globalCompositeOperation即Canvas中的合成操作。
1、source-over
这是默认值，他表示绘制的图形将画在现有画布之上
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<script type="text/javascript" src="http://www.jq22.com/jquery/jquery-2.1.1.js"></script>
<script type="text/javascript">
$(document).ready(function(){
var canvas=document.getElementById("myCanvas");
var context=canvas.getContext("2d");
context.fillStyle="red";
context.fillRect(50,50,100,100);
context.globalCompositeOperation="source-over"
context.fillStyle="green";
context.fillRect(100,100,100,100);
});
</script>
</head>
<body>
<canvas id="myCanvas" width="1000" height="1000"></canvas>
</body>
</html>
2、destination-over
这个操作的值与前一个值相反，所以现在目标绘制在源之上
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<script type="text/javascript" src="http://www.jq22.com/jquery/jquery-2.1.1.js"></script>
<script type="text/javascript">
$(document).ready(function(){
var canvas=document.getElementById("myCanvas");
var context=canvas.getContext("2d");
context.fillStyle="red";
context.fillRect(50,50,100,100);
context.globalCompositeOperation="destination-over"
context.fillStyle="green";
context.fillRect(100,100,100,100);
});
</script>
</head>
<body>
<canvas id="myCanvas" width="1000" height="1000"></canvas>
</body>
</html>
3、source-atop
这个操作会将源绘制在目标之上，但是在重叠区域上两者都是不透明的。绘制在其他位置的目标是不透明的，但源是透明的。
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<script type="text/javascript" src="http://www.jq22.com/jquery/jquery-2.1.1.js"></script>
<script type="text/javascript">
$(document).ready(function(){
var canvas=document.getElementById("myCanvas");
var context=canvas.getContext("2d");
context.fillStyle="red";
context.fillRect(50,50,100,100);
context.globalCompositeOperation="source-atop"
context.fillStyle="green";
context.fillRect(100,100,100,100);
});
</script>
</head>
<body>
<canvas id="myCanvas" width="1000" height="1000"></canvas>
</body>
</html>
4、destination-atop
这个操作与source-atop相反，目标绘制在源之上
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<script type="text/javascript" src="http://www.jq22.com/jquery/jquery-2.1.1.js"></script>
<script type="text/javascript">
$(document).ready(function(){
var canvas=document.getElementById("myCanvas");
var context=canvas.getContext("2d");
context.fillStyle="red";
context.fillRect(50,50,100,100);
context.globalCompositeOperation="destination-atop"
context.fillStyle="green";
context.fillRect(100,100,100,100);
});
</script>
</head>
<body>
<canvas id="myCanvas" width="1000" height="1000"></canvas>
</body>
</html>
5、source-in
在源于目标重叠的区域只绘制源，而不重叠的部分编程透明的。
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<script type="text/javascript" src="http://www.jq22.com/jquery/jquery-2.1.1.js"></script>
<script type="text/javascript">
$(document).ready(function(){
var canvas=document.getElementById("myCanvas");
var context=canvas.getContext("2d");
context.fillStyle="red";
context.fillRect(50,50,100,100);
context.globalCompositeOperation="source-in"
context.fillStyle="green";
context.fillRect(100,100,100,100);
});
</script>
</head>
<body>
<canvas id="myCanvas" width="1000" height="1000"></canvas>
</body>
</html>
6、destination-in
这个操作与source-in相反，在源于目标重叠的区域保留目标。而不重叠的部分都变成透明的。
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<script type="text/javascript" src="http://www.jq22.com/jquery/jquery-2.1.1.js"></script>
<script type="text/javascript">
$(document).ready(function(){
var canvas=document.getElementById("myCanvas");
var context=canvas.getContext("2d");
context.fillStyle="red";
