `i`     | `[0, 0.581]`≈     | Intensity             |
-| `t`    | `[-0.282, 0.278]`≈ | Blue-yellow component |
-| `p`    | `[-0.162, 0.279]`≈ | Green–red component   |
+| `t`    | `[-0.282, 0.278]`≈ | Blue-yellow component (“tritanopia”) |
+| `p`    | `[-0.162, 0.279]`≈ | Green–red component (“protanopia”)  |
 
 Serialized as `color(--ictcp i t p)`, with the `none` keyword for any missing color channel. An explicit `alpha < 1` is included as ` / alpha`.
 

src/hdr/constants.js

@@ -0,0 +1,6 @@
+/*
+	Relative XYZ has Y=1 for media white,
+	BT.2048 says media white Y=203 (at PQ 58).
+	See: https://www.itu.int/dms_pub/itu-r/opb/rep/R-REP-BT.2408-3-2019-PDF-E.pdf
+*/
+export const YW = 203;

src/itp/constants.js

@@ -1,22 +1,42 @@
-import { YW } from './constants.js';
+import { YW } from '../hdr/constants.js';
 import { transferPqDecode } from '../hdr/transfer.js';
 
+const toRel = c => Math.max(c / YW, 0);
+
 const convertItpToXyz65 = ({ i, t, p, alpha }) => {
-	const [l, m, s] = [
-		i + 0.008609037037932761 * t + 0.11102962500302593 * p,
-		i - 0.00860903703793275 * t - 0.11102962500302599 * p,
+	if (i === undefined) i = 0;
+	if (t === undefined) t = 0;
+	if (p === undefined) p = 0;
+
+	const l = transferPqDecode(
+		i + 0.008609037037932761 * t + 0.11102962500302593 * p
+	);
+	const m = transferPqDecode(
+		i - 0.00860903703793275 * t - 0.11102962500302599 * p
+	);
+	const s = transferPqDecode(
 		i + 0.5600313357106791 * t - 0.32062717498731885 * p
-	].map(transferPqDecode);
+	);
 
-	const [x, y, z] = [
-		2.0701522183894219 * l -
-			1.3263473389671556 * m +
-			0.2066510476294051 * s,
-		0.3647385209748074 * l + 0.680566024947227 * m - 0.0453045459220346 * s,
-		-0.049747207535812 * l - 0.0492609666966138 * m + 1.1880659249923042 * s
-	].map(c => Math.max(c / YW, 0));
+	const res = {
+		mode: 'xyz65',
+		x: toRel(
+			2.0701522183894219 * l -
+				1.3263473389671556 * m +
+				0.2066510476294051 * s
+		),
+		y: toRel(
+			0.3647385209748074 * l +
+				0.680566024947227 * m -
+				0.0453045459220346 * s
+		),
+		z: toRel(
+			-0.049747207535812 * l -
+				0.0492609666966138 * m +
+				1.1880659249923042 * s
+		)
+	};
 
-	const res = { mode: 'xyz65', x, y, z };
 	if (alpha !== undefined) {
 		res.alpha = alpha;
 	}

src/itp/convertItpToXyz65.js

@@ -1,23 +1,31 @@
-import { YW } from './constants.js';
+import { YW } from '../hdr/constants.js';
 import { transferPqEncode } from '../hdr/transfer.js';
 
+const toAbs = (c = 0) => Math.max(c * YW, 0);
+
 const convertXyz65ToItp = ({ x, y, z, alpha }) => {
-	const [absX, absY, absZ] = [x, y, z].map(c => Math.max(c * YW, 0));
-	const [l, m, s] = [
+	const absX = toAbs(x);
+	const absY = toAbs(y);
+	const absZ = toAbs(z);
+	const l = transferPqEncode(
 		0.3592832590121217 * absX +
 			0.6976051147779502 * absY -
-			0.0358915932320289 * absZ,
+			0.0358915932320289 * absZ
+	);
+	const m = transferPqEncode(
 		-0.1920808463704995 * absX +
 			1.1004767970374323 * absY +
-			0.0753748658519118 * absZ,
+			0.0753748658519118 * absZ
+	);
+	const s = transferPqEncode(
 		0.0070797844607477 * absX +
 			0.0748396662186366 * absY +
 			0.8433265453898765 * absZ
-	].map(transferPqEncode);
+	);
 
 	const i = 0.5 * l + 0.5 * m;
-	const t = 1.61376953125 * l + -3.323486328125 * m + 1.709716796875 * s;
-	const p = 4.378173828125 * l + -4.24560546875 * m + -0.132568359375 * s;
+	const t = 1.61376953125 * l - 3.323486328125 * m + 1.709716796875 * s;
+	const p = 4.378173828125 * l - 4.24560546875 * m - 0.132568359375 * s;
 
 	const res = { mode: 'itp', i, t, p };
 	if (alpha !== undefined) {