Add perceptual 16-colour mapping

Dark shades map to black and greys to the grey/white entries while
saturated category colours keep their hue, instead of the stock
conversion's bright yellow/cyan shadows.

internal/tui/canvas.go

@@ -232,7 +232,16 @@ func (cv *Canvas) Line(y int) string {
 	return b.String()
 }
 
-func sgr(b *strings.Builder, st Style, p256 bool) {
+// colorDepth selects how colours are serialised.
+type colorDepth uint8
+
+const (
+	depthTrue colorDepth = iota
+	depth256
+	depth16
+)
+
+func sgr(b *strings.Builder, st Style, depth colorDepth) {
 	b.WriteString("\x1b[0")
 	if st.Attr&Bold != 0 {
 		b.WriteString(";1")
@@ -253,10 +262,22 @@ func sgr(b *strings.Builder, st Style, p256 bool) {
 		if c == 0 {
 			return
 		}
-		if p256 {
+		switch depth {
+		case depth256:
 			b.WriteString(";" + base + ";5;")
 			b.WriteString(strconv.Itoa(int(quant256(c))))
 			return
+		case depth16:
+			n := int(quant16(c))
+			code := 30 + n
+			if n >= 8 {
+				code = 90 + n - 8
+			}
+			if base == "48" {
+				code += 10
+			}
+			b.WriteString(";" + strconv.Itoa(code))
+			return
 		}
 		b.WriteString(";" + base + ";2;")
 		b.WriteString(strconv.Itoa(int(c >> 16 & 0xff)))
@@ -270,14 +291,14 @@ func sgr(b *strings.Builder, st Style, p256 bool) {
 	b.WriteByte('m')
 }
 
-// String serialises the canvas as lines of text with SGR styling. Colours
-// are emitted as 24-bit, or as 256-colour indices from our own quantizer
-// when p256 is set; Bubble Tea downsamples further (16 colours, none) to
-// the terminal's profile.
-func (cv *Canvas) String() string { return cv.Render(false) }
+// String serialises the canvas as lines of text with SGR styling and
+// 24-bit colours.
+func (cv *Canvas) String() string { return cv.Render(depthTrue) }
 
-// Render serialises the canvas; see String.
-func (cv *Canvas) Render(p256 bool) string {
+// Render serialises the canvas. For 256 and 16 colours it quantizes with
+// its own perceptual mapping (see color256.go); Bubble Tea strips colour
+// entirely for colourless terminals.
+func (cv *Canvas) Render(depth colorDepth) string {
 	var b strings.Builder
 	b.Grow(cv.W * cv.H * 4)
 	for y := 0; y < cv.H; y++ {
@@ -292,7 +313,7 @@ func (cv *Canvas) Render(p256 bool) string {
 				continue
 			}
 			if first || c.st != cur {
-				sgr(&b, c.st, p256)
+				sgr(&b, c.st, depth)
 				cur, first = c.st, false
 			}
 			b.WriteString(c.s)

internal/tui/color256.go

@@ -43,3 +43,54 @@ func quant256(c Color) uint8 {
 	q256Cache.Store(c, uint8(best))
 	return uint8(best)
 }
+
+var q16Cache sync.Map // Color -> uint8
+
+// quant16 maps a colour to a basic colour. Dark shades become black and
+// unsaturated colours grey/white, so tints and shadows stay quiet; saturated
+// colours keep their hue (bright variant when light), so category colours
+// survive. The stock conversion instead turns dark tints into bright
+// yellow/cyan.
+func quant16(c Color) uint8 {
+	if v, ok := q16Cache.Load(c); ok {
+		return v.(uint8)
+	}
+	rf, gf, bf := c.rgb()
+	mx, mn := max(rf, gf, bf), min(rf, gf, bf)
+	luma := c.Luma()
+	var n uint8
+	switch {
+	case luma < 0.2:
+		n = 0
+	case mx-mn < 0.12: // grey
+		switch {
+		case luma < 0.45:
+			n = 8
+		case luma < 0.8:
+			n = 7
+		default:
+			n = 15
+		}
+	default:
+		// Hue in sixths: red, yellow, green, cyan, blue, magenta.
+		var h float64
+		switch mx {
+		case rf:
+			h = math.Mod((gf-bf)/(mx-mn), 6)
+		case gf:
+			h = (bf-rf)/(mx-mn) + 2
+		default:
+			h = (rf-gf)/(mx-mn) + 4
+		}
+		if h < 0 {
+			h += 6
+		}
+		ansi := [6]uint8{1, 3, 2, 6, 4, 5}[int(math.Round(h))%6]
+		n = ansi
+		if luma > 0.5 {
+			n += 8
+		}
+	}
+	q16Cache.Store(c, n)
+	return n
+}

internal/tui/render.go

@@ -76,7 +76,7 @@ func (m *Model) render() string {
 	g := computeGeometry(m.w, m.h)
 	if g.class == tooSmall {
 		m.renderTooSmall(cv)
-		return cv.Render(m.profile == colorprofile.ANSI256)
+		return cv.Render(m.depth())
 	}
 	m.tree.RLock()
 	defer m.tree.RUnlock()
@@ -100,7 +100,17 @@ func (m *Model) render() string {
 	if len(m.modals) > 0 {
 		m.paintModal(cv)
 	}
-	return cv.Render(m.profile == colorprofile.ANSI256)
+	return cv.Render(m.depth())
+}
+
+func (m *Model) depth() colorDepth {
+	switch m.profile {
+	case colorprofile.ANSI256:
+		return depth256
+	case colorprofile.ANSI:
+		return depth16
+	}
+	return depthTrue
 }
 
 func (m *Model) renderTooSmall(cv *Canvas) {

internal/tui/tui_test.go

@@ -454,3 +454,29 @@ func TestMouse(t *testing.T) {
 		t.Fatal("click outside modal should close it")
 	}
 }
+
+func TestQuantizers(t *testing.T) {
+	if quant16(RGB(10, 12, 8)) != 0 {
+		t.Fatal("near-black should map to black")
+	}
+	if n := quant16(RGB(0xe0, 0x6c, 0x75)); n != 1 && n != 9 {
+		t.Fatalf("red-ish mapped to %d", n)
+	}
+	if n := quant256(RGB(40, 48, 30)); n < 16 {
+		t.Fatalf("256 quantizer used palette-dependent index %d", n)
+	}
+	// A dark, slightly tinted shade must land on the grey ramp or a dark
+	// cube entry, not a saturated one like olive (58) or navy (17).
+	if n := quant256(RGB(40, 48, 30)); n == 58 || n == 17 || n == 22 {
+		t.Fatalf("dark tint mapped to saturated %d", n)
+	}
+}
+
+func TestColourDepthsRenderSameGeometry(t *testing.T) {
+	for _, p := range []colorprofile.Profile{colorprofile.TrueColor, colorprofile.ANSI256, colorprofile.ANSI, colorprofile.ASCII} {
+		m := newTestModel(t)
+		m.profile = p
+		send(m, size(100, 30))
+		checkFrame(t, m, 100, 30)
+	}
+}