fix(theme): meet WCAG-AA contrast in light theme; gate contrast + token distinctness
Computing actual WCAG ratios surfaced two real accessibility defects in the shipped v0.2.0 light theme: tok_string (4.42:1) and tok_flag (3.15:1) fell below the 4.5:1 the scheme promises for normal text (NFR-5). Darken both to clear it with headroom (5.18 / 5.98). Also widen two dark-theme token pairs that were perceptually close despite distinct hex (hard to separate on good screens): tok_type and tok_function move from ΔE2000 ~14 to ~18-19 vs their neighbours. Add four gated tests in theme.rs: - all_text_colours_meet_wcag_aa_contrast: every text fg >= 4.5:1 on bg, both themes (the regression gate that would have caught this). - advanced_mode_border_meets_ui_contrast: 3:1 for the mode-alert border; plain border is decorative chrome, exempt. - delta_e_2000_matches_reference_vectors: validates the CIEDE2000 metric against Sharma et al. reference data. - syntax_token_colours_are_perceptually_distinct: ΔE2000 >= 15 for every token-class pair, both themes. Persist scripts/palette-preview.py: the true-colour swatch + contrast + ΔE2000 audit tool used to drive these changes (reads the live palette from theme.rs), for future palette work. Tests: 2513 passed / 0 failed / 1 ignored (was 2509; +4). clippy and fmt clean. Relates to NFR-5/NFR-7 (ADR-0057).
This commit is contained in:
claude@clouddev1
Executable
+168
@@ -0,0 +1,168 @@
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#!/usr/bin/env python3
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"""Palette preview + WCAG contrast + CIEDE2000 perceptual audit.
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A dev tool for working on the colour palette (`src/theme.rs`). It reads
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the live `dark()` / `light()` constructors so it never drifts from the
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code, renders a true-colour swatch + sample for every palette entry on
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the theme background, prints the WCAG-AA contrast ratio, and reports the
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pairwise CIEDE2000 (ΔE2000) distance between the syntax-token colours so
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near-duplicates (distinct hex, indistinguishable on screen) are obvious.
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Usage:
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scripts/palette-preview.py
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scripts/palette-preview.py dark:tok_type=F58AAE light:tok_flag=7A5C00
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Each `theme:key=HEX` argument previews a change without editing the
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source — handy for trying candidate colours. The gates mirrored here are
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enforced for real by the tests in `src/theme.rs` (NFR-5/NFR-7, ADR-0057):
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text foregrounds must clear 4.5:1; token pairs must clear ΔE2000 15.
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"""
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import math
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import os
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import re
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import sys
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THEME_RS = os.path.join(os.path.dirname(__file__), "..", "src", "theme.rs")
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# --- WCAG contrast -----------------------------------------------------
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def _lin(c):
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c = c / 255.0
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return c / 12.92 if c <= 0.03928 else ((c + 0.055) / 1.055) ** 2.4
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def _luminance(rgb):
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r, g, b = rgb
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return 0.2126 * _lin(r) + 0.7152 * _lin(g) + 0.0722 * _lin(b)
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def contrast(fg, bg):
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a, b = _luminance(fg), _luminance(bg)
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hi, lo = max(a, b), min(a, b)
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return (hi + 0.05) / (lo + 0.05)
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# --- sRGB -> CIELAB (D65) + CIEDE2000 ----------------------------------
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def _f(t):
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return t ** (1 / 3) if t > 0.008856 else 7.787 * t + 16 / 116
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def rgb_to_lab(rgb):
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r, g, b = (_lin(c) for c in rgb)
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x = (r * 0.4124 + g * 0.3576 + b * 0.1805) / 0.95047
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y = r * 0.2126 + g * 0.7152 + b * 0.0722
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z = (r * 0.0193 + g * 0.1192 + b * 0.9505) / 1.08883
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fx, fy, fz = _f(x), _f(y), _f(z)
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return (116 * fy - 16, 500 * (fx - fy), 200 * (fy - fz))
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def de2000(lab1, lab2):
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L1, a1, b1 = lab1
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L2, a2, b2 = lab2
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avg_Lp = (L1 + L2) / 2
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C1, C2 = math.hypot(a1, b1), math.hypot(a2, b2)
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avg_C = (C1 + C2) / 2
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G = 0.5 * (1 - math.sqrt(avg_C ** 7 / (avg_C ** 7 + 25 ** 7))) if avg_C > 0 else 0
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a1p, a2p = (1 + G) * a1, (1 + G) * a2
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C1p, C2p = math.hypot(a1p, b1), math.hypot(a2p, b2)
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avg_Cp = (C1p + C2p) / 2
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def hp(ap, b):
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if ap == 0 and b == 0:
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return 0
