Add offset functions

package.json

@@ -68,6 +68,7 @@
 		"@types/bezier-js": "^4.1.3",
 		"bezier-js": "^6.1.4",
 		"linearly": "^0.21.0",
-		"paper": "^0.12.17"
+		"paper": "^0.12.17",
+		"paperjs-offset": "^1.0.8"
 	}
 }

src/Path.ts

@@ -1,5 +1,6 @@
 import {mat2d, scalar, vec2} from 'linearly'
 import paper from 'paper'
+import {OffsetOptions as PaperOffsetOptions, PaperOffset} from 'paperjs-offset'
 
 import {Bezier} from './Bezier'
 import {memoize, toFixedSimple} from './utils'
@@ -208,6 +209,72 @@ export namespace Path {
 		return length
 	})
 
+	interface OffsetOptions {
+		/**
+		 * The join style of offset path
+		 * @defaultValue 'miter'
+		 */
+		join?: CanvasLineJoin
+		/**
+		 * The limit for miter style
+		 * @see https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/miterLimit
+		 * @defaultValue 10
+		 */
+		miterLimit?: number
+	}
+
+	/**
+	 * Creates an offset path from the given path.
+	 * @param path The path to offset
+	 * @param offset The width of offset
+	 * @param options The options
+	 * @returns The newly created path
+	 */
+	export function offset(
+		path: Path,
+		offset: number,
+		options?: OffsetOptions
+	): Path {
+		const paperPath = toPaperPath(path)
+
+		const _options: PaperOffsetOptions = {
+			...options,
+			limit: options?.miterLimit,
+		}
+
+		return fromPaperPath(PaperOffset.offset(paperPath, offset, _options))
+	}
+
+	interface OffsetStrokeOptions extends OffsetOptions {
+		/**
+		 * The cap style of offset path (`'square'` will be supported in future)
+		 * @defaultValue 'butt'
+		 */
+		cap?: 'butt' | 'round'
+	}
+
+	/**
+	 * Creates an offset path from the given path.
+	 * @param path The path to offset
+	 * @param offset The width of stroke
+	 * @param options The options
+	 * @returns The newly created path
+	 */
+	export function offsetStroke(
+		path: Path,
+		offset: number,
+		options?: OffsetStrokeOptions
+	) {
+		const paperPath = toPaperPath(path)
+
+		const _options: PaperOffsetOptions = {
+			...options,
+			limit: options?.miterLimit,
+		}
+
+		return fromPaperPath(PaperOffset.offsetStroke(paperPath, offset, _options))
+	}
+
 	/**
 	 * Creates a rectangle path from the given two points.
 	 * @param start The first point defining the rectangle

yarn.lock

@@ -3084,11 +3084,18 @@ p-try@^2.0.0:
   resolved "https://registry.yarnpkg.com/p-try/-/p-try-2.2.0.tgz#cb2868540e313d61de58fafbe35ce9004d5540e6"
   integrity sha512-R4nPAVTAU0B9D35/Gk3uJf/7XYbQcyohSKdvAxIRSNghFl4e71hVoGnBNQz9cWaXxO2I10KTC+3jMdvvoKw6dQ==
 
-paper@^0.12.17:
+paper@^0.12.17, paper@^0.12.4:
   version "0.12.17"
   resolved "https://registry.yarnpkg.com/paper/-/paper-0.12.17.tgz#57215615f3db5a32826ab60d3e08bc7953614052"
   integrity sha512-oCe+e1C2w8hKIcGoAqUjD0GGxGPv+itrRXlEFUmp3H8tY/NTnHOkYgpJFPGw6OJ8Q1Wa6+RgzlY7Dx/2WWHtkA==
 
+paperjs-offset@^1.0.8:
+  version "1.0.8"
+  resolved "https://registry.yarnpkg.com/paperjs-offset/-/paperjs-offset-1.0.8.tgz#3ddb297785a45b99126e3da3bca852e3612b3d68"
+  integrity sha512-rUXMtHH4bTNVrCgDcu+x71qyGoPe+01yZtrPD7bowji4VfWiDWzl4EUdRxiZnG5ooPg7b07mk6jVx33hNSojaA==
+  dependencies:
+    paper "^0.12.4"
+
 parent-module@^1.0.0:
   version "1.0.1"
   resolved "https://registry.yarnpkg.com/parent-module/-/parent-module-1.0.1.tgz#691d2709e78c79fae3a156622452d00762caaaa2"