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canvasで画像をトリミングする

Last updated at Posted at 2016-05-04

デモ:http://mo49.tokyo/qiita/20160504/trim.html

imgは読み込んだ後でないとwidth, heightがとれないので、img.onloadの中で描画する。
もしくはdomとしてあらかじめ用意。display:none;で消しておく。

trim.js
// トリミングしてcanvas要素に描画して返す関数
function trimImage (imgSrc) {

    var TRIM_SIZE = 300;
    var canvas = document.createElement('canvas');
    var ctx = canvas.getContext('2d');

    canvas.width = canvas.height = TRIM_SIZE;
    canvas.id = 'canvas-photo';

    var img = new Image();
    img.src = imgSrc + '?' + (new Date).getHours(); // IE対策

    // imgは読み込んだ後でないとwidth,heightが0
    img.onload = function() {
	    // 横長か縦長かで場合分けして描画位置を調整
	    var width, height, xOffset, yOffset;
	    if (img.width > img.height) {
	        height = TRIM_SIZE;
	        width = img.width * (TRIM_SIZE / img.height);
	        xOffset = -(width - TRIM_SIZE) / 2;
	        yOffset = 0;
	    } else {
	        width = TRIM_SIZE;
	        height = img.height * (TRIM_SIZE / img.width);
	        yOffset = -(height - TRIM_SIZE) / 2;
	        xOffset = 0;
	    }
    	ctx.drawImage(img, xOffset, yOffset, width, height);
    };
    
    return canvas;
}

// トリミングしたcanvasを追加
var imgSrc = './img/test.jpg';
$('body').append( trimImage(imgSrc) );

正方形に切り抜くのではなく、画像の比率を保ったまま縮小させる場合は以下のようにやる。

resize.js
img.onload = function() {
	//縦横の縮小率を求める	
	var ratioWidth = canvas.width / img.width;
	var ratioHeight = canvas.height / img.width;
	//縮小率の小さい方を使う
	if (ratioWidth <= ratioHeight) {
		var ratio = ratioWidth;
	} else {
		var ratio = ratioHeight;
	}
	
	//リサイズ後のピクセル数を計算(縦横に同じ比率をかける)	
	var sizeWidth = Math.round(img.width * ratio);
	var sizeHeight = Math.round(img.height * ratio);		
	//中央値を求める
	var drawX = Math.round((canvas.width - sizeWidth) / 2);
	var drawY = Math.round((canvas.height - sizeHeight) / 2);
	
	//写真をcanvasに表示する
	ctx.drawImage(img,drawX,drawY,sizeWidth,sizeHeight);
}

参考
drawImage() メソッド
canvasに画像を描画しよう(トリミングやクリッピングも)

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