Javascript 如何将水印添加到已裁剪的图片？

基本上，人们用手机拍一张照片，我应该裁剪照片并添加水印。 如何在图片中添加水印？ 下面是我的代码

function takeSnapshot(){
    // Here we're using a trick that involves a hidden canvas element.  

    var hidden_canvas = document.querySelector('canvas'),
        context = hidden_canvas.getContext('2d');

        var width = 480,
        height = 480;
    
        hidden_canvas.width = width;
        hidden_canvas.height = height;
    
        // Make a copy of the current frame in the video on the canvas.
        context.drawImage(video, 100, 0, 480, 480, 0, 0, hidden_canvas.width, hidden_canvas.width);

        // Turn the canvas image into a dataURL that can be used as a src for our photo.
        return hidden_canvas.toDataURL('image/png');
   
}

html

html

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使用JS无法更改图像本身，任何人都可以关闭JS以删除任何重叠的JS水印，但您可以在图像上覆盖水印，然后使用重叠的水印捕获图像的屏幕截图，一旦完成，显示现在嵌入水印的屏幕截图，而不是图像。谢谢，我用html和css更新了代码，我怎么能按你说的做？使用JS无法更改图像本身，任何人都可以关闭JS以删除任何重叠的JS水印，但是你可以在图像上覆盖水印，然后用覆盖的水印捕获图像的屏幕截图，完成后，显示嵌入水印的屏幕截图，而不是图像。谢谢，我用html和css更新了代码，我该怎么做？

<div class="container">

  <div class="app">

    <a href="#" id="start-camera" class="visible">Touch here to start the app.</a>
    
    <video  id="camera-stream" width="640" height="480"></video>
    
    
    <img id="snap">
      <img src="http://localhost/selfie/face-pic.png" style="z-index:100;position:absolute; top:0;left:0;"/>

    <p id="error-message"></p>

    <div class="controls" style="z-index:200;">
      <a href="#" id="delete-photo" title="Delete Photo" class="disabled"><i class="material-icons">delete</i></a>
      <a href="#" id="take-photo" title="Take Photo"><i class="material-icons">camera_alt</i></a>
      <a href="#" id="download-photo" download="selfie.png" title="Save Photo" class="disabled"><i class="material-icons">file_download</i></a>  
    </div>

    <!-- Hidden canvas element. Used for taking snapshot of video. -->
    <canvas>
      
    </canvas>

  </div>

</div>

document.addEventListener('DOMContentLoaded', function () {

    // References to all the element we will need.
    var video = document.querySelector('#camera-stream'),
        image = document.querySelector('#snap'),
        start_camera = document.querySelector('#start-camera'),
        controls = document.querySelector('.controls'),
        take_photo_btn = document.querySelector('#take-photo'),
        delete_photo_btn = document.querySelector('#delete-photo'),
        download_photo_btn = document.querySelector('#download-photo'),
        error_message = document.querySelector('#error-message');


    // The getUserMedia interface is used for handling camera input.
    // Some browsers need a prefix so here we're covering all the options
    navigator.getMedia = ( navigator.getUserMedia ||
    navigator.webkitGetUserMedia ||
    navigator.mozGetUserMedia ||
    navigator.msGetUserMedia);


    if(!navigator.getMedia){
        displayErrorMessage("Your browser doesn't have support for the navigator.getUserMedia interface.");
    }
    else{

        // Request the camera.
        navigator.getMedia(
            {
                video: true
            },
            // Success Callback
            function(stream){

                // Create an object URL for the video stream and
                // set it as src of our HTLM video element.
                video.srcObject=stream;
                // Play the video element to start the stream.
                video.play();
                video.onplay = function() {
                    showVideo();
                };
         
            },
            // Error Callback
            function(err){
                displayErrorMessage("There was an error with accessing the camera stream: " + err.name, err);
            }
        );

    }



    // Mobile browsers cannot play video without user input,
    // so here we're using a button to start it manually.
    start_camera.addEventListener("click", function(e){

        e.preventDefault();

        // Start video playback manually.
        video.play();
        showVideo();

    });


    take_photo_btn.addEventListener("click", function(e){

        e.preventDefault();

        var snap = takeSnapshot();

        // Show image. 
        image.setAttribute('src', snap);
        image.classList.add("visible");

        // Enable delete and save buttons
        delete_photo_btn.classList.remove("disabled");
        download_photo_btn.classList.remove("disabled");

        // Set the href attribute of the download button to the snap url.
        download_photo_btn.href = snap;

        // Pause video playback of stream.
        video.pause();

    });


    delete_photo_btn.addEventListener("click", function(e){

        e.preventDefault();

        // Hide image.
        image.setAttribute('src', "");
        image.classList.remove("visible");

        // Disable delete and save buttons
        delete_photo_btn.classList.add("disabled");
        download_photo_btn.classList.add("disabled");

        // Resume playback of stream.
        video.play();

    });


  
    function showVideo(){
        // Display the video stream and the controls.

        hideUI();
        video.classList.add("visible");
        controls.classList.add("visible");
    }


    function takeSnapshot(){
        // Here we're using a trick that involves a hidden canvas element.  

        var hidden_canvas = document.querySelector('canvas'),
            context = hidden_canvas.getContext('2d');

            var width = 480,
            height = 480;
        
            hidden_canvas.width = width;
            hidden_canvas.height = height;
        
            // Make a copy of the current frame in the video on the canvas.
            context.drawImage(video, 100, 0, 480, 480, 0, 0, hidden_canvas.width, hidden_canvas.width);
    
            // Turn the canvas image into a dataURL that can be used as a src for our photo.
            return hidden_canvas.toDataURL('image/png');
       
    }


    function displayErrorMessage(error_msg, error){
        error = error || "";
        if(error){
            console.error(error);
        }

        error_message.innerText = error_msg;

        hideUI();
        error_message.classList.add("visible");
    }

   
    function hideUI(){
        // Helper function for clearing the app UI.

        controls.classList.remove("visible");
        start_camera.classList.remove("visible");
        video.classList.remove("visible");
        snap.classList.remove("visible");
        error_message.classList.remove("visible");
    }

});