屬性值：
- shadowOffsetX: 陰影的水平偏移距離
- shadowOffsetY: 陰影的豎直偏移距離
- shadowBlur: 陰影的模糊效果，數字越大越模糊
- shadowColor: 陰影顏色
示例：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<canvas id="c2d">瀏覽器不支持canvas</canvas>
<script>
const canvas = document.querySelector('#c2d');
if(canvas.getContext) {
const ctx = canvas.getContext('2d');
// 設置陰影效果
ctx.shadowOffsetX = -7;
ctx.shadowOffsetY = 5;
ctx.shadowBlur = 3;
ctx.shadowColor = 'rgba(255,255,0,0.7)';
ctx.fillStyle = 'red';
ctx.fillRect(15,30,130,40);
}
</script>
</body>
</html>
