HTML5 Canvas toDataURL 返回空白

2022-01-17 00:00:00 jquery javascript html html5-canvas

我想上传图像文件，将它们绘制到画布中，进行更改并将其保存在数据库中.我试图测试画布图像 (Pic) 返回的 base64 值，它是空白的.但是，当我将画布 (Pic) 附加到文档时，我看到了结果.我在这里做错了什么?

I wanted to upload the image files, draw them into canvas, make changes and save it in the database. I tried to test the base64 value that the canvas image (Pic) returned, and it is blank. However, I see the result when I append the canvas (Pic) to the document. What am I doing wrong here?

function handleFileSelect(evt) {
  var files = evt.target.files; // FileList object                
  for (var i = 0, f; f = files[i]; i++) {

    if (!f.type.match('image.*')) {
      continue;
    }
    // read contents of files asynchronously
    var reader = new FileReader();

    // Closure to capture the file information.
    reader.onload = (function(theFile) {
      return function(e) {

        var canvas = document.createElement("canvas");

        var datauri = event.target.result,
          ctx = canvas.getContext("2d"),
          img = new Image();

        img.onload = function() {

          canvas.width = width;
          canvas.height = height;
          ctx.drawImage(img, 0, 0, width, height);
        };
        img.src = datauri;
        var imageData = ctx.getImageData(0, 0, canvas.width, canvas.height);

        document.body.appendChild(canvas); //picture gets uploaded                    

        // Generate the image data

        var Pic = canvas.toDataURL("image/png");

        console.log(Pic); // => returns base64 value which when tested equivalent to blank                              
        Pic = Pic.replace(/^data:image/(png|jpg);base64,/, "")

        // Sending image to Server
        $.ajax({
          // …
        });

      };
    })(f);
    reader.readAsDataURL(f);
  }
}

推荐答案

我的直觉是 var imageData = ... 中的所有内容都应该进入 img.onload 函数.

My intuition says that everything from var imageData = … should go into the img.onload function.

这意味着，在相关部分，代码变为:

That means, at the relevant part the code becomes:

img.onload = function() {
  canvas.width = width;
  canvas.height = height;
  ctx.drawImage(img, 0, 0, width, height);

  var imageData = ctx.getImageData(0, 0, canvas.width, canvas.height);

  document.body.appendChild(canvas); //picture gets uploaded                    

  // Generate the image data

  var Pic = canvas.toDataURL("image/png");

  console.log(Pic); // => returns base64 value which when tested equivalent to blank                              
  Pic = Pic.replace(/^data:image/(png|jpg);base64,/, "")

  // Sending image to Server
  $.ajax({
    // …
  });
};
img.src = datauri;

原因是行

ctx.drawImage(img, 0, 0, width, height);

图像加载后正确执行.但不幸的是，执行此行时您无需等待加载:

correctly executes after the image has been loaded. But unfortunately, you don’t wait for loading when this line gets executed:

var imageData = ctx.getImageData(0, 0, canvas.width, canvas.height);

以及所有后续行.

需要加载图像才能在画布上绘制它.画布需要包含加载的图像才能调用 getImageData.

The image needs to be loaded in order to draw it on the canvas. The canvas needs to contain the loaded image in order to call getImageData.

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