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ang = math.degrees(math.atan2(b, ap))
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return ang + 360 if ang < 0 else ang
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h1p, h2p = hp(a1p, b1), hp(a2p, b2)
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dLp, dCp = L2 - L1, C2p - C1p
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if C1p * C2p == 0:
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dhp = 0
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elif abs(h2p - h1p) <= 180:
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dhp = h2p - h1p
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elif h2p - h1p > 180:
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dhp = h2p - h1p - 360
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else:
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dhp = h2p - h1p + 360
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dHp = 2 * math.sqrt(C1p * C2p) * math.sin(math.radians(dhp) / 2)
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if C1p * C2p == 0:
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avg_hp = h1p + h2p
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elif abs(h1p - h2p) <= 180:
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avg_hp = (h1p + h2p) / 2
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elif h1p + h2p < 360:
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avg_hp = (h1p + h2p + 360) / 2
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else:
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avg_hp = (h1p + h2p - 360) / 2
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T = (1 - 0.17 * math.cos(math.radians(avg_hp - 30))
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+ 0.24 * math.cos(math.radians(2 * avg_hp))
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+ 0.32 * math.cos(math.radians(3 * avg_hp + 6))
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- 0.20 * math.cos(math.radians(4 * avg_hp - 63)))
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d_ro = 30 * math.exp(-((avg_hp - 275) / 25) ** 2)
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Rc = 2 * math.sqrt(avg_Cp ** 7 / (avg_Cp ** 7 + 25 ** 7)) if avg_Cp > 0 else 0
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Sl = 1 + (0.015 * (avg_Lp - 50) ** 2) / math.sqrt(20 + (avg_Lp - 50) ** 2)
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Sc, Sh = 1 + 0.045 * avg_Cp, 1 + 0.015 * avg_Cp * T
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Rt = -math.sin(math.radians(2 * d_ro)) * Rc
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return math.sqrt((dLp / Sl) ** 2 + (dCp / Sc) ** 2 + (dHp / Sh) ** 2
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+ Rt * (dCp / Sc) * (dHp / Sh))
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# --- parse the live palette from src/theme.rs --------------------------
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def parse_palette(path):
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text = open(path, encoding="utf-8").read()
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field = re.compile(
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r"(\w+):\s*Color::Rgb\(0x([0-9A-Fa-f]{2}),\s*0x([0-9A-Fa-f]{2}),\s*0x([0-9A-Fa-f]{2})\)"
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)
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def block(start, end):
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s = text.index(start)
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e = text.index(end, s)
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return {m.group(1): (m.group(2) + m.group(3) + m.group(4)).upper()
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for m in field.finditer(text[s:e])}
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return {"dark": block("fn dark()", "fn light()"),
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"light": block("fn light()", "fn highlight_class_color")}
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def h(s):
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return tuple(int(s[i:i + 2], 16) for i in (0, 2, 4))
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def fg(rgb):
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r, g, b = rgb
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return f"\x1b[38;2;{r};{g};{b}m"
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def bg(rgb):
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r, g, b = rgb
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return f"\x1b[48;2;{r};{g};{b}m"
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R = "\x1b[0m"
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TOKENS = ["tok_keyword", "tok_identifier", "tok_type", "tok_number",
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"tok_string", "tok_flag", "tok_function"]
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NONTEXT = {"border", "border_advanced"}
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SAMPLE = {
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"tok_keyword": "create table", "tok_identifier": "Customers",
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"tok_type": "serial", "tok_number": "42", "tok_string": "'hello'",
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"tok_flag": "--all-rows", "tok_function": "count(", "tok_punct": ", ;",
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"tok_error": "bad", "fg": "body text", "muted": "(none yet)",
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"system": "done.", "error": "error: nope", "warning": "[WRN] slow",
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"plan_efficient": "SEARCH idx", "mode_simple": "SIMPLE",
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"mode_advanced": "ADVANCED", "border": "──────", "border_advanced": "──────",
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}
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def main():
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themes = parse_palette(THEME_RS)
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for arg in sys.argv[1:]:
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th, rest = arg.split(":")
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k, v = rest.split("=")
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themes[th][k] = v.upper()
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for tn, t in themes.items():
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B = h(t["bg"])
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print(f"\n{'=' * 64}\n {tn.upper()} THEME (bg #{t['bg']})\n{'=' * 64}")
