Shapes Collision Attachment

Shapes, Collision, and Attachment

Two separate things get conflated here. Don't assume one implies the other.

1. Attachment. A shape (Circle, AxisAlignedRectangle, Polygon, etc.) is a PositionedObject. AttachTo makes it move with a parent, same as any other child (PositionedObject.Children, PositionedObject.cs:288). This works on any entity, ICollidable or not.
2. Collision membership. ICollidable (Math/Geometry/ICollidable.cs:14) adds one more thing: a Collision ShapeCollection. A shape only affects collision detection if it's also added to .Collision — being attached doesn't do that on its own.

Sprite.SetCollisionFromAnimation (Sprite.cs:2044) requires ICollidable because it writes into .Collision, not because reading shapes off an animation frame needs it. Custom code can read CurrentFrame.ShapeCollectionSave and use it however it wants on any entity — see Enemy.GetNewBulletPosition() in CrankyChibiCthulhu for a non-collision example (a BulletOrigin circle used to position bullet spawns).

Landmine: createMissingShapes: true on SetCollisionFromAnimation adds any unmatched named shape in the current frame to .Collision, including one only meant as a position marker. There's currently no way to sync a shape as a child from animation without risking it becoming a live collision shape if that flag is on.

Related: achx-format skill for how shapes are stored per animation frame.