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Runtime MVP

Crab2D now has a playable runtime MVP for small top-down prototypes. The loop is kept intentionally small:

InputState -> PlayerController -> Velocity -> collision resolution -> camera follow -> RenderList

The runtime app is separate from the editor and can open saved projects:

cargo run -p crab2d-runtime-app -- project.crab2d.json

What Runs Per Tick

Engine::tick(delta_seconds) runs scene systems with empty input. Engine::tick_with_input(delta_seconds, input) runs the playable path and returns a FrameStep:

  • reads PlayerControllerComponent and InputState
  • writes movement into Velocity2DComponent
  • moves nodes that have Velocity2DComponent
  • detects AABB overlaps between Collider2DComponent instances
  • resolves non-sensor AABB collisions by moving on X, resolving, then moving on Y
  • filters entity collisions and triggers with collision_layer / collision_mask
  • supports one_way entity platforms for downward Y-axis collision only
  • resolves against solid tilemap cells from TilesetCollision
  • reports sensor/trigger events through TriggerEvent
  • applies CameraFollowComponent
  • reports explicit EngineTickError::InvalidDelta for non-finite or negative frame deltas

The core systems accept InputState as data, so they remain testable without opening a window.

Input Boundary

crab2d-platform owns InputState, which is derived from PlatformEvent. It tracks:

  • currently pressed keys
  • keys pressed this frame
  • keys released this frame
  • cursor position

This keeps OS/window input separate from crab2d-core and lets headless tests drive gameplay without a real window.

Scene Data

Runtime components live in crab2d-scene and serialize with ProjectDocument:

  • Velocity2DComponent
  • Collider2DComponent
  • PlayerControllerComponent
  • CameraFollowComponent
  • TriggerComponent
  • TilesetCollision inside TilemapComponent

The starter scene gives the player both components, so saved starter projects already contain the minimum data needed for movement, camera follow, collision, and a trigger example.

Render Boundary

crab2d-render can extract a RenderList from a Scene. It contains:

  • active camera command
  • sprite draw commands
  • tilemap draw commands

apps/crab2d-runtime uses an isolated egui painter backend for the MVP. It opens a real window, loads sprite and tileset images from the project/assets paths, applies camera transforms, respects render order from RenderList, and falls back to colored tiles when image assets are missing.

NullRenderer still avoids opening a graphics backend and remains useful for tests or headless validation.

Intentional Limits

This is not a full physics engine. Collision is kinematic AABB only: no forces, mass, rotations, polygons, or rigid bodies. Entity colliders support layer/mask filtering and one-way platform behavior; solid tilemap cells are still treated as world geometry. Trigger events are reported in FrameStep and can be forwarded to runtime scripts.

Audio is also outside this MVP. The next minimal step is a small audio command API in crab2d-render or a dedicated audio crate, with runtime-only playback by asset path and global volume.