javascript - HTML5 Canvas toDataURL 返回空白

Question

我想上传图像文件，将它们绘制到 Canvas 中，进行更改并将其保存在数据库中。我尝试测试 Canvas 图像(Pic)返回的 base64 值，它是空白的。但是，当我将 Canvas (Pic) 附加到文档时，我看到了结果。我在这里做错了什么？

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);
  }
}

Answer 1

我的直觉告诉我们，var imageData = … 中的所有内容都应该进入 img.onload 函数。

这意味着，在相关部分，代码变为:

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);

在加载图像后正确执行。但不幸的是，当这一行被执行时，你不会等待加载:

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

以及所有后续行。

图像需要加载才能在 Canvas 上绘制。 Canvas 需要包含加载的图像才能调用 getImageData。