light: the HIGHLIGHT knob rides half its anchor too, so a lift stops drawing a cloud to paper and a pull stops flattening the quarter under it — the top quarter came back at 0.26x of its own contrast, 0.62x now

docker/frontend/scripts/highlight-knee-check.mjs

@@ -16,15 +16,16 @@
 // THE TONE PASS builds the luma a new ramp instead. The four knobs
 // (HIGHLIGHT, SHADOW, WHITE, BLACK) are the four zones of the tone-mapping doc —
 // one tent each, one per quarter of the ramp — and each knob moves the knot it
-// owns by TONE_ANCHOR of the ramp, held inside the knot before it; SHADOW, whose
-// knot is the head of the band above it rather than an end of the ramp, moves
-// half that. The 0.50 midpoint is the one value all four leave where it was.
+// owns by TONE_ANCHOR of the ramp, held inside the knot before it; SHADOW and
+// HIGHLIGHT, whose knots are the HEAD of a quarter rather than an end of the
+// ramp, move half that. The 0.50 midpoint is the one value all four leave where
+// it was.
 //
 // Both are SkSL, so the SHAPE is pinned on the source; the arithmetic is then
 // checked against the source's own constants, and the ramp re-run here as a twin
 // so monotonicity, the neutral identity, the partition of the four masks and the
-// slope SHADOW leaves the band above it are checked rather than asserted in a
-// comment.
+// slopes SHADOW and HIGHLIGHT leave the quarters around them are checked rather
+// than asserted in a comment.
 //
 //   node scripts/highlight-knee-check.mjs
 import assert from 'node:assert/strict';
@@ -84,7 +85,7 @@ assert.match(sksl, /float hlMask = clamp\(smoothstep\(0\.50, 0\.75, t\) - whMask
 // pair of masked terms (0.12 at t = 0, 0.18 at t = 1) and half of each at the
 // knots next to them.
 assert.match(sksl, /float a4 = 1\.0 \+ 0\.25 \* wh - dr \* 0\.18;/);
-assert.match(sksl, /float a3 = clamp\(0\.75 \+ 0\.25 \* hl - dr \* 0\.09, 0\.5, a4\);/);
+assert.match(sksl, /float a3 = clamp\(0\.75 \+ 0\.25 \* 0\.5 \* hl - dr \* 0\.09, 0\.5, a4\);/);
 assert.match(sksl, /float a1 = clamp\(0\.25 \+ 0\.25 \* 0\.5 \* sh \+ dr \* 0\.06, 0\.0, 0\.5\);/);
 assert.match(sksl, /float a0 = clamp\(0\.25 \* bl \+ dr \* 0\.12, 0\.0, a1\);/);
 assert.doesNotMatch(sksl, /o \+= dr \* 0\.12/, 'DR is an additive term again — it folds the flat stretch at 0.238');
@@ -174,7 +175,7 @@ function ramp(t, k) {
   const whMask = smoothstep(0.75, 1, t);
   const hlMask = clamp01(smoothstep(0.5, 0.75, t) - whMask);
   const a4 = 1 + A * wh - dr * 0.18;
-  const a3 = Math.min(a4, Math.max(0.5, 0.75 + A * hl - dr * 0.09));
+  const a3 = Math.min(a4, Math.max(0.5, 0.75 + A * 0.5 * hl - dr * 0.09));
   const a1 = Math.min(0.5, Math.max(0, 0.25 + A * 0.5 * sh + dr * 0.06));
   const a0 = Math.min(a1, Math.max(0, A * bl + dr * 0.12));
   let o = mix(a0, a1, lin(0, 0.25, t));
@@ -237,16 +238,18 @@ for (const k of combos) {
 }
 assert.ok(worst > -1e-12, `worst step ${worst} — the ramp is folded`);
 // A knob moves its own quarter, and only its own: +BLACK takes the toe off the
-// floor, -HIGHLIGHT pulls the 0.75 knot onto the midpoint, and WHITE - rolls the
-// head under 1.0. That is the reach a tonal-range slider has — a quarter of the
-// ramp, so the middle stays a middle. SHADOW is the exception and moves HALF of
-// it: its knot is the HEAD of the 0.25..0.50 band, so a whole quarter would draw
-// that band flat, which is the wash-out the knob was reported for.
+// floor, -HIGHLIGHT rolls the 0.75 knot down a quarter of the way to the
+// midpoint, and WHITE - rolls the head under 1.0. That is the reach a
+// tonal-range slider has — a quarter of the ramp, so the middle stays a middle.
