Shapes

Shapes are provided by PhysicsShape and define collision geometry. The engine derives actual dimensions from the Composable's measured size unless specified.

Box

PhysicsShape.Box uses the Composable bounds as a rectangle.

Modifier.physicsBody(key = "box", shape = PhysicsShape.Box)

Circle

PhysicsShape.Circle(radiusPx) optionally specifies a radius in pixels. If radiusPx is null, the runtime derives it from the Composable size.

Modifier.physicsBody(
    key = "ball",
    shape = PhysicsShape.Circle(radiusPx = 40f)
)

Polygon

PhysicsShape.Polygon uses a list of vertices in either normalized or pixel space.

val verts = listOf(
    PhysicsVector2(-0.5f, -0.4f),
    PhysicsVector2(0.5f, -0.4f),
    PhysicsVector2(0.2f, 0.5f),
    PhysicsVector2(-0.2f, 0.5f),
)

Modifier.physicsBody(
    key = "poly",
    shape = PhysicsShape.Polygon(
        vertices = verts,
        space = PhysicsShape.Polygon.VertexSpace.Normalized,
    )
)

Constraints

• Convex only (no concave or self‑intersecting shapes).

• 3..8 vertices recommended (backend limit is typically 8).

• Vertices should not be degenerate (zero area).

• Counter‑clockwise winding is recommended. The current backend auto‑corrects winding if needed.

Common pitfalls

Polygon validity

If the polygon is invalid (concave, too many vertices, zero area), the fixture creation fails and the body will not collide as expected.