Playground: clearer pixel-diff failure diagnostics

[bkaradzic-microsoft]

Summary

Makes the Playground validation runner's pixel-comparison failures faster to triage, and flags the common transient network/font-load flake (e.g. Parse GUI json with unicode), which is a playgroundId snippet test whose GUI text renders with the fallback font until droidsans.ttf finishes loading over the network.

Changes (Apps/Playground/Scripts/validation_native.js)

• Fix a bug in the "First pixel off" line. It printed the blue channel as renderData[index] + 2 (red + 2) instead of renderData[index + 2], so the reported "Value" blue was wrong. It now prints the true value and the offending pixel's x,y coordinates.
• Report the diff ratio vs. the allowance. Pixel difference: now shows N / total pixels (X.XXX%, per-channel threshold T); allowed errorRatio R%, so it's immediately clear how close a diff is to the pass line.
• Actionable failure diagnostics. On a pixel-comparison failure the runner prints the rendered/diff PNG paths, and for playgroundId (snippet) tests only a note that the scene + its GUI/assets/fonts load over the network (so the diff is often a transient async font/asset-load flake) plus the exact isolation re-run command. Local scene-file tests get only the PNG paths — a diff there is a real regression, not a flake.

Example output

Passing snippet test (new ratio line + fixed blue channel + coords):

First pixel off at 277192 (pixel 69298 @ x=298, y=115): Value: (81, 81, 100) - Expected: (51, 51, 76)
Pixel difference: 2031 / 240000 pixels (0.846%, per-channel threshold 25); allowed errorRatio 2.5%.
Test 'Parse GUI json with unicode' validated

Failing snippet test (new hint):

Test 'Parse GUI json with unicode' failed (pixel comparison)
  Rendered result: .../Results/parseGuiJsonWithUnicode.png
  Diff overlay:    .../Errors/parseGuiJsonWithUnicode.png
  Note: this test loads playgroundId #ERVGT5#0 from the snippet server and pulls GUI/assets/fonts over the network, so a pixel diff is often a transient async asset/font-load timing flake.
  Re-run in isolation to confirm a real regression:
    Playground --headless --once --test "Parse GUI json with unicode" app:///Scripts/validation_native.js

Validation

• node --check passes.
• Ran Parse GUI json with unicode in isolation — new output verified, test still passes (ran=1 passed=1).
• Diagnostics-only change; no effect on pass/fail logic.

[Copilot]

Pull request overview

Improves the Babylon Native Playground validation runner’s pixel-diff failure output to make image-comparison failures faster to diagnose, especially for snippet-based (playgroundId) tests that can flake due to async network font/asset loading.

Changes:

• Adds a dedicated pixel-comparison failure diagnostics helper that prints output PNG paths and (for snippet tests) an isolation re-run command plus a transient network/font-load hint.
• Fixes incorrect “First pixel off” reporting (blue channel) and augments it with pixel index and x/y coordinates.
• Enhances “Pixel difference” output to include diff ratio vs. the allowed error ratio and the per-channel threshold.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.