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for k, hexv in t.items():
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if k == "bg":
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continue
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c = h(hexv)
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cr = contrast(c, B)
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swatch = f"{bg(c)} {R}"
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text = f"{bg(B)}{fg(c)} {SAMPLE.get(k, k):14}{R}"
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floor = 3.0 if k in NONTEXT else 4.5
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warn = "" if cr >= floor else f" !! < {floor}"
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print(f" {swatch} {text} #{hexv} {cr:5.2f}:1{warn} {k}")
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print("\n -- token ΔE2000 (<12 hard to distinguish; gate is >=15) --")
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labs = {k: rgb_to_lab(h(t[k])) for k in TOKENS if k in t}
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pairs = sorted(
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(de2000(labs[a], labs[b]), a, b)
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for i, a in enumerate(labs) for b in list(labs)[i + 1:]
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)
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for d, a, b in pairs[:6]:
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flag = " !! TOO CLOSE" if d < 15 else (" ~ close" if d < 18 else "")
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print(f" {d:5.1f} {a:14} vs {b:14}{flag}")
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if __name__ == "__main__":
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main()
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+275
-4
@@ -106,13 +106,13 @@ impl Theme {
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// distinct from the mode-banner blue.
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tok_keyword: Color::Rgb(0xC7, 0x92, 0xEA), // muted purple
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tok_identifier: Color::Rgb(0x56, 0xB6, 0xC2), // cyan-teal — identifiers are the user's content, deserve a vivid distinct colour
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tok_type: Color::Rgb(0xF0, 0x8F, 0xC0), // pink — types sit in the red-purple range, clearly apart from the lavender keyword and teal identifier
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tok_type: Color::Rgb(0xF5, 0x8A, 0xAE), // rose-pink — types sit in the red-purple range, clearly apart from the lavender keyword and teal identifier (ADR-0057: widened ΔE from keyword 14.5→18.4)
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tok_number: Color::Rgb(0xF7, 0x8C, 0x6C), // warm orange
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tok_string: Color::Rgb(0xC3, 0xE8, 0x8D), // soft green
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tok_punct: Color::Rgb(0x8B, 0x90, 0x9A), // == muted
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tok_flag: Color::Rgb(0xFF, 0xCB, 0x6B), // amber
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tok_error: Color::Rgb(0xFF, 0x6B, 0x6B), // == error
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tok_function: Color::Rgb(0x82, 0xCF, 0xFD), // sky blue — cool like keyword but bluer, clearly apart from purple keyword + teal identifier + pink type
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tok_function: Color::Rgb(0x6C, 0xB2, 0xFF), // sky blue — cool like keyword but bluer, clearly apart from purple keyword + teal identifier + rose type (ADR-0057: widened ΔE from identifier 14.3→19.5)
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}
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}
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@@ -139,9 +139,9 @@ impl Theme {
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tok_identifier: Color::Rgb(0x0F, 0x6B, 0x76), // deep teal — same role as dark variant: identifiers stand out
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tok_type: Color::Rgb(0xA8, 0x2D, 0x73), // deep magenta — red-purple, distinct from royal-purple keyword + teal identifier
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tok_number: Color::Rgb(0xBC, 0x4F, 0x1F), // burnt orange
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tok_string: Color::Rgb(0x22, 0x86, 0x3A), // forest green
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tok_string: Color::Rgb(0x1B, 0x7A, 0x33), // forest green (ADR-0057: darkened for WCAG-AA, 4.42→5.18:1)
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tok_punct: Color::Rgb(0x60, 0x66, 0x73), // == muted
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tok_flag: Color::Rgb(0xB0, 0x88, 0x00), // mustard
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tok_flag: Color::Rgb(0x7A, 0x5C, 0x00), // dark mustard/olive (ADR-0057: darkened for WCAG-AA, 3.15→5.98:1)
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tok_error: Color::Rgb(0xC0, 0x39, 0x2B), // == error
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tok_function: Color::Rgb(0x1A, 0x5F, 0xB0), // strong blue — cool like keyword but bluer, apart from royal-purple keyword + teal identifier + magenta type
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}
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@@ -264,4 +264,275 @@ mod tests {
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assert_ne!(t.tok_function, t.tok_type);
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}
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}
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// ---- NFR-5 / NFR-7 (ADR-0057): WCAG-AA contrast gate ----
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//
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// Every text-bearing foreground must clear WCAG-AA 4.5:1 against
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// the panel background, in BOTH themes, so the colour scheme stays
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// legible on light and dark terminals (no dark-on-dark / light-on-
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// light failures). Borders are non-text structural chrome and are
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// handled by `advanced_mode_border_meets_ui_contrast` below; the
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// plain `border` is intentionally exempt (decorative — see ADR-0057).
