如何在OpenCV.js中绘制文档的轮廓？

<script async src="https://docs.opencv.org/master/opencv.js" onload="onOpenCvReady();" type="text/javascript"></script>

const img = cv.imread('path/to/image.jpg');

const gray = new cv.Mat();
cv.cvtColor(img, gray, cv.COLOR_RGBA2GRAY);
const binary = new cv.Mat();
cv.threshold(gray, binary, 0, 255, cv.THRESH_BINARY | cv.THRESH_OTSU);

const contours = new cv.MatVector();
const hierarchy = new cv.Mat();
cv.findContours(binary, contours, hierarchy, cv.RETR_EXTERNAL, cv.CHAIN_APPROX_SIMPLE);

cv.drawContours(img, contours, -1, new cv.Scalar(0, 255, 0), 2);

const canvas = document.createElement('canvas');
cv.imshow(canvas, img);
document.body.appendChild(canvas);