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moonlock/DECISIONS.md
nevaforget 4026f6dafa fix: audit fixes — double-unlock guard, PAM OOM code, GPU blur, async fp stop (v0.5.1) 
Security: prevent double unlock() when PAM and fingerprint succeed
simultaneously (ext-session-lock protocol error). Fix PAM callback
returning PAM_AUTH_ERR instead of PAM_BUF_ERR on calloc OOM.

Performance: replace CPU-side Gaussian blur (image crate) with GPU blur
via GskBlurNode + GskRenderer::render_texture(). Eliminates 500ms-2s
main-thread blocking on cold cache for 4K wallpapers. Remove image and
dirs dependencies (~15 transitive crates). Make fingerprint stop()
fire-and-forget async to avoid 6s UI block after successful auth.
2026-03-28 22:06:38 +01:00

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Decisions

Architectural and design decisions for Moonlock, in reverse chronological order.

2026-03-28 – GPU blur via GskBlurNode replaces CPU blur

  • Who: Nyx, Dom
  • Why: CPU-side Gaussian blur (image crate) blocked the GTK main thread for 500ms–2s on 4K wallpapers at cold cache. Disk cache mitigated repeat starts but added ~100 lines of complexity.
  • Tradeoffs: GPU blur quality is slightly different (box-blur approximation vs true Gaussian), acceptable for wallpaper. Removes image and dirs dependencies entirely. No disk cache needed.
  • How: Snapshot::push_blur() + GskRenderer::render_texture() on connect_realize. Blur happens once on the GPU when the widget gets its renderer, producing a concrete gdk::Texture. Zero startup latency.

2026-03-28 – Optional background blur via image crate (superseded)

  • Who: Nyx, Dom
  • Why: Consistent with moonset/moongreet — blurred wallpaper as lockscreen background is a common UX pattern
  • Tradeoffs: Adds image crate dependency (~15 transitive crates); CPU-side Gaussian blur at load time adds startup latency proportional to image size and sigma. Acceptable because blur runs once and the texture is shared across monitors.
  • How: load_background_texture(bg_path, blur_radius) loads texture, optionally applies imageops::blur(), returns gdk::Texture. Config option background_blur: Option<f32> in TOML.

2026-03-28 – Shared wallpaper texture pattern (aligned with moonset/moongreet)

  • Who: Nyx, Dom
  • Why: Previously loaded wallpaper per-window via Picture::for_filename(). Multi-monitor setups decoded the JPEG redundantly. Blur feature requires texture pixel access anyway.
  • Tradeoffs: Slightly more code in main.rs (texture loaded before window creation), but avoids redundant decoding and enables the blur feature.
  • How: load_background_texture() in lockscreen.rs decodes once, create_background_picture() wraps shared gdk::Texture in gtk::Picture. Same pattern as moonset/moongreet.