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//
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// These checks rely on every `Theme` colour being `Color::Rgb`
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// (true-colour). `rgb_channels` panics on any other variant, which
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// also guards against a future regression to a named palette colour
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// (whose real contrast can't be known).
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fn rgb_channels(c: Color) -> (f64, f64, f64) {
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match c {
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Color::Rgb(r, g, b) => (f64::from(r), f64::from(g), f64::from(b)),
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other => panic!("theme colours must be Color::Rgb for contrast checks; got {other:?}"),
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}
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}
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/// WCAG 2.x relative luminance of an sRGB colour.
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// The luminance/Lab/CIEDE2000 maths below are published standards; we
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// keep them in canonical `a*x + b*y` form (verified against reference
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// vectors) rather than refactoring into `mul_add`, which would obscure
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// the formula for a negligible test-only gain.
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#[allow(clippy::suboptimal_flops)]
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fn relative_luminance(c: Color) -> f64 {
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let chan = |v: f64| {
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let v = v / 255.0;
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if v <= 0.03928 {
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v / 12.92
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} else {
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((v + 0.055) / 1.055).powf(2.4)
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}
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};
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let (r, g, b) = rgb_channels(c);
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0.2126 * chan(r) + 0.7152 * chan(g) + 0.0722 * chan(b)
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}
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/// WCAG 2.x contrast ratio between two colours (1.0 ..= 21.0).
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fn contrast_ratio(a: Color, b: Color) -> f64 {
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let (la, lb) = (relative_luminance(a), relative_luminance(b));
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let (hi, lo) = if la >= lb { (la, lb) } else { (lb, la) };
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(hi + 0.05) / (lo + 0.05)
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}
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fn theme_label(t: &Theme) -> &'static str {
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match t.background {
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Background::Dark => "dark",
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Background::Light => "light",
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}
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}
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/// Every text foreground (incl. dimmed `muted`/`tok_punct` and the
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/// syntax-token classes) on `bg`, both themes.
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fn text_foregrounds(t: &Theme) -> [(&'static str, Color); 17] {
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[
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("fg", t.fg),
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("muted", t.muted),
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("mode_simple", t.mode_simple),
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("mode_advanced", t.mode_advanced),
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("system", t.system),
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("error", t.error),
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("warning", t.warning),
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("plan_efficient", t.plan_efficient),
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("tok_keyword", t.tok_keyword),
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("tok_identifier", t.tok_identifier),
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("tok_type", t.tok_type),
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("tok_number", t.tok_number),
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("tok_string", t.tok_string),
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("tok_punct", t.tok_punct),
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("tok_flag", t.tok_flag),
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("tok_error", t.tok_error),
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("tok_function", t.tok_function),
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]
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}
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#[test]
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fn all_text_colours_meet_wcag_aa_contrast() {
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for t in [Theme::dark(), Theme::light()] {
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let label = theme_label(&t);
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for (name, c) in text_foregrounds(&t) {
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let ratio = contrast_ratio(c, t.bg);
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assert!(
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ratio >= 4.5,
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"{label} theme: {name} contrast {ratio:.2}:1 on bg is below WCAG-AA 4.5:1",
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);
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}
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}
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}
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#[test]
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fn advanced_mode_border_meets_ui_contrast() {
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// The advanced-mode border carries a mode-warning signal, so it
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// meets WCAG's 3:1 non-text / UI-component threshold in both
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// themes. The plain `border` is decorative structural chrome and
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// is intentionally exempt (recorded in ADR-0057).
