Opaque sRGB text color pairs on white

#!/usr/bin/env python3 """WCAG 2.2 sRGB text contrast arithmetic, with a rounding failure case. Run: python3 text-contrast-lab.py --json text-contrast-results.json \ --svg text-contrast-samples.svg Only opaque #RRGGBB sRGB colors are supported. No CSS parsing, alpha compositing, DOM inspection, browser rendering, or accessibility audit. Uses the formula documented at https://www.w3.org/WAI/WCAG22/Techniques/general/G18 """ import argparse from datetime import datetime import html import json import math from pathlib import Path import platform import re def parse_hex(color): if not isinstance(color, str) or re.fullmatch(r"#[0-9a-fA-F]{6}", color) is None: raise ValueError("expected one opaque sRGB color in #RRGGBB format") return tuple(int(color[index:index + 2], 16) for index in (1, 3, 5)) def linear_channel(channel): value = channel / 255 return value / 12.92 if value <= 0.04045 else ((value + 0.055) / 1.055) ** 2.4 def luminance(color): red, green, blue = (linear_channel(channel) for channel in parse_hex(color)) return 0.2126 * red + 0.7152 * green + 0.0722 * blue def contrast_ratio(foreground, background): darker, lighter = sorted((luminance(foreground), luminance(background))) return (lighter + 0.05) / (darker + 0.05) def meets_threshold(raw_ratio, threshold=4.5): return raw_ratio >= threshold def rejected(color): try: parse_hex(color) except ValueError: return raise AssertionError("unsupported color was accepted: " + repr(color)) def write_samples(path, cases): selected = [case for case in cases if case["case"] in ("black", "gray_118", "gray_119", "rounding_trap", "red", "blue")] height = 148 + 56 * len(selected) parts = [f'']) path.write_text('\n'.join(parts) + '\n', encoding="utf-8") def main(): parser = argparse.ArgumentParser(description=__doc__) parser.add_argument("--json", type=Path) parser.add_argument("--svg", type=Path) options = parser.parse_args() pairs = [ ("black", "#000000", 21.0, True), ("gray_118", "#767676", 4.542224959605253, True), ("gray_119", "#777777", 4.478089453577214, False), ("rounding_trap", "#707972", 4.49979546274635, False), ("gray_136", "#888888", 3.5448862152994005, False), ("red", "#FF0000", 3.9984767707539985, False), ("green", "#00FF00", 1.3721902770517513, False), ("blue", "#0000FF", 8.592471358428805, True), ] cases = [] for name, foreground, expected, normal_pass in pairs: raw = contrast_ratio(foreground, "#FFFFFF") assert math.isclose(raw, expected, rel_tol=0, abs_tol=1e-12) assert meets_threshold(raw) is normal_pass cases.append({"case": name, "foreground": foreground, "background": "#FFFFFF", "foreground_luminance": luminance(foreground), "ratio_raw": raw, "ratio_display_2dp": f"{raw:.2f}", "meets_4_5": meets_threshold(raw), "meets_3_0": meets_threshold(raw, 3.0)}) trap = next(case for case in cases if case["case"] == "rounding_trap") assert trap["ratio_raw"] < 4.5 and float(trap["ratio_display_2dp"]) >= 4.5 assert meets_threshold(4.5) and not meets_threshold(4.499) assert not meets_threshold(math.nextafter(4.5, 0.0)) assert contrast_ratio("#FFFFFF", "#FFFFFF") == 1.0 assert contrast_ratio("#000000", "#FFFFFF") == 21.0 assert contrast_ratio("#707972", "#FFFFFF") == contrast_ratio("#FFFFFF", "#707972") assert math.isclose(linear_channel(10), (10 / 255) / 12.92, abs_tol=1e-15) assert math.isclose(linear_channel(11), ((11 / 255 + 0.055) / 1.055) ** 2.4, abs_tol=1e-15) gray_ratios = [contrast_ratio(f"#{channel:02x}{channel:02x}{channel:02x}", "#FFFFFF") for channel in range(256)] assert all(a > b for a, b in zip(gray_ratios, gray_ratios[1:])) assert contrast_ratio("#ffffff", "#000000") == 21.0 for color in ("#GG0000", "#000", "000000", None): rejected(color) rejected("#00000080") rejected("color(display-p3 1 0 0)") primary_means = [sum(parse_hex(color)) / 3 for color in ("#FF0000", "#00FF00", "#0000FF")] assert primary_means == [85.0, 85.0, 85.0] encoded_example = {"color": "#808080", "normalized_channel": 128 / 255, "relative_luminance": luminance("#808080"), "correct_ratio_on_white": contrast_ratio("#808080", "#FFFFFF"), "incorrect_ratio_using_encoded_channel": 1.05 / (128 / 255 + 0.05)} assert math.isclose(encoded_example["correct_ratio_on_white"], 3.9494396480491156, abs_tol=1e-12) assert math.isclose(encoded_example["incorrect_ratio_using_encoded_channel"], 1.9023090586145648, abs_tol=1e-12) checks = ["exact 4.5 accepted", "4.499 rejected", "next float below 4.5 rejected", "identical colors give 1", "black and white give 21", "foreground/background symmetry", "both transfer-function branches", "256 gray levels decrease monotonically", "lowercase hex accepted", "malformed or unsupported hex rejected", "alpha input rejected", "Display P3 input rejected", "rounded-display trap reproduced", "RGB mean counterexample", "encoded-channel counterexample"] report = {"run_date": datetime.now().astimezone().date().isoformat(), "python": platform.python_version(), "platform": platform.system(), "standard": "WCAG 2.2", "scope": "opaque 8-bit #RRGGBB sRGB pairs; arithmetic only", "formula_source": "https://www.w3.org/WAI/WCAG22/Techniques/general/G18", "cases": cases, "boundary_checks": checks, "rgb_mean_counterexample": {"colors": ["#FF0000", "#00FF00", "#0000FF"], "means": primary_means}, "encoding_counterexample": encoded_example} print(f"Python {report['python']} on {report['platform']}") for case in cases: print(f"{case['foreground']} on white: {case['ratio_raw']:.12f}:1; " f"shown={case['ratio_display_2dp']}; >=4.5={case['meets_4_5']}; >=3={case['meets_3_0']}") print(f"All {len(cases)} color cases and {len(checks)} boundary checks passed.") if options.json: options.json.write_text(json.dumps(report, ensure_ascii=False, indent=2) + '\n', encoding="utf-8") print("JSON written:", options.json) if options.svg: write_samples(options.svg, cases) print("SVG written:", options.svg) if __name__ == "__main__": main()