LLM生成ALTテキスト統制 2025 — 品質スコアリングと署名付与の実践

// pipelines/alt/scoring.ts
import { scoreRelevance } from './models/clip'
import { scoreToxicity } from './models/toxicity'
import { evaluatePolicy } from './rules/policy'

export async function scoreAlt({ imageVector, altText }: { imageVector: Float32Array; altText: string }) {
  const [relevance, toxicity, compliance] = await Promise.all([
    scoreRelevance(imageVector, altText),
    scoreToxicity(altText),
    evaluatePolicy(altText)
  ])
  return { relevance, toxicity, compliance }
}

graph TD
    A[Prompt Builder] --> B[LLM Generation]
    B --> C[Scoring]
    C -->|Pass| D[Reviewer Workbench]
    C -->|Fail| E[Prompt Adjuster]
    D --> F[C2PA Signer]
    F --> G[CDN Delivery]

// components/AltWorkbench.tsx
function AltWorkbench({ imageUrl, generatedAlt }: Props) {
  const [value, setValue] = useState(generatedAlt)
  return (
    <div className="grid gap-4 md:grid-cols-2">
      <img src={imageUrl} alt="preview" className="rounded-lg" />
      <textarea value={value} onChange={e => setValue(e.target.value)} className="h-64 font-mono" />
      <aside>
        <h3>禁則ワード</h3>
        <ForbiddenList text={value} />
        <h3>アクセシビリティ診断</h3>
        <AltQualityScore text={value} />
      </aside>
    </div>
  )
}

// pipelines/alt/sign.ts
import { sign } from '@contentauth/toolkit'

export async function signAlt({ altText, manifest }: { altText: string; manifest: any }) {
  const signed = await sign(Buffer.from(altText), {
    signer: {
      name: 'Unified Image Tools ALT Review',
      certificate: process.env.C2PA_CERT!,
      privateKey: process.env.C2PA_KEY!
    },
    assertions: [
      {
        label: 'org.unified.alt-text',
        data: { altText, version: manifest.version, reviewer: manifest.reviewer }
      }
    ]
  })
  return signed
}

// components/OptimizedImage.tsx
import manifest from '../../data/c2pa-manifest.json'

export function OptimizedImage({ id }: { id: string }) {
  const data = manifest[id]
  return (
    <figure>
      <img src={data.src} alt={data.alt.text} data-alt-signature={data.alt.signature} />
      <figcaption>{data.caption}</figcaption>
      <link rel="alternate" type="application/c2pa" href={data.manifestUrl} />
    </figure>
  )
}

CREATE TABLE alt_audit (
  asset_id TEXT,
  alt_text TEXT,
  relevance NUMERIC,
  toxicity NUMERIC,
  compliance NUMERIC,
  reviewer TEXT,
  signed_at TIMESTAMP DEFAULT now()
);

prompts:
  default: |
    画像を説明するALTテキストを120文字以内で生成してください。
    禁止: 人種・性別の推測、推測が必要な感情表現。
    必須: 主題、背景、色、構図に言及。
  product: |
    商品画像のALTを80文字以内で生成。
    必須: 商品名、主要な特徴、色。

# Windows (NVDA)
nvda --speech-log --playback optimized-image.html > logs/nvda-20250929.log