What exactly does WCAG 2.2 SC 1.4.3 require for text contrast, and how do I apply it as a check to AI-generated posters, title cards and thumbnails?

Problem. Generated backgrounds are busy, and text laid over them often fails contrast. Conditions. WCAG 2.2 Understanding document for SC 1.4.3 (Level AA); it governs web content, so for posters and thumbnails it is a benchmark, not a legal rule. Method (documented). Text and images of text need a contrast ratio of at least 4.5:1, except large-scale text at 3:1; the Understanding text says 18 point text or 14 point bold text is judged to be large; computed values are not rounded (4.499:1 fails 4.5:1); text that is part of a logo or logotype is exempt; the document notes images of text do not scale well because they pixelate. Relative-luminance formula details are in C3-B066-G25-04 and C2-B22-E3-03. Checks on AI graphics [inference]. (1) After setting type (C3-B593-03), sample the background pixels under each glyph box, not the average of the image, and compute the ratio for the lightest and darkest background pixel against the text colour; the worst case must pass. (2) Gradients and photographs give a range of ratios; add a solid scrim or shadow band behind the type when any sample fails. (3) Large-scale threshold applies by size and weight, so title-card words at poster size can use 3:1, while captions and credits need 4.5:1. (4) Text baked into the generated image is an 'image of text': you cannot repair contrast without regenerating, another reason to composite type afterwards. Failure modes. Passing the ratio does not guarantee legibility on busy textures (inference); colour-vision issues are separate (C3-B140-G50-05). Limitations. WCAG has its own status as a spec; APCA is discussed in C2-B22-E3-04. Version/date. WCAG 2.2. Cross-reference correction 2026-10-23 (discovery C3-B697-01): worker provisional ids replaced by published C3 ids: P-L1-5-03 -> C3-B593-03 (same_batch; at link verification 2026-10-23T23:35:19Z this producer identifier had no verified public Hub record; no public link is asserted here). Curator qualifications: All cited documentation excerpts located on the live pages by curator GET; no video sources. Limitations: The threshold values are normative text in WCAG 2.2; the application to posters and thumbnails and the sampling method are inference. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

{
  "citations": [
    {
      "url": "https://www.w3.org/WAI/WCAG22/Understanding/contrast-minimum.html",
      "note": "SC 1.4.3 text; SC 1.4.3 large-scale; Large scale definition; Understanding: rounding; Logotypes; Images of text; check design"
    }
  ]
}