Cura 5.13: preview thumbnail not injected due to Snapshot.snapshot() returning None (fix + PR ready)

[hobibilgisi]

Environment

• OS: Windows 10/11
• Ultimaker Cura: 5.13.0
• MKS WiFi Plugin: 1.4.6-dev (this repo, develop branch)
• Printer: MKS Robin Nano v1.x + Marlin 2.0 (Bluer / Blue Whale variant), Chitu 480×320 LCD

Summary

On Cura 5.13 the plugin silently fails to inject the ;simage: / ;;gimage:
preview header into the generated gcode. The printer's LCD shows only the
file name — the model thumbnail never appears. Affects both local
Save-to-File and WiFi upload paths.

Reproduction

1. Cura 5.13.0, this plugin installed.
2. Any printer profile.
3. Load a model, slice, then File → Save to File (or Print over WiFi).
4. Open the produced .gcode in a text editor — no ;simage: block.
5. Printer LCD shows the file name without a thumbnail.

Log evidence

WARNING - cura.Snapshot.snapshot [200]: Failed to crop the snapshot even after 10 attempts!
DEBUG   - MKSWifiPlugin.MKSPreview.add_preview: Skipping adding screenshot

Root cause

Four cascading defects — all four need to be addressed for the LCD
thumbnail to appear reliably:

1. cura.Snapshot.snapshot() returns None on Cura 5.13 — OpenGL /
   QQuickView timing issue. After 10 retries the crop step gives up and
   returns None. add_preview immediately bails out with
   Skipping adding screenshot.
2. CuraEngineBackend.getLatestSnapshot() uses the same broken code
   internally — not a valid fallback on its own.
3. MKSOutputDevice.requestWrite only fires its own writeStarted signal,
   not OutputDeviceManager.writeStarted, so add_preview is never
   invoked on the WiFi upload path (only local save triggers it).
4. mks_simage / mks_gimage default to 0 for community printer
   profiles that don't declare them. int(None) never happens because
   the if ... in meta_data guard skips it — but when a profile does
   declare an invalid or zero value, an empty preview is produced.

Fix (submitted as PR)

• MKSPreview.take_screenshot() — new fallback chain
  Snapshot.snapshot → Snapshot.isometricSnapshot → backend.getLatestSnapshot.
  Snapshot.isometricSnapshot() builds its own offscreen Camera +
  PreviewPass + QtRenderer and does not depend on viewport render state,
  so it works reliably on Cura 5.13.
• MKSPreview.generate_preview() — sensible defaults (simage=100,
  gimage=200) and clamp non-positive values.
• MKSOutputDevice.requestWrite() — direct MKSPreview.add_preview(self)
  call right after self.writeStarted.emit(self) so WiFi uploads inject
  the preview too.

After the fix

Cura log now contains:

DEBUG - MKSWifiPlugin.MKSPreview.take_screenshot: MKSPreview: using Snapshot.isometricSnapshot fallback

Produced gcode starts with:

;simage:...
;;gimage:...
;MKSPREVIEWPROCESSED

Thumbnail appears on the printer's LCD for both Save-to-File and
Print-over-WiFi paths. Verified on real hardware (MKS Robin Nano v1.x +
Marlin 2.0 Bluer variant).

References

• Reference implementation and full write-up:
  https://github.com/hobibilgisi/MKS_Wifi_Plugin_Thumbnail_Error_Fix
• Related older issues that may be the same root cause on newer Cura:
  No image preview on printing after upload #354, Can't save thumb in Cura 5.0.0 #299, [BUG] MKS Marlin 2.0 - Plugin 'develop' and 1.1.2 do not render preview in Cura 4.8.0 #142

Disclosure

Root-cause analysis, patch and documentation developed with AI-assisted
coding (GitHub Copilot + Claude). All patches were validated on real
hardware by the human contributor before this report was filed.