Arc

Arc (from scenerystack/kite) is a Segment describing a piece of a circle — every point on it is equidistant (radius) from a center, swept from startAngle to endAngle. It's what shape.arc( centerX, centerY, radius, startAngle, endAngle, anticlockwise ) appends to the current subpath, and it's also what LineStyles uses internally to build 'round' line caps and joins (see LineStyles).

import { Arc, Shape } from 'scenerystack/kite';
import { Vector2 } from 'scenerystack/dot';

const halfCircle = new Arc( new Vector2( 0, 0 ), 50, 0, Math.PI, false );
halfCircle.start;    // Vector2( 50, 0 )
halfCircle.end;      // Vector2( -50, 0 ) (approximately)
halfCircle.bounds;   // Bounds2 tightly containing the arc

// The usual way you'll get one: build it through Shape
const shape = new Shape().arc( 0, 0, 50, 0, Math.PI, false );

Angles are in radians, and anticlockwise follows the Canvas 2D convention

startAngle/endAngle are radians, not degrees, and anticlockwise matches the HTML Canvas arc() parameter of the same name — it's easy to get a swept direction backwards by assuming it means "counterclockwise in screen space," when scenery's y-axis points down, so anticlockwise: true sweeps what looks like clockwise on screen. If an arc looks like it's going the wrong way, flip this flag before touching the angles.

Constructor

new Arc( center: Vector2, radius: number, startAngle: number, endAngle: number, anticlockwise: boolean )

If startAngle/endAngle differ by (approximately) a full turn, the Arc represents a complete circle rather than a partial arc — see isFullPerimeter below.

Public API

Member	Description
center, radius, startAngle, endAngle, anticlockwise	The five defining values, each with a getter and a setter (setters recompute cached bounds/tangents)
start / end	The endpoint Vector2s, derived from center + radius at startAngle/endAngle
startTangent / endTangent	Normalized tangent direction at each endpoint
actualEndAngle	endAngle remapped relative to startAngle so it always represents the actual angular sweep direction
angleDifference	The signed angular sweep of the arc, accounting for anticlockwise
isFullPerimeter	true if the arc is (approximately) a complete circle rather than a partial sweep
bounds	The tight Bounds2 around the swept arc (not the full circle, unless isFullPerimeter)
positionAtAngle( angle ) / tangentAtAngle( angle )	Position/tangent at a raw angle, rather than a parametric t
transformed( matrix )	Returns Arc | EllipticalArc — a uniform-scale/rotation transform stays a circular Arc, but a non-uniform scale turns it into an EllipticalArc, since circles don't stay circular under non-uniform scaling
reversed()	A new Arc tracing the same points in the opposite direction

Arc also implements the full Segment contract shared by every segment type — positionAt( t ), tangentAt( t ), curvatureAt( t ), subdivided( t ), and getArcLength() all work the same way they do on Line, Quadratic, and Cubic.

Related

• Shape — how Arc and other segments compose into subpaths and shapes; also has the fluent arc()/static Shape.circle() builders.
• EllipticalArc — the more general segment Arc.transformed() can produce under non-uniform scaling.
• LineStyles — 'round' caps/joins are built from Arc segments.