Qiita Teams that are logged in
You are not logged in to any team

Log in to Qiita Team
Community
OrganizationAdvent CalendarQiitadon (β)
Service
Qiita JobsQiita ZineQiita Blog
10
Help us understand the problem. What is going on with this article?
@ms2sato

Canvasで画像を縮小したいのだけど、MobileSafariでは大きな画像に関して取得される高さが小さくなっているらしい時

More than 5 years have passed since last update.

大きな画像を選んでCanvasで縮小すると、下の方が真っ黒になってあれあれ?となってしまうと思う。

※コメント戴いた内容から、下記の話題へ続く。
http://qiita.com/ms2sato/items/d1e6a58b23af41e0090a

iOS6からのバグの様子。下記を参考にすべし。
http://stackoverflow.com/questions/11929099/html5-canvas-drawimage-ratio-bug-ios

コード保全のため一応転載。

/**
 * Detecting vertical squash in loaded image.
 * Fixes a bug which squash image vertically while drawing into canvas for some images.
 * This is a bug in iOS6 devices. This function from https://github.com/stomita/ios-imagefile-megapixel
 * 
 */
function detectVerticalSquash(img) {
    var iw = img.naturalWidth, ih = img.naturalHeight;
    var canvas = document.createElement('canvas');
    canvas.width = 1;
    canvas.height = ih;
    var ctx = canvas.getContext('2d');
    ctx.drawImage(img, 0, 0);
    var data = ctx.getImageData(0, 0, 1, ih).data;
    // search image edge pixel position in case it is squashed vertically.
    var sy = 0;
    var ey = ih;
    var py = ih;
    while (py > sy) {
        var alpha = data[(py - 1) * 4 + 3];
        if (alpha === 0) {
            ey = py;
        } else {
            sy = py;
        }
        py = (ey + sy) >> 1;
    }
    var ratio = (py / ih);
    return (ratio===0)?1:ratio;
}

/**
 * A replacement for context.drawImage
 * (args are for source and destination).
 */
function drawImageIOSFix(ctx, img, sx, sy, sw, sh, dx, dy, dw, dh) {
    var vertSquashRatio = detectVerticalSquash(img);
    ctx.drawImage(img, sx, sy, sw, sh, dx, dy, dw, dh / vertSquashRatio);
}

上記をどこかに定義しておいて下記のように変更してみると良い。

    //context.drawImage(img, 0, 0, canvaswidth, canvasheight); //これはダメだった
    drawImageIOSFix(context, img, 0, 0, width, height, 0, 0, canvaswidth, canvasheight);
10
Help us understand the problem. What is going on with this article?
Why not register and get more from Qiita?
  1. We will deliver articles that match you
    By following users and tags, you can catch up information on technical fields that you are interested in as a whole
  2. you can read useful information later efficiently
    By "stocking" the articles you like, you can search right away
ms2sato
何でも屋だが、最近はRails中心。JavaScript好き。レビューやタスク分解が多くて近頃仕事ではコード書けていない。仕事でもプライベートでもコラボできる人が常に欲しい人。 会社では人を育てるの中心で https://circlearound.co.jp/training/ こんなのやってます。 独り言はこっち https://ms2sato.circlearound.co.jp

Comments

No comments
Sign up for free and join this conversation.
Sign Up
If you already have a Qiita account Login
10
Help us understand the problem. What is going on with this article?