1 files changed, 102 insertions, 0 deletions
diff --git a/apps/web/src/graphics/stroke.ts b/apps/web/src/graphics/stroke.ts
new file mode 100644
index 00000000..230d9904
--- /dev/null
+++ b/apps/web/src/graphics/stroke.ts
@@ -0,0 +1,102 @@
+import type { GraphicStrokeAlign } from "./definitions/shared";
+
+type GraphicRenderContext =
+	| CanvasRenderingContext2D
+	| OffscreenCanvasRenderingContext2D;
+
+function createTempCanvas({
+	width,
+	height,
+}: {
+	width: number;
+	height: number;
+}): OffscreenCanvas | HTMLCanvasElement {
+	try {
+		return new OffscreenCanvas(width, height);
+	} catch {
+		const canvas = document.createElement("canvas");
+		canvas.width = width;
+		canvas.height = height;
+		return canvas;
+	}
+}
+
+function applyStroke({
+	ctx,
+	path,
+	strokeWidth,
+	strokeColor,
+}: {
+	ctx: GraphicRenderContext;
+	path: Path2D;
+	strokeWidth: number;
+	strokeColor: string;
+}) {
+	ctx.strokeStyle = strokeColor;
+	ctx.lineWidth = strokeWidth;
+	ctx.stroke(path);
+}
+
+export function applyAlignedStroke({
+	ctx,
+	path,
+	strokeWidth,
+	strokeAlign,
+	strokeColor,
+}: {
+	ctx: GraphicRenderContext;
+	path: Path2D;
+	strokeWidth: number;
+	strokeAlign: GraphicStrokeAlign;
+	strokeColor: string;
+}): void {
+	if (strokeWidth <= 0) {
+		return;
+	}
+
+	if (strokeAlign === "inside") {
+		ctx.save();
+		ctx.clip(path);
+		applyStroke({
+			ctx,
+			path,
+			strokeWidth: strokeWidth * 2,
+			strokeColor,
+		});
+		ctx.restore();
+		return;
+	}
+
+	if (strokeAlign === "outside") {
+		const strokeCanvas = createTempCanvas({
+			width: ctx.canvas.width,
+			height: ctx.canvas.height,
+		});
+		const strokeCtx = strokeCanvas.getContext("2d") as GraphicRenderContext | null;
+		if (!strokeCtx) {
+			return;
+		}
+
+		applyStroke({
+			ctx: strokeCtx,
+			path,
+			strokeWidth: strokeWidth * 2,
+			strokeColor,
+		});
+
+		// Keep only the outer half of the doubled stroke so alpha fills do not
+		// leave a visible inner stroke behind.
+		strokeCtx.globalCompositeOperation = "destination-out";
+		strokeCtx.fill(path);
+
+		ctx.drawImage(strokeCanvas, 0, 0);
+		return;
+	}
+
+	applyStroke({
+		ctx,
+		path,
+		strokeWidth,
+		strokeColor,
+	});
+}