javascript - Canvas drawImage() 不工作

Question

此 Javascript 代码在 Canvas 上绘制部分图像:

var img = document.getElementById('img');
var canvas = document.getElementById('can');
//canvas.width = img.width;
//canvas.height = img.height;
var ctx = canvas.getContext("2d");
ctx.drawImage(img, 0, 0);

但是当我取消中间线的注释以设置 Canvas 的宽度和高度时，drawImage() 不起作用。 我应该检查什么才能发现问题？

Answer 1

您需要等待浏览器加载图片。

如果图像尚未加载，请使用 onload 事件并将您的代码更改为如下内容:

var img = document.getElementById('img');
var canvas = document.getElementById('can');
var ctx = canvas.getContext("2d");

var callback = function(image) {
   if(!image) image = this;
   canvas.width = img.width;
   canvas.height = img.height;
   ctx.drawImage(image, 0, 0);
}

if(img.complete) { //check if image was already loaded by the browser
   callback(img);
}else {
   img.onload = callback;
}

See this working demo