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for t in [Theme::dark(), Theme::light()] {
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let label = theme_label(&t);
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let ratio = contrast_ratio(t.border_advanced, t.bg);
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assert!(
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ratio >= 3.0,
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"{label} theme: border_advanced contrast {ratio:.2}:1 below the 3:1 UI bar",
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);
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}
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}
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// ---- NFR-5 perceptual distinctness (ADR-0057) ----
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//
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// Contrast-against-background is necessary but not sufficient: two
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// token colours can each clear 4.5:1 yet be hard to tell APART from
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// each other (a real pain reported on high-quality screens). We lock
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// that in with a CIEDE2000 (ΔE2000) floor between every pair of
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// syntax-token classes that can share a line. The metric itself is
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// validated against the Sharma et al. reference vectors.
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/// sRGB `Color` → CIELAB (D65), for perceptual-difference checks.
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#[allow(clippy::suboptimal_flops)]
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fn rgb_to_lab(c: Color) -> (f64, f64, f64) {
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let lin = |v: f64| {
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let v = v / 255.0;
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if v <= 0.03928 {
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v / 12.92
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} else {
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((v + 0.055) / 1.055).powf(2.4)
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}
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};
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let (r0, g0, b0) = rgb_channels(c);
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let (r, g, b) = (lin(r0), lin(g0), lin(b0));
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let x = (r * 0.4124 + g * 0.3576 + b * 0.1805) / 0.950_47;
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let y = r * 0.2126 + g * 0.7152 + b * 0.0722;
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let z = (r * 0.0193 + g * 0.1192 + b * 0.9505) / 1.088_83;
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let f = |t: f64| {
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if t > 0.008_856 {
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t.cbrt()
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} else {
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7.787 * t + 16.0 / 116.0
|
||||
}
|
||||
};
|
||||
let (fx, fy, fz) = (f(x), f(y), f(z));
|
||||
(116.0 * fy - 16.0, 500.0 * (fx - fy), 200.0 * (fy - fz))
|
||||
}
|
||||
|
||||
/// CIEDE2000 colour difference between two CIELAB values.
|
||||
#[allow(clippy::suboptimal_flops)]
|
||||
fn delta_e_2000(lab1: (f64, f64, f64), lab2: (f64, f64, f64)) -> f64 {
|
||||
let (l1, a1, b1) = lab1;
|
||||
let (l2, a2, b2) = lab2;
|
||||
let pow7 = |v: f64| v.powi(7);
|
||||
let k_l = pow7(25.0);
|
||||
|
||||
let avg_lp = (l1 + l2) / 2.0;
|
||||
let c1 = a1.hypot(b1);
|
||||
let c2 = a2.hypot(b2);
|
||||
let avg_c = (c1 + c2) / 2.0;
|
||||
let g = if avg_c > 0.0 {
|
||||