context.fillRect(50,50,100,100);
context.globalCompositeOperation="destination-in"
context.fillStyle="green";
context.fillRect(100,100,100,100);
});
</script>
</head>
<body>
<canvas id="myCanvas" width="1000" height="1000"></canvas>
</body>
</html>
7、source-out
在与目标不重叠的区域上绘制源，其他部分都变成透明的。
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<script type="text/javascript" src="http://www.jq22.com/jquery/jquery-2.1.1.js"></script>
<script type="text/javascript">
$(document).ready(function(){
var canvas=document.getElementById("myCanvas");
var context=canvas.getContext("2d");
context.fillStyle="red";
context.fillRect(50,50,100,100);
context.globalCompositeOperation="source-out"
context.fillStyle="green";
context.fillRect(100,100,100,100);
});
</script>
</head>
<body>
<canvas id="myCanvas" width="1000" height="1000"></canvas>
</body>
</html>
8、destination-out
在与源不重叠的区域上保留目标。其他部分都变成透明的。
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<script type="text/javascript" src="http://www.jq22.com/jquery/jquery-2.1.1.js"></script>
<script type="text/javascript">
$(document).ready(function(){
var canvas=document.getElementById("myCanvas");
var context=canvas.getContext("2d");
context.fillStyle="red";
context.fillRect(50,50,100,100);
context.globalCompositeOperation="destination-out"
context.fillStyle="green";
context.fillRect(100,100,100,100);
});
</script>
</head>
<body>
<canvas id="myCanvas" width="1000" height="1000"></canvas>
</body>
</html>
9、lighter
这个值与顺序无关，如果源与目标重叠，就将两者的颜色值相加。得到的颜色值的最大取值为255。如下例中，红色的rgb值为rgb（255,0,0），绿色的rgb值为rgb（0,128,0），两者相加为rgb（255,128,0），即为图中橘色。如果一个rgb值为rgb（63,169,245），另一个rgb值为rgb（255,123,172），两者相加取最大值255，即rgb（255,255,255）为白色。
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<script type="text/javascript" src="http://www.jq22.com/jquery/jquery-2.1.1.js"></script>
<script type="text/javascript">
$(document).ready(function(){
var canvas=document.getElementById("myCanvas");
var context=canvas.getContext("2d");
context.fillStyle="red";
context.fillRect(50,50,100,100);
context.globalCompositeOperation="lighter"
context.fillStyle="green";
context.fillRect(100,100,100,100);
});
</script>
</head>
<body>
<canvas id="myCanvas" width="1000" height="1000"></canvas>
</body>
</html>
10、copy
这个值与顺序无关，只绘制源，覆盖掉目标。
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<script type="text/javascript" src="http://www.jq22.com/jquery/jquery-2.1.1.js"></script>
<script type="text/javascript">
$(document).ready(function(){
var canvas=document.getElementById("myCanvas");
var context=canvas.getContext("2d");
context.fillStyle="red";
context.fillRect(50,50,100,100);
context.globalCompositeOperation="copy"
context.fillStyle="green";
context.fillRect(100,100,100,100);
});
</script>
</head>
<body>
<canvas id="myCanvas" width="1000" height="1000"></canvas>
</body>
</html>
11、xor
这个值与顺序无关，只绘制出不重叠的源与目标区域。所有重叠的部分都变成透明的
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<script type="text/javascript" src="http://www.jq22.com/jquery/jquery-2.1.1.js"></script>
<script type="text/javascript">
$(document).ready(function(){
var canvas=document.getElementById("myCanvas");
var context=canvas.getContext("2d");
context.fillStyle="red";
context.fillRect(50,50,100,100);
context.globalCompositeOperation="xor"
context.fillStyle="green";
context.fillRect(100,100,100,100);
});
</script>
</head>
<body>
<canvas id="myCanvas" width="1000" height="1000"></canvas>
</body>
</html>
注：图中“看不见的区域”为透明的源。
参考网站：https://blog.csdn.net/laijieyao/article/details/41862473