What fills the window behind the game when the world view does not cover it (a phone in landscape with the world letterboxed, a desktop window larger than the game area). Source of truth: ADR-0016 (Accepted) and tools\bg_videos\README.md.

Upstream paints flat grey there. GUO draws a CanvasLayer at layer -1 (src\Render\CanvasBackground.cs) under everything, cover-scaled to the window. In the default mode the node is inactive and nothing changes.

A path that does not exist falls back to builtin-grey. The background is sampled Nearest for the wood tile (it is pixel art) and Linear for a player's picture, video or frames; nothing else in the client changes its filter.

Original procedural art, CC0, rendered by python tools\bg_videos\run.py into godot\GUO\assets\backgrounds\ as <name>.ogv (1280x720, 24 fps, Theora, no audio) plus <name>.png, the exact first frame used as the low-power still. backgrounds.json there is the manifest the client reads. Every theme is a pure function of the loop phase, so the last frame equals the first and the loop has no seam.

moongate-shimmer, candle-parchment, starlit-sea, drifting-fog, ember-drift, rain-on-stone, aurora-night, snowfall-pines, sunken-caustics, twin-moons.

To re-render one, run.py --only ember-drift; to render masters at another size, run.py --size 1920x1080 --out <folder outside the repo>. A contact sheet and a seam report land in build\bg_videos\.

In the game, Options, the Display page, the Background section: pick a mode, browse for a file, set the frame rate for frames, tick low power. (ADR-0016 places the section under Display; the commit that built it says Video. Look on both pages.) The choice is saved in the character's profile:

CanvasBackgroundMode   "builtin-grey"
CanvasBackgroundPath   ""
CanvasBackgroundFps    12
CanvasBackgroundLowPower  false

For a scripted run, --background <mode>[:<path>][@fps][,lowpower] overrides the profile and is never saved, for example --background builtin:starlit-sea or --background frames:D:\loops\rain@8,lowpower.

• A picture: any image Godot loads; mode image.
• A video: encode to Ogg Theora (.ogv), the one video format Godot plays without a plugin; mode video. It loops and is muted.
• Frames: a folder of PNGs in natural order (frame_2 before frame_10), or one PNG whose name ends in a strip suffix and holds square frames side by side; mode frames. On a phone a picked file is copied into the user directory, because a content URI cannot be reopened later; a folder cannot be, so on a phone frames means a sheet.
• A new built-in: add a theme to tools\bg_videos, render it, and it appears in backgrounds.json; the client lists whatever the manifest lists.

With CanvasBackgroundLowPower on, a video pauses once its first frame has decoded and frames never advances: the background becomes a still. Mobile and Web profiles get it on through the per-platform table at profile version 6 (see Mobile UI); the desktop leaves it off.

ADR-0016's validation section covers the mode switch, the profile round trip, the fallback on a missing path and the migration to version 6. The loops' seamlessness is the tool's seam report, checked when they were rendered.