0.5 * (1.0 - (pow7(avg_c) / (pow7(avg_c) + k_l)).sqrt())
|
||||
} else {
|
||||
0.0
|
||||
};
|
||||
let a1p = (1.0 + g) * a1;
|
||||
let a2p = (1.0 + g) * a2;
|
||||
let c1p = a1p.hypot(b1);
|
||||
let c2p = a2p.hypot(b2);
|
||||
let avg_cp = (c1p + c2p) / 2.0;
|
||||
|
||||
let hp = |ap: f64, b: f64| -> f64 {
|
||||
if ap == 0.0 && b == 0.0 {
|
||||
return 0.0;
|
||||
}
|
||||
let ang = b.atan2(ap).to_degrees();
|
||||
if ang < 0.0 { ang + 360.0 } else { ang }
|
||||
};
|
||||
let h1p = hp(a1p, b1);
|
||||
let h2p = hp(a2p, b2);
|
||||
|
||||
let dlp = l2 - l1;
|
||||
let dcp = c2p - c1p;
|
||||
let dhp = if c1p * c2p == 0.0 {
|
||||
0.0
|
||||
} else if (h2p - h1p).abs() <= 180.0 {
|
||||
h2p - h1p
|
||||
} else if h2p - h1p > 180.0 {
|
||||
h2p - h1p - 360.0
|
||||
} else {
|
||||
h2p - h1p + 360.0
|
||||
};
|
||||
let d_big_hp = 2.0 * (c1p * c2p).sqrt() * (dhp.to_radians() / 2.0).sin();
|
||||
|
||||
let avg_hp = if c1p * c2p == 0.0 {
|
||||
h1p + h2p
|
||||
} else if (h1p - h2p).abs() <= 180.0 {
|
||||
(h1p + h2p) / 2.0
|
||||
} else if h1p + h2p < 360.0 {
|
||||
(h1p + h2p + 360.0) / 2.0
|
||||
} else {
|
||||
(h1p + h2p - 360.0) / 2.0
|
||||
};
|
||||
let t = 1.0 - 0.17 * (avg_hp - 30.0).to_radians().cos()
|
||||
+ 0.24 * (2.0 * avg_hp).to_radians().cos()
|
||||
+ 0.32 * (3.0 * avg_hp + 6.0).to_radians().cos()
|
||||
- 0.20 * (4.0 * avg_hp - 63.0).to_radians().cos();
|
||||
let d_ro = 30.0 * (-((avg_hp - 275.0) / 25.0).powi(2)).exp();
|
||||
let rc = if avg_cp > 0.0 {
|
||||
2.0 * (pow7(avg_cp) / (pow7(avg_cp) + k_l)).sqrt()
|
||||
} else {
|
||||
0.0
|
||||
};
|
||||
let sl = 1.0 + (0.015 * (avg_lp - 50.0).powi(2)) / (20.0 + (avg_lp - 50.0).powi(2)).sqrt();
|
||||
let sc = 1.0 + 0.045 * avg_cp;
|
||||
let sh = 1.0 + 0.015 * avg_cp * t;
|
||||
let rt = -(2.0 * d_ro.to_radians()).sin() * rc;
|
||||
|
||||
((dlp / sl).powi(2)
|
||||
+ (dcp / sc).powi(2)
|
||||
+ (d_big_hp / sh).powi(2)
|
||||
+ rt * (dcp / sc) * (d_big_hp / sh))
|
||||
.sqrt()
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn delta_e_2000_matches_reference_vectors() {
|
||||
// Sharma, Wu & Dalal (2005) CIEDE2000 test data — validates the
|
||||
// metric so the distinctness gate below rests on a correct base.
|
||||
let cases = [
|
||||
((50.0, 2.6772, -79.7751), (50.0, 0.0, -82.7485), 2.0425),
|
||||
((50.0, 3.1571, -77.2803), (50.0, 0.0, -82.7485), 2.8615),
|
||||
((50.0, 2.4900, -0.0010), (50.0, -2.4900, 0.0009), 7.1792),
|
||||
((50.0, -1.0, 2.0), (50.0, 0.0, 0.0), 2.3669),
|
||||
];
|
||||
for (l1, l2, exp) in cases {
|
||||
let got = delta_e_2000(l1, l2);
|
||||
assert!(
|
||||
(got - exp).abs() < 0.01,
|
||||
"ΔE2000 {got:.4} != reference {exp:.4}",
|
||||
);
|
||||
}
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn syntax_token_colours_are_perceptually_distinct() {
|
||||
// Every pair of syntax-token classes that can appear together on
|
||||
// one line must be perceptually separable, not merely unequal in
|
||||
// hex. Floor at ΔE2000 >= 15 (post-ADR-0057 minimum is ~18).
|
||||
// Excludes tok_punct (== muted) and tok_error (== error), which
|
||||
// deliberately alias other roles.
|
||||
const MIN_DE: f64 = 15.0;
|
||||
for t in [Theme::dark(), Theme::light()] {
|
||||
let label = theme_label(&t);
|
||||
let toks = [
|
||||
("tok_keyword", t.tok_keyword),
|
||||
("tok_identifier", t.tok_identifier),
|
||||
("tok_type", t.tok_type),
|
||||
("tok_number", t.tok_number),
|
||||
("tok_string", t.tok_string),
|
||||
("tok_flag", t.tok_flag),
|
||||
("tok_function", t.tok_function),
|
||||
];
|
||||
for (i, (name_a, ca)) in toks.iter().enumerate() {
|
||||
for (name_b, cb) in toks.iter().skip(i + 1) {
|
||||
let de = delta_e_2000(rgb_to_lab(*ca), rgb_to_lab(*cb));
|
||||
assert!(
|
||||
de >= MIN_DE,
|
||||
"{label} theme: {name_a} vs {name_b} ΔE2000 {de:.1} below {MIN_DE} — too similar",
|
||||
);
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
Reference in New Issue
Block a user