code: async function highlightFaces(inputFile, faces, outputFile, Canvas) {
    const {promisify} = require('util');
    const readFile = promisify(fs.readFile);
    const image = await readFile(inputFile);
    const Image = Canvas.Image;
    // Open the original image into a canvas
    const img = new Image();
    img.src = image;
    const canvas = new Canvas.Canvas(img.width, img.height);
    const context = canvas.getContext('2d');
    context.drawImage(img, 0, 0, img.width, img.height);
  
    // Now draw boxes around all the faces
    context.strokeStyle = 'rgba(0,255,0,0.8)';
    context.lineWidth = '5';
  
    faces.forEach(face => {
      context.beginPath();
      let origX = 0;
      let origY = 0;
      face.boundingPoly.vertices.forEach((bounds, i) => {
        if (i === 0) {
          origX = bounds.x;
          origY = bounds.y;
        }
        context.lineTo(bounds.x, bounds.y);
      });
      context.lineTo(origX, origY);
      context.stroke();
    });
  
    // Write the result to a file
    console.log(`Writing to file ${outputFile}`);
    const writeStream = fs.createWriteStream(outputFile);
    const pngStream = canvas.pngStream();
  
    await new Promise((resolve, reject) => {
      pngStream
        .on('data', chunk => writeStream.write(chunk))
        .on('error', reject)
        .on('end', resolve);
    });
  }
docs: Draws a polygon around the faces, then saves to outputFile.
   [START vision_face_detection_tutorial_process_response]