Can not export the modified image correctly

[superlin]

I'm trying to export an image from the canvas using canvas.toDataURL(). However I got a transparent image, and I find a way to fix it(http://stackoverflow.com/questions/12538193/why-does-my-canvas-go-blank-after-converting-to-image).

exports.canvas = function () {
  var canvas = document.createElement('canvas');
  var setting = { premultipliedAlpha: false, preserveDrawingBuffer: true };
  try {
    gl = canvas.getContext('webgl', setting) || canvas.getContext('experimental-webgl', setting);
  } catch (e) {
    gl = null;
  }
  if (!gl) {
    throw 'This browser does not support WebGL';
  }
  // ...
}

[superlin]

Still not correct in chrome of ios 8, always get the last image, not the current image.

but ok in chrome of android