javascript - 如何在 Canvas 中缩放图像？

Question

我正在尝试在 Canvas 中使用 cover 模拟来显示图像。我找到了一些很酷的 answer关于如何去做。

到目前为止，我的图像会根据屏幕分辨率发生变化，但只有在刷新页面后才会发生变化。

我怎样才能得到以下 scaling effect不刷新页面？ 尝试调整那里的窗口大小。

HTML

<canvas id="canvas"></canvas> 

JS

var ctx = canvas.getContext('2d'),
    img = new Image;

canvas.setAttribute('width', window.innerWidth);
canvas.setAttribute('height', window.innerHeight);

img.onload = draw;
img.src = 'https://upload.wikimedia.org/wikipedia/commons/0/0f/2010-02-19_3000x2000_chicago_skyline.jpg';

function draw() {
    drawImageProp(ctx, this, 0, 0, canvas.width, canvas.height);
}

/**
 * By Ken Fyrstenberg
 *
 * drawImageProp(context, image [, x, y, width, height [,offsetX, offsetY]])
 *
 * If image and context are only arguments rectangle will equal canvas
*/
function drawImageProp(ctx, img, x, y, w, h, offsetX, offsetY) {

    if (arguments.length === 2) {
        x = y = 0;
        w = ctx.canvas.width;
        h = ctx.canvas.height;
    }

    /// default offset is center
    offsetX = offsetX ? offsetX : 0.5;
    offsetY = offsetY ? offsetY : 0.5;

    /// keep bounds [0.0, 1.0]
    if (offsetX < 0) offsetX = 0;
    if (offsetY < 0) offsetY = 0;
    if (offsetX > 1) offsetX = 1;
    if (offsetY > 1) offsetY = 1;

    var iw = img.width,
        ih = img.height,
        r = Math.min(w / iw, h / ih),
        nw = iw * r,   /// new prop. width
        nh = ih * r,   /// new prop. height
        cx, cy, cw, ch, ar = 1;

    /// decide which gap to fill    
    if (nw < w) ar = w / nw;
    if (nh < h) ar = h / nh;
    nw *= ar;
    nh *= ar;

    /// calc source rectangle
    cw = iw / (nw / w);
    ch = ih / (nh / h);

    cx = (iw - cw) * offsetX;
    cy = (ih - ch) * offsetY;

    /// make sure source rectangle is valid
    if (cx < 0) cx = 0;
    if (cy < 0) cy = 0;
    if (cw > iw) cw = iw;
    if (ch > ih) ch = ih;

    /// fill image in dest. rectangle
    ctx.drawImage(img, cx, cy, cw, ch,  x, y, w, h);
}

Answer 1

在 window resize event 上添加对 draw 的调用:

window.onresize = draw;

这应该可以解决问题。

代替:

drawImageProp(ctx, this, 0, 0, canvas.width, canvas.height);

在draw函数中传递img:

drawImageProp(ctx, img, 0, 0, canvas.width, canvas.height);

然后，将宽度/高度设置移动到draw 函数中，导致:

function draw() {
    canvas.setAttribute('width', window.innerWidth);
    canvas.setAttribute('height', window.innerHeight);
    drawImageProp(ctx, img, 0, 0, canvas.width, canvas.height);
}

您可能还想对重绘进行去抖动处理:

var timeOut;
window.onresize = function(){
    if(timeOut)
        clearTimeout(timeOut);
    timeOut = setTimeout(draw, 10);
}

这可以防止 draw 函数在调整窗口大小时每秒被调用几次。

这是 a working example ，从您的 codepen 派生。