+// SHADOW and HIGHLIGHT are the exceptions and move HALF of it: their knots (a1,
+// a3) are the HEAD of the quarter in front of them, so a whole quarter would draw
+// that quarter flat, which is the wash-out both knobs were reported for.
 close(ramp(0, {}).o, 0, 'a neutral toe moved');
 close(ramp(0, { bl: 1 }).o, A, 'BLACK no longer reaches a quarter of the ramp');
 close(ramp(0.25, { sh: 1 }).o, 0.375, 'SHADOW no longer stops halfway to the midpoint');
 close(ramp(0.25, { sh: -1 }).o, 0.125, 'SHADOW no longer stops halfway to the floor');
-close(ramp(0.75, { hl: -1 }).o, 0.5, 'HIGHLIGHT no longer reaches the midpoint');
+close(ramp(0.75, { hl: -1 }).o, 0.625, 'HIGHLIGHT no longer stops halfway to the midpoint');
+close(ramp(0.75, { hl: 1 }).o, 0.875, 'HIGHLIGHT no longer stops halfway to the white knot');
 close(ramp(1, { wh: -1 }).o, 0.75, 'WHITE no longer rolls the head under 1.0');
 close(ramp(0.25, {}).o, 0.25, 'a neutral knot moved');
 close(ramp(0.75, {}).o, 0.75, 'a neutral knot moved');
@@ -285,26 +288,45 @@ for (let sh = -1; sh <= 1.0001; sh += 1 / 64) {
   assert.ok(below >= 0.5 - 1e-12, `SHADOW drew the quarter below it flat at ${sh}: slope ${below}`);
   assert.ok(band <= 1.5 + 1e-12, `SHADOW stretched the band above it at ${sh}: slope ${band}`);
 }
-// The film stocks ride the same knot, so a halved SHADOW would have halved their
-// crush with it. They are written at DOUBLE for that reason, and the look they
-// were tuned to is the knot, not the unit: these three land where they always
-// did (0.18 Classic Chrome/Vivid, 0.22 Acros, 0.17 Acros HC).
+// HIGHLIGHT pays the same way and at BOTH ends, which is why its knot is the
+// worse of the two: a3 is the head of the top quarter, so lifting it draws
+// 0.75..1.00 flat (a cloud to paper) and pulling it draws 0.50..0.75 flat. At the
+// whole anchor the sweep in scratchpad/knob-sweep.mjs read the quarter slopes as
+// 1.00 1.00 2.00 0.00 at +100 and 1.00 1.00 0.00 2.00 at -100 — a quarter flat
+// either way. Half draws neither, over the whole travel.
+for (let hl = -1; hl <= 1.0001; hl += 1 / 64) {
+  const mid = (ramp(0.75, { hl }).o - ramp(0.5, { hl }).o) / 0.25;
+  const top = (ramp(1, { hl }).o - ramp(0.75, { hl }).o) / 0.25;
+  assert.ok(mid >= 0.5 - 1e-12, `HIGHLIGHT drew the quarter below it flat at ${hl}: slope ${mid}`);
+  assert.ok(top >= 0.5 - 1e-12, `HIGHLIGHT drew the top quarter flat at ${hl}: slope ${top}`);
+  assert.ok(mid <= 1.5 + 1e-12, `HIGHLIGHT stretched the quarter below it at ${hl}: slope ${mid}`);
+  assert.ok(top <= 1.5 + 1e-12, `HIGHLIGHT stretched the top quarter at ${hl}: slope ${top}`);
+}
+// The film stocks ride the same knots, so a halved SHADOW or HIGHLIGHT would have
+// halved their crush and their shoulder with it. They are written at DOUBLE for
+// that reason, and the look they were tuned to is the knot, not the unit: these
+// land where they always did (0.18 Classic Chrome/Vivid, 0.22 Acros, 0.17 Acros
+// HC; the head 0.7375 on Acros and 0.815 on Acros HC).
 const filmTone = tone.match(/const FILM_TONE[\s\S]*?\n};/)?.[0];
 assert.ok(filmTone, 'FILM_TONE is gone — the stocks no longer shape the ramp at all');
 // The keys are quoted or not depending on whether they are identifiers, so the
 // quotes come off before the lookup.
 const filmFlat = filmTone.replace(/['"]/g, '');
-for (const [name, sh, knot] of [
+for (const [name, sh, knot, hlSrc, hl, head] of [
   ['classic-chrome', -0.56, 0.18],
   ['classic-vivid', -0.56, 0.18],
-  ['monochrome', -0.24, 0.22],
-  ['mono-high-contrast', -0.64, 0.17],
+  ['monochrome', -0.24, 0.22, '-0.10', -0.1, 0.7375],
+  ['mono-high-contrast', -0.64, 0.17, '0.52', 0.52, 0.815],
 ]) {
   assert.ok(
     filmFlat.includes(`${name}: { sh: ${sh}`),
     `${name} is not on the doubled ${sh} — the stock's crush moved with the knob's reach`
   );
   close(ramp(0.25, { sh }).o, knot, `${name}'s shadow knot moved`);
+  if (hlSrc) {
+    assert.ok(filmFlat.includes(`hl: ${hlSrc}`), `${name} is not on the doubled ${hlSrc} — its shoulder moved with the knob's reach`);
+    close(ramp(0.75, { hl }).o, head, `${name}'s highlight knot moved`);
+  }
 }
 // The two ends stay ordered even at full deflection against each other: the toe
 // can never climb past the head.