Quality Ladder：PSNR/SSIM/VMAF 指標の実務的な使い分け

import sharp from 'sharp';
import { getSSIM } from 'ssim.js'; // npm i ssim.js

// PSNR を計算 (単純二乗誤差から導出)
async function psnr(a: Buffer, b: Buffer) {
  if (a.length !== b.length) throw new Error('dimension mismatch');
  let mse = 0;
  for (let i = 0; i < a.length; i++) {
    const d = a[i] - b[i];
    mse += d * d;
  }
  mse /= a.length;
  if (mse === 0) return Infinity;
  const PIXEL_MAX = 255;
  return 10 * Math.log10((PIXEL_MAX * PIXEL_MAX) / mse);
}

async function main() {
  const ref = sharp('ref.png').raw().ensureAlpha().toBuffer({ resolveWithObject: true });
  const cand = sharp('q85.png').raw().ensureAlpha().toBuffer({ resolveWithObject: true });
  const [r, c] = await Promise.all([ref, cand]);
  const psnrValue = await psnr(r.data, c.data);
  const { mssim } = getSSIM(
    { data: r.data, width: r.info.width, height: r.info.height },
    { data: c.data, width: c.info.width, height: c.info.height },
    { ssim: 'fast' }
  );
  console.log({ psnrValue, ssim: mssim });
}
main();

name: image-quality
on: [pull_request]
jobs:
  ladder:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: 20
      - run: npm ci
      - name: Compute metrics
        run: node scripts/compute-quality-ladder.mjs --out report.json
      - name: Validate thresholds
        run: node scripts/validate-thresholds.mjs report.json thresholds.json
      - name: Upload artifacts
        if: always()
        uses: actions/upload-artifact@v4
        with:
          name: ladder-report
          path: report/*