如何从画布查找图像文件的大小？

user938363 发表于 Dev

用户名

这是一个调整jpeg大小的js函数image。通过功能将原稿image尺寸调整为width x height。将image.size在alert返回undefined。是mainCanvas.toDataURL.length调整后的图像文件的大小？如果没有，如何在调整大小后查找图像文件大小？

    function resize(image, width, height) {
      var mainCanvas = document.createElement("canvas");
      mainCanvas.width = width;
      mainCanvas.height = height;
      var ctx = mainCanvas.getContext("2d");
      ctx.drawImage(image, 0, 0, width, height);
      $('#uploaded_file_hidden_file').val(mainCanvas.toDataURL("image/jpeg")); 
      $('#file_size').val(Math.ceil(image.size/1024));
      alert(image.size);
    };

1cgonza

如果按大小表示文件大小（以字节为单位），则图像元素将不会具有的大小属性image.size。您需要将画布转换为Blob，然后可以获取尺寸：

 // canvas.toBlob() is not well supported, so here is the polyfill just in case.
 // https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/toBlob#Polyfill
 if (!HTMLCanvasElement.prototype.toBlob) {
 Object.defineProperty(HTMLCanvasElement.prototype, 'toBlob', {
  value: function (callback, type, quality) {

    var binStr = atob( this.toDataURL(type, quality).split(',')[1] ),
        len = binStr.length,
        arr = new Uint8Array(len);

    for (var i=0; i<len; i++ ) {
     arr[i] = binStr.charCodeAt(i);
    }

    callback( new Blob( [arr], {type: type || 'image/png'} ) );
  }
 });
}

function resize(image, width, height) {
  var mainCanvas = document.createElement("canvas");
  mainCanvas.width = width;
  mainCanvas.height = height;
  var ctx = mainCanvas.getContext("2d");
  ctx.drawImage(image, 0, 0, width, height);
  $('#uploaded_file_hidden_file').val(mainCanvas.toDataURL("image/jpeg"));

  // Canvas to blob so we can get size.
  mainCanvas.toBlob(function(blob) {
    $('#file_size').val(Math.ceil(blob.size/1024));
    alert(blob.size);
  }, 'image/jpeg', 1);
};