android/app/src/main/java/com/fabledsword/thoughtsync/ui/DerivedTint.kt

@@ -158,7 +158,7 @@ fun noteColorIsChosen(
 // at most a 1.03 contrast ratio, which is to say not at all. Nine tints that look
 // like three is exactly the wall the tint was added to break up.
 //
-// So this hashes to a colour directly rather than to a key. 324 distinct fills in
+// So this hashes to a colour directly rather than to a key. 338 distinct fills in
 // dark, 193 in light, against nine.
 //
 // TWO AXES, AND THE SECOND ONE IS THE FIX. The old ramp varied hue while pinning
@@ -185,12 +185,23 @@ private const val DARK_SATURATION = 0.25
 private const val LIGHT_SATURATION = 0.60
 
 /**
- * Dark starts at 0.090 — a hair under `neutral-900`, the plain card surface — and
+ * HSL LIGHTNESS IS NOT LUMINANCE, and the floor here is set by the difference.
- * climbs. Nothing is ever darker than an untinted card, so no note recedes into the
+ *
- * board; they only ever rise off it. Top of the range measures 1.54 against the
+ * The obvious floor is `neutral-900`'s own lightness, 0.090 — "start at the plain card
- * board where the old single level managed 1.14.
+ * surface and climb, so no note ever recedes into the board". That was the first
+ * attempt and it was wrong, because at a FIXED HSL lightness the eye sees wildly
+ * different brightnesses depending on hue: green carries 71% of the luminance formula
+ * and blue only 7%, so at L=0.090 a yellow measures 0.0118 and a blue 0.0061 — the
+ * blue landing 1.41x DARKER than the card it was supposed to match. A sixth of the
+ * board would have been holes rather than variety.
+ *
+ * 0.113 is the lowest floor at which EVERY hue clears the card surface, solved for
+ * rather than guessed. The range then measures 1.11–1.71 against the board where the
+ * old single level managed 1.14, so the floor is unchanged and the ceiling is much
+ * higher. `no generated fill sinks below the card surface` in DerivedTintTest is what
+ * caught the original mistake and what holds this.
  */
-private val DARK_LIGHTNESS = doubleArrayOf(0.090, 0.104, 0.118, 0.132, 0.146, 0.160)
+private val DARK_LIGHTNESS = doubleArrayOf(0.113, 0.127, 0.141, 0.155, 0.169, 0.183)
 
 /**
  * Light runs the other way, from white down toward the `neutral-50` board and just

android/app/src/test/java/com/fabledsword/thoughtsync/ui/DerivedTintTest.kt

@@ -196,10 +196,10 @@ class DerivedTintTest {
      *  actually compared. The same four ids and hexes are a comment in colors.ts. */
     @Test
     fun `generated fills match the fixture shared with the web`() {
-        assertEquals("#192a29", hex(derivedFillArgb("00000000-0000-0000-0000-000000000000", dark = true)))
+        assertEquals("#1e3130", hex(derivedFillArgb("00000000-0000-0000-0000-000000000000", dark = true)))
-        assertEquals("#152114", hex(derivedFillArgb("11111111-1111-1111-1111-111111111111", dark = true)))
+        assertEquals("#1a2818", hex(derivedFillArgb("11111111-1111-1111-1111-111111111111", dark = true)))
-        assertEquals("#111d14", hex(derivedFillArgb("6ba7b810-9dad-11d1-80b4-00c04fd430c8", dark = true)))
+        assertEquals("#162419", hex(derivedFillArgb("6ba7b810-9dad-11d1-80b4-00c04fd430c8", dark = true)))
-        assertEquals("#2a191b", hex(derivedFillArgb("f47ac10b-58cc-4372-a567-0e02b2c3d479", dark = true)))
+        assertEquals("#311e20", hex(derivedFillArgb("f47ac10b-58cc-4372-a567-0e02b2c3d479", dark = true)))
 
         assertEquals("#f3fcfb", hex(derivedFillArgb("00000000-0000-0000-0000-000000000000", dark = false)))
         assertEquals("#fbfefb", hex(derivedFillArgb("11111111-1111-1111-1111-111111111111", dark = false)))
@@ -234,14 +234,35 @@ class DerivedTintTest {
         assertEquals(true, levels.max() / levels.min() > 2.0)
     }
 
