要使用HTML5 Canvas裁剪图像，我们可以向drawImage（）方法，sourceX，sourceY，sourceWidth，sourceHeight，destWidth和destHeight添加六个额外的参数。 这些参数定义了我们想要从图像中剪切出来的矩形的位置和大小。
html代码：
<!DOCTYPE HTML>
<html>
<head>
<style>
body {
margin: 0px;
padding: 0px;
}
</style>
</head>
<body>
<canvas id="myCanvas" width="578" height="200"></canvas>
<script>
var canvas = document.getElementById('myCanvas');
var context = canvas.getContext('2d');
var imageObj = new Image();
imageObj.onload = function() {
// draw cropped image
var sourceX = 150;
var sourceY = 0;
var sourceWidth = 150;
var sourceHeight = 150;
var destWidth = sourceWidth;
var destHeight = sourceHeight;
var destX = canvas.width / 2 - destWidth / 2;
var destY = canvas.height / 2 - destHeight / 2;
context.drawImage(imageObj, sourceX, sourceY, sourceWidth, sourceHeight, destX, destY, destWidth, destHeight);
};
imageObj.src = 'https://www.html5canvastutorials.com/demos/assets/darth-vader.jpg';
</script>
</body>
</html>
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