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Sub Apps ​

Sub apps are offscreen-rendered mini apps that keep their own renderers, state, and update logic, then get composited into the main scene as textured quads. They are useful for embedding self-contained visuals inside dialogs or panels without mixing their draw calls into the main scene buffers.

Architecture Overview ​

  • sub-app owns its own render targets and rendering state.
  • sub-app-view owns layout, sizing, and wiring a sub app into the main renderers.
  • renderers is responsible for prerendering all sub apps and then compositing their quads into the main scene.

Key Modules ​

  • assets/lua/sub-app.fnl
    • Creates an offscreen framebuffer (FBO) with a color texture and depth buffer.
    • Owns a local TriangleRenderer and VectorBuffer instances.
    • Renders content into its FBO in :prerender.
    • Renders the final textured quad using ImageRenderer in :render.
  • assets/lua/sub-app-view.fnl
    • Creates the sub app and registers it with app.renderers.
    • Converts layout size in world units to pixel size using units-per-pixel.
    • Updates the quad geometry based on layout position/rotation/size.
  • assets/lua/renderers.fnl
    • :prerender-sub-apps runs before main scene rendering.
    • :draw-sub-apps draws the textured quads after scene content and before the HUD.
    • :add-sub-app / :remove-sub-app manage the list.

Size Matching (Sharpness) ​

The sub app’s FBO size is recomputed from its layout size in world units:

pixel_width  = layout.size.x / units_per_pixel
pixel_height = layout.size.y / units_per_pixel

This ensures the offscreen texture matches the on-screen size, keeping the output sharp. The FBO is only recreated when the computed pixel size changes.

Demo Wiring ​

  • Button: sub-app-one in assets/lua/hud-control-panel.fnl.
  • Opens a dialog with a SubAppView named sub-world-one.
  • The demo sub app uses raw-gradient-triangle.fnl to render a gradient triangle.

E2E Snapshot ​

  • Test: assets/lua/tests/e2e/test-sub-app.fnl
  • Snapshot: assets/lua/tests/data/snapshots/sub-app-dialog.png

To update:

SPACE_SNAPSHOT_UPDATE=sub-app-dialog SPACE_DISABLE_AUDIO=1 SPACE_ASSETS_PATH=$(pwd)/assets make test-e2e