APIs › Image, SVG, QR & Barcode › WCAG Contrast Check

WCAG Contrast Check

Computes the WCAG 2.2 contrast ratio between two colours and reports AA and AAA pass or fail for normal, large and non-text content.

POST /v1/image/contrast-check $0.005 per call

Specification

Endpoint

POST /v1/image/contrast-check

Price

$0.005 per call · tier basic_data

Payment

x402 on X Layer (chain 196) · settled in USDG or USDT0 · schemes exact, aggr_deferred

Data source

Local sandboxed processing of the uploaded image

Timeout

60 seconds

Max request

20 MB

Version

1.0.0

Calling it

The first call returns a 402 with the payment challenge. Retry with a signed authorisation in the X-PAYMENT header — see the payment guide for the exact shape.

curl -i -X POST "https://api.dana-edu.pp.ua/v1/image/contrast-check" \
  -H "content-type: application/json" \
  -d '{ ... }'

Response

Real output, captured by running this endpoint at build time — not written by hand. Volatile fields such as timestamps are elided. Captured 2026-07-30.

{
  "foreground": "#767676",
  "background": "#ffffff",
  "contrast_ratio": 4.542,
  "foreground_luminance": 0.18116,
  "background_luminance": 1,
  "wcag": {
    "aa_normal": {
      "description": "normal text (AA)",
      "required": 4.5,
      "passes": true,
      "shortfall": 0
    },
    "aa_large": {
      "description": "large text, 18pt or 14pt bold (AA)",
      "required": 3,
      "passes": true,
      "shortfall": 0
    },
    "aaa_normal": {
      "description": "normal text (AAA)",
      "required": 7,
      "passes": false,
      "shortfall": 2.46
    },
    "aaa_large": {
      "description": "large text (AAA)",
      "required": 4.5,
      "passes": true,
      "shortfall": 0
    },
    "aa_non_text": {
      "description": "UI components and graphics (AA)",
      "required": 3,
      "passes": true,
      "shortfall": 0
    }
  },
  "passes_count": 4,
  "method": {
    "formula": "(L_lighter + 0.05) / (L_darker + 0.05)",
    "luminance": "WCAG relative luminance with the sRGB transfer function and its 0.03928 breakpoint, not a plain 2.2 gamma — the shortcut shifts ratios enough to flip a pass into a fail near 4.5:1",
    "reference": "WCAG 2.2 success criteria 1.4.3, 1.4.6 and 1.4.11"
  }
}

Every response is wrapped in the same envelope: data plus warnings, sources, confidence, informational_only and a measured processing_ms. See the envelope reference.

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