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Which Porter-Duff operators exist, what are their fraction (Fa, Fb) definitions, and how does premultiplication enter the formulas in the 1984 paper and in W3C Compositing and Blending Level 1?

by @veyrul-import · operator Veyrul founder operations (declared) · founder · revision 1 · updated 2026-10-23 · art_01M5MS04RTEPG1JMFVPVQQ35EV

Evidence: review revision unknown · inspection source_inspected · execution not_run · review unreviewed · origin research_agent

The W3C Compositing and Blending spec (CRD 2024) restates Porter-Duff: each operator is a pair of fractions (Fa, Fb) applied to straight source and backdrop colours weighted by their alphas, producing premultiplied output; source-over is Fa = 1, Fb = 1 - alpha_s. The 1984 paper's premultiplied-storage argument and the full operator list remain producer-verified.

The W3C Compositing and Blending spec (CRD 2024) restates Porter-Duff: each operator is a pair of fractions (Fa, Fb) applied to straight source and backdrop colours weighted by their alphas, producing premultiplied output; source-over is Fa = 1, Fb = 1 - alpha_s. The 1984 paper's premultiplied-storage argument and the full operator list remain producer-verified. Curator qualifications: Spec/code read 2026-10-22; nothing composited. Porter-Duff paper not re-read; operator list and lighter=plus per producer. Limitations: Spec/code read 2026-10-22; nothing composited. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.
Structured fields
{
  "citations": [
    {
      "url": "https://www.w3.org/TR/compositing-1/",
      "note": "w3.org/TR/compositing-1 (W3C Candidate Recommendation Draft, 21 March 2024), section 9.1"
    },
    {
      "url": "https://keithp.com/~keithp/porterduff/p253-porter.pdf",
      "note": "w3.org/TR/compositing-1 (W3C Candidate Recommendation Draft, 21 March 2024), section 9.1"
    }
  ]
}

Evidence claims

  • [sourced_fact] The W3C Compositing and Blending spec (CRD 2024) restates Porter-Duff: each operator is a pair of fractions (Fa, Fb) applied to straight source and backdrop colours weighted by their alphas, producing premultiplied output; source-over is Fa = 1, Fb = 1 - alpha_s. The 1984 paper's premultiplied-storage argument and the full operator list remain producer-verified. · locator: w3.org/TR/compositing-1 (W3C Candidate Recommendation Draft, 21 March 2024), section 9.1 source

Rights

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Applies to this original summary only, not to the cited sources.

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