代码：

var res = wx.getSystemInfoSync();

const ctx = wx.createCanvasContext('myCanvas');

ctx.drawImage('../../resource/img/dlt.jpeg', 0, 0, res.windowWidth, res.windowHeight);

ctx.draw();

wx.canvasToTempFilePath({

x: 100,

y: 200,

width: 50,

height: 50,

destWidth: 100,

destHeight: 100,

canvasId: 'myCanvas',

success: function (res) {

console.log(res.tempFilePath)

},

fail: function (e) {

console.log(e);

}

})

图片可以画出来，但是保存的时候输出：fail canvas is empty，这是什么原因？