name: platformer-physics
description: How FRB's platformer entity movement/physics works — MovementType state machine, ground/air value sets, collision-driven terrain switching. Triggers: PlatformerValues, MovementType, GroundMovement, AirMovement, AfterDoubleJump, CurrentMovement, GroundCollidedAgainst, IsOnGround, PlatformerPlugin.
Platformer Physics
Platformer entities are generated code, not engine classes: FRBDK/Glue/PlatformerPlugin (the Glue plugin) reads Models/PlatformerValues.cs field values from the Glue project and emits the actual movement logic into the user's project via CodeGenerators/EntityCodeGenerator.cs. To change runtime platformer behavior, edit the templates in EntityCodeGenerator.cs; to change what values a user can configure, edit Models/PlatformerValues.cs + the plugin's ViewModels/Views.
Movement value sets and state machine
A platformer entity holds up to three PlatformerValues sets — GroundMovement, AirMovement, AfterDoubleJump — plus an enum MovementType {Ground, Air, AfterDoubleJump}. CurrentMovementType setter calls UpdateCurrentMovement() (EntityCodeGenerator.cs:337) which resolves CurrentMovement to the matching set (falls back to AirMovement if AfterDoubleJump is unset) and applies Gravity→YAcceleration. DetermineMovementValues() (EntityCodeGenerator.cs:666) drives the Ground/Air/AfterDoubleJump transitions each frame off mIsOnGround and mHasDoubleJumped.
mIsOnGround is set only inside CollideAgainst(...) during solid-collision resolution (EntityCodeGenerator.cs:882), not by any standalone "am I touching the floor" check — it's a side effect of that frame's collision pass.
Terrain-specific values (ice, water, sticky) aren't automatic
Swapping GroundMovement/AirMovement per terrain type is user code, not plugin-generated: check GroundCollidedAgainst/ItemsCollidedAgainst (populated after all of that frame's collision resolves) against named TileShapeCollection relationships in CustomActivity, then reassign e.g. this.GroundMovement = PlatformerValuesStatic[...IceGround]. Full pattern: FlatRedBallDocs/tutorials/platformer-plugin/groundcollidedagainst-for-movement-values.md.
Value meanings
Full explanations (with tuning guidance and video examples) live in FlatRedBallDocs/tutorials/platformer-plugin/platformer-basics/04-control-values.md. Field reference: Models/PlatformerValues.cs. Key gotcha: AccelerationTimeX/DecelerationTimeX of 0 means Immediate movement (no easing); JumpApplyByButtonHold implements variable jump height by turning gravity off while held, not by delaying the jump.
Related docs
FlatRedBallDocs/tutorials/platformer-plugin/ has the full tutorial tree (basics, animations, ladders, moving platforms, multiplayer, etc.) — check there before re-deriving behavior from code.
