Is the APCA versus WCAG luminance difference really just the exponent, or does APCA add a near-black clamp, and has the WCAG threshold value changed?

Problem. C2-B90-U3-04 says APCA uses channel^2.4 with no linear segment while WCAG 2.2 uses the piecewise curve with a 0.04045 threshold. Conditions. Comparing contrast outputs from APCA and WCAG tools. Method. The APCA reference code at 6c5c066 computes Ys as three coefficients times (channel/255)^2.4 (confirmed). The APCA doc adds that the pre-processing stage includes a soft clamp at black, and that this clamp is why the piecewise segment is called redundant; the earlier finding states the redundancy without the reason, which could suggest near-black values are untreated. The WCAG 2.2 value is not covered by any cited URL; I fetched WCAG 2.2 and it confirms 0.04045 and notes it was 0.03928 before May 2021, so tools built on older text may differ slightly. Failure modes. Comparing APCA to old WCAG calculators near black without accounting for the clamp. Limitations. I did not read the clamp code. Version and date. apca-w3 6c5c066 and WCAG 2.2 TR page, fetched 2026-10-23. Curator qualifications: All cited documentation excerpts located on the live pages by curator GET; no video sources. Correction of earlier record(s) C2-B90-U3-04 from a discovery audit; the curator re-located the cited excerpts by timed GET. Limitations: The main claim of the earlier finding (different exponent handling) is supported; this is a completeness correction. The clamp function itself was not inspected. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

{
  "citations": [
    {
      "url": "https://github.com/Myndex/apca-w3/blob/6c5c066cf0c853507930a4239b129283b8e05353/docs/regardingexponents.md",
      "note": "Input Stage Notes; relative luminance Note 2"
    },
    {
      "url": "https://www.w3.org/TR/WCAG22",
      "note": "cited at https://www.w3.org/TR/WCAG22/; Input Stage Notes; relative luminance Note 2"
    },
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      "url": "https://www.w3.org/TR/WCAG22/",
      "note": "observed URL (dated/alias form of the cited catalogue source; canonical identity retained)"
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