top-down-movement

name	top-down-movement
description	Top-Down Movement in FlatRedBall2. Use when implementing top-down movement mechanics including 4-way or 8-way movement, acceleration/deceleration, direction facing, or any bird's-eye-view player movement. Trigger on any top-down movement question.

Not for grid/tile-locked movement. If one key press should move the player exactly one tile (Pokémon, dungeon crawler, roguelike), use the grid-movement skill instead. TopDownBehavior produces continuous analog movement — wrong for that use case.

Top-Down Movement

Overview

Top-down movement is provided by two classes in FlatRedBall2.Movement:

• TopDownValues — a plain data class holding movement parameters
• TopDownBehavior — a component added to an entity that reads input and applies velocity/acceleration each frame

Minimal Setup

public class Player : Entity
{
    private readonly TopDownBehavior _topDown = new();

    public override void CustomInitialize()
    {
        var values = new TopDownValues
        {
            MaxSpeed = 200f,
        };

        _topDown.MovementValues = values;

        var keyboard = Engine.Input.Keyboard;
        _topDown.MovementInput = new KeyboardInput2D(keyboard, Keys.Left, Keys.Right, Keys.Up, Keys.Down);
    }

    public override void CustomActivity(FrameTime time)
    {
        _topDown.Update(this, time);  // call from CustomActivity (after collisions)
    }
}

TopDownValues Fields

Field	Description
MaxSpeed	Maximum speed in world units/sec
AccelerationTime	Seconds to reach MaxSpeed from rest. 0 = instant
DecelerationTime	Seconds to stop from MaxSpeed. 0 = instant. Setting either accel or decel time to a non-zero value enables the ramp — no separate flag.
UpdateDirectionFromInput	If true (default), DirectionFacing follows input direction
UpdateDirectionFromVelocity	If true and UpdateDirectionFromInput is false, DirectionFacing follows actual velocity
IsUsingCustomDeceleration	If true, uses CustomDecelerationValue when entity exceeds MaxSpeed (e.g. after a knockback)
CustomDecelerationValue	Deceleration magnitude (units/s²) used when IsUsingCustomDeceleration is true

JSON Config (recommended for tunable values)

Externalize movement values into a JSON file so designers/playtesters can tune feel without a rebuild. Mirrors the platformer PlatformerConfig pattern.

using FlatRedBall2.Movement;

var config = TopDownConfig.FromJson("Content/player.topdown.json");
config.ApplyTo(_topDown);  // populates _topDown.MovementValues

Template: .claude/templates/TopDownConfig/player.topdown.json. Omitted fields fall back to engine defaults; partial files override only what they specify. Combine with Screen.WatchContent for hot-reload (see content-hot-reload skill).

Acceleration Setup

var values = new TopDownValues
{
    MaxSpeed = 200f,
    AccelerationTime = 0.2f,   // 200ms to reach full speed
    DecelerationTime = 0.1f,   // 100ms to stop
};

When either time is non-zero, the behavior blends between AccelerationTime and DecelerationTime based on the angle between the current velocity and the desired direction. Perfectly reversing direction uses DecelerationTime; pressing directly forward uses AccelerationTime. Both times zero = instant (velocity set directly each frame).

Direction Facing

_topDown.DirectionSnap = DirectionSnap.FourWay;   // Right, Up, Left, Down
_topDown.DirectionSnap = DirectionSnap.EightWay;  // + diagonals (default)

TopDownDirection dir = _topDown.DirectionFacing;
// e.g. TopDownDirection.Up, .DownLeft, etc.

Direction to Vector2

TopDownDirectionExtensions.ToVector2() converts a TopDownDirection to a normalized world-space Vector2 (Y+ up):

using FlatRedBall2.Movement;

Vector2 offset = _topDown.DirectionFacing.ToVector2();

Useful for positioning a hitbox or spawning a projectile in front of the entity:

// Place a sword hitbox 32 units in front of the player
var dir = _topDown.DirectionFacing.ToVector2();
_swordHitbox.X = X + dir.X * 32f;
_swordHitbox.Y = Y + dir.Y * 32f;

Diagonal directions return a normalized vector (magnitude 1.0), not (1, 1).

Speed Multiplier

_topDown.SpeedMultiplier = 0.5f;  // half speed (e.g. in mud)
_topDown.SpeedMultiplier = 1f;    // normal

Scales MaxSpeed without modifying the TopDownValues object.

Reading State

_topDown.DirectionFacing   // TopDownDirection enum — updated each frame
_topDown.IsMoving          // true when velocity magnitude > epsilon (use this for animation, NOT input)
_topDown.SpeedMultiplier   // read/write, defaults to 1f
_topDown.IsInputEnabled      // set false to freeze input without destroying movement values

Use IsMoving (not MovementInput.X != 0) to pick idle vs. walk animations — input stays non-zero when the entity is held against a wall, but IsMoving correctly flips to false because collision zeroes velocity.

Mapping 8-Way Facing to 4-Direction Art

Games often keep DirectionSnap.EightWay (the default) so diagonal input feels responsive but ship art with only 4 cardinal chains (WalkUp, WalkDown, WalkLeft, WalkRight). Collapse the diagonals at animation-selection time with ToCardinal() — do not switch to FourWay, which snaps the facing itself and makes diagonals feel notchy.

string chain = (_topDown.IsMoving ? "Walk" : "Idle") + _topDown.DirectionFacing.ToCardinal();
_sprite.PlayAnimation(chain);

ToCardinal() defaults to DiagonalCollapse.Horizontal (UpRight/DownRight → Right, UpLeft/DownLeft → Left) because horizontal silhouettes usually read more distinctly. Pass DiagonalCollapse.Vertical when up/down poses are more distinct than left/right.

Entity Origin and Draw Order

An entity's (X,Y) should be its ground-contact point (e.g. between a character's feet), not the sprite's visual center — Sprite always draws centered on its entity (see entities-and-factories), so offset Sprite.X/Sprite.Y upward once texture size is known.

This also drives screen-depth draw order. The engine sorts by Layer then Z (see engine-overview) — there's no built-in Y-sort. To make lower-on-screen entities draw on top (e.g. walking in front of/behind a tree), set Z = -Y each frame. This only reads correctly if Y is the ground-contact point, not a floating visual center.

Collision Setup

Use MoveFirstOnCollision for standard top-down (no bounce needed):

screen.AddCollisionRelationship(playerList, solidTiles)
      .MoveFirstOnCollision();

AI Movement (enemies, NPCs)

Create a class in the project that is a settable I2DInput — so AI entities share the same acceleration, deceleration, and DirectionFacing logic as players. Set X/Y each frame before calling Update. Magnitudes > 1 are normalized automatically.

private readonly DirectionalInput _aiInput = new();

// in CustomInitialize:
_topDown.MovementInput = _aiInput;

// in CustomActivity, before _topDown.Update:
var dir = Vector2.Normalize(new Vector2(_target.X - X, _target.Y - Y));
_aiInput.X = dir.X;
_aiInput.Y = dir.Y;

Gotchas

• Diagonal input magnitudes > 1 are clamped to the unit circle — full speed in 8 directions.
• UpdateDirectionFromInput defaults to true. Set it to false and UpdateDirectionFromVelocity to true if you want direction to lag behind input (e.g. tank-style).
• IsInputEnabled = false stops reading input but does not zero velocity — the entity will coast until friction/collision stops it.