canvas 的图为啥显示不了 base64的显示不出来

let promiseImg = new Promise(function (resolve, reject) {

wx.getImageInfo({

src: cover,

success: function (cover) {

resolve(cover.path)

console.log(cover)

}

})

})

promiseImg.then(function (cover) {

that.setData({

cover:cover

})

ctx.drawImage(cover, 0, 0, 320, 170);

})

getImageInfo返回本地的路径也显示不到canvas上如果吧路径放到img上就能显示