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                <div class="canvas__wrap">
  <canvas id="js-canvas"></canvas>


                .canvas__wrap {
  width: 100%;
  background: #000;


                // canvas要素を取得
var $canvas = document.querySelector('#js-canvas');

// 2Dコンテキストを取得
var ctx = $canvas.getContext('2d');

// canvasのサイズを指定
var canvasWidth = 400;
var canvasHeight = 250;
$canvas.width = canvasWidth;
$canvas.height = canvasHeight;

// マスクの画像
var mask = new Image();
mask.src = '';

// 背景の画像
var bg = new Image();
bg.src = '';

// マスクの大きさの初期値
var expandVal = 200;

// 繰り返しの処理
function render() {
	// 現在の描画状態を保存;

	// 背景の画像
	ctx.drawImage(bg, 0, 0, canvasWidth, canvasHeight);

	// 合成方法の指定
	ctx.globalCompositeOperation = 'destination-in';

	// マスクの画像
	ctx.drawImage(mask, canvasWidth/2 - expandVal/2, canvasHeight/2 - expandVal/2, expandVal, expandVal);

	// save(); 時点での描画状態を復元

	// マスクを大きくする
	if (expandVal > canvasWidth) {
		// 0にするとマスクがかからない
		expandVal = 1;


window.addEventListener('load', function() {