-    /** Nothing may be darker than the plain card surface (`neutral-900`, #171717) in
+    /**
-     *  dark, or the note recedes into the near-black board instead of sitting on it. */
+     * Nothing may be darker than the plain card surface (`neutral-900`, #171717) in
+     * dark, or the note reads as a hole in the board rather than a card on it.
+     *
+     * THIS TEST HAS ALREADY EARNED ITS KEEP. The first floor was 0.090 — `neutral-900`'s
+     * own HSL lightness — on the reasoning that starting at the card surface and
+     * climbing could not possibly go below it. That reasoning confuses HSL lightness
+     * with luminance. At one fixed lightness the eye sees very different brightnesses
+     * by hue, because green carries 71% of the luminance formula and blue 7%: at 0.090
+     * a yellow measures 0.0118 and a blue 0.0061, so every blue-ish untagged note was
+     * 1.41x darker than the card it was supposed to match. Solved for, the floor is
+     * 0.113.
+     *
+     * Which is why this asserts on LUMINANCE and not on the input lightness — testing
+     * the number that was put in would have agreed with the bug.
+     */
     @Test
     fun `no generated fill sinks below the card surface`() {
         val surface = luminance(0xFF171717.toInt())
         for (n in 0 until 500) {
-            val l = luminance(derivedFillArgb("note-$n", dark = true))
+            assertEquals(true, luminance(derivedFillArgb("note-$n", dark = true)) >= surface)
-            assertEquals(true, l >= surface * 0.98)
+        }
+        // The worst case is a HUE — blue, around 240 — and 500 ids are not enough to
+        // be sure of having visited it at the darkest level. There are 360 x 6 = 2160
+        // reachable combinations; by the coupon-collector bound ~16,600 draws covers
+        // them, so 40,000 makes a miss vanishingly unlikely. Cheap on the JVM, and the
+        // alternative is a test that would have PASSED against the original bug.
+        for (n in 0 until 40_000) {
+            assertEquals(true, luminance(derivedFillArgb("hue-sweep-$n", dark = true)) >= surface)
         }
     }
 

frontend/src/notes/colors.ts

@@ -154,7 +154,7 @@ export function derivedTint(id: string): NoteColor {
 // at most a 1.03 contrast ratio, which is to say not at all. Nine tints that look
 // like three is exactly the wall the tint was added to break up.
 //
-// So this hashes to a colour directly rather than to a key. 324 distinct fills in
+// So this hashes to a colour directly rather than to a key. 338 distinct fills in
 // dark, 193 in light, against nine.
 //
 // TWO AXES, AND THE SECOND ONE IS THE FIX. The old ramp varied hue while pinning
@@ -181,13 +181,23 @@ const TINT_LEVELS = 6;
 const DARK_SATURATION = 0.25;
 const LIGHT_SATURATION = 0.6;
 
-// Dark starts a hair under `neutral-900`, the plain card surface, and climbs — so
+// HSL LIGHTNESS IS NOT LUMINANCE, and the dark floor is set by the difference.
-// nothing is ever darker than an untinted card and no note recedes into the board.
+//
-// The top of the range measures 1.54 against the board where the old single level
+// The obvious floor is `neutral-900`'s own lightness, 0.090 — start at the plain card
+// surface and climb, so no note ever recedes into the board. That was the first
+// attempt and it was wrong: at a FIXED HSL lightness the eye sees wildly different
+// brightnesses by hue, because green carries 71% of the luminance formula and blue
+// only 7%. At L=0.090 a yellow measures 0.0118 and a blue 0.0061 — the blue landing
+// 1.41x DARKER than the card it was meant to match, so a sixth of the board would
+// have been holes rather than variety.
+//
+// 0.113 is the lowest floor at which every hue clears the card surface, solved for
+// rather than guessed: 1.11-1.71 against the board, where the old single level
 // managed 1.14. Light runs the other way, from white down past the `neutral-50`
 // board; a card slightly darker than the board still reads as one because the edge
-// says so.
+// says so, and near white the hue barely moves luminance at all so it needs no
-const DARK_LIGHTNESS = [0.09, 0.104, 0.118, 0.132, 0.146, 0.16];
+// equivalent correction.
+const DARK_LIGHTNESS = [0.113, 0.127, 0.141, 0.155, 0.169, 0.183];
 const LIGHT_LIGHTNESS = [1.0, 0.99, 0.98, 0.97, 0.96, 0.95];
 
 /**
@@ -282,10 +292,10 @@ export function resolveLabelColor(label: { name: string; color?: string | null }
 //
 // And for derivedFill, which uses the same hash on two axes (dark / light):
 //
-//   00000000-0000-0000-0000-000000000000  hue 177 lev 3  #192a29  #f3fcfb
+//   00000000-0000-0000-0000-000000000000  hue 177 lev 3  #1e3130  #f3fcfb
-//   11111111-1111-1111-1111-111111111111  hue 113 lev 1  #152114  #fbfefb
+//   11111111-1111-1111-1111-111111111111  hue 113 lev 1  #1a2818  #fbfefb
-//   6ba7b810-9dad-11d1-80b4-00c04fd430c8  hue 136 lev 0  #111d14  #ffffff
+//   6ba7b810-9dad-11d1-80b4-00c04fd430c8  hue 136 lev 0  #162419  #ffffff
-//   f47ac10b-58cc-4372-a567-0e02b2c3d479  hue 352 lev 3  #2a191b  #fcf3f4
+//   f47ac10b-58cc-4372-a567-0e02b2c3d479  hue 352 lev 3  #311e20  #fcf3f4
 //
 // Note `work`/`ideas` and `home`/`reading` collide. Nine keys makes that unavoidable
 // and it is not a bug: colour hints that two notes are related, it never claims they