pcbjam/tests/tools/screenshots/README.md

Screenshot regression + Discord review tooling

One comparison engine + a churn-free updater + a Discord reporter for the e2e screenshots. Design and rationale: ~/.claude/plans/…snowglobe.md (or ask).

Source of truth = CI's Linux render. The dev never authors baselines on the Mac (Mac fonts/GL ≠ CI). Instead, CI renders on every push; when a render change is intentional you promote CI's artifact into the committed baselines. The environment isn't pinned — if the host's Mesa/fonts drift, the gate lights up in Discord and you just re-promote (broad + low-intensity change ⇒ likely drift).

Everything is per-engine. Specs write test-results/<engine>/<name>.png (engine derived from the running browser via stableShot/shotPath); baselines live in baseline-screenshots/<engine>/; the canonical key everywhere in this tooling is <engine>/<name> and captions/attachments carry the engine label. The same spec on chromium + firefox is two independent gated screenshots.

Files

• config.ts — thresholds, baseline dirs, per-engine floors (calibrate!), clustering knobs.
• image-ops.ts — PNG load/save, pixelmatch diff (AA-excluded), connected-component boxes, triptych compositing, size-cap resize.
• compare.ts — the comparison engine: classify per-engine baselines vs test-results/<engine>/ → test-results/screenshot-diff/report.json + triptych/heatmap PNGs. --pair diffs two files.
• promote.ts — churn-free updater: pull a CI run's shots (gh run download) or --from DIR; overwrite a baseline only when pixels differ beyond the floor (verbatim bytes, no re-encode) → no git churn.
• perf-report.ts — renders the track-only runtime-perf table (loadMs/openMs/FPS) with Δ vs the previous main run (fetched via gh).
• post-discord.ts — the always-on CI-on-main report: SHA + e2e status + perf table, then screenshot triptychs (batched, size-capped, flood-collapsed).
• changelog.ts — Discord trigger B: git-history diff of committed baseline PNGs (no build/GPU).
• noise.ts — calibration: diff two identical-input renders → per-engine noise floor.
• gen-manifest.ts — regenerate screenshot-manifest.json (canonical {name, engine} list) from the committed per-engine baseline tree; --check (gating in CI) fails if it drifts.
• spec-map.ts — best-effort screenshot-name → spec-file attribution for captions (scans stableShot/shotPath literals).

npm scripts (run from tests/)

npm run screenshots:check      # gate: baselines vs test-results → report.json (exit 0; add --fail-on-change to gate)
npm run screenshots:promote -- --run <ci-run-id>   # churn-free re-baseline from a CI run (or --from DIR)
npm run screenshots:report -- --e2e pass           # post the CI report to Discord (main+push only; needs DISCORD_WEBHOOK_URL)
npm run screenshots:changelog                       # post the baseline changelog (main+push only)
npm run screenshots:noise -- run1/ run2/            # calibrate floors
npm run screenshots:manifest                        # regenerate the manifest (--check to verify it's fresh)

Activation checklist

• screenshot-manifest.json generated ({name, engine} authoritative — derived from the per-engine baseline tree).
• scale:'device'→'css' normalized (no-op at CI's DSF=1).

1. Add the DISCORD_WEBHOOK_URL repo secret — until then everything is inert.
2. Calibrate: run the suite twice in CI, screenshots:noise the two dirs, set FLOORS in config.ts.
3. First re-baseline: promote a clean CI run's render, commit (expect a big, one-time chrome-font diff vs the Mac baselines).
4. Delete the old scripts/{compare,update-baseline}-screenshots.sh.
5. Once floors are proven stable, flip the gate to --fail-on-change.