canvas.drawImage 将裁剪后的图像不成比例

Question

渲染以下图像时（右键单击 + 查看图像以查看完整尺寸）：

大小为 4800x320，html5 canvas.drawImage() 并试图从中裁剪部分 480x320 图像，canvas 将图像放大。

这是我的代码：

var canvas = document.createElement('canvas');
canvas.setAttribute('id', 'my-canvas');
canvas.style.width = 480;
canvas.style.height = 320;
canvas.style.display = 'block';

document.body.appendChild(canvas);

var img=document.createElement('img');
img.src='http://cdn.cocimg.com/bbs/attachment/Fid_14/14_29173_56b5563d9a81a86.png';
img.onload = function () {
  var c=document.getElementById('my-canvas');
  var ctx=c.getContext('2d');
  ctx.drawImage(img,480,0,480,320,0,0,480,320);
}

另外，当我将图像绘制到较小的比例时，它突然适合：

var canvas = document.createElement('canvas');
canvas.setAttribute('id', 'my-canvas');
canvas.style.width = 480;
canvas.style.height = 320;
canvas.style.display = 'block';

document.body.appendChild(canvas);

var img=document.createElement('img');
img.src='http://cdn.cocimg.com/bbs/attachment/Fid_14/14_29173_56b5563d9a81a86.png';
img.onload = function () {
  var c=document.getElementById('my-canvas');
  var ctx=c.getContext('2d');
  ctx.drawImage(img,480,0,480,320,0,0,240,160);
}

有人知道这是为什么吗？

Answer 1

Canvas' 默认位图大小为 300x150 像素。设置 CSS 大小不会改变位图大小。位图直接在元素上设置了自己的 width/height properties/attributes。如果不是，位图将被拉伸以填充元素 CSS 大小。

更改这些：

canvas.style.width = 480;
canvas.style.height = 320;

至

canvas.width = 480;
canvas.height = 320;

您还可以设置代码以重用上下文：

//var c=document.getElementById('my-canvas'); // you already have this as canvas
var ctx=canvas.getContext('2d');              // global/parent scope

var img=document.createElement('img');
img.src='http://cdn.cocimg.com/bbs/attachment/Fid_14/14_29173_56b5563d9a81a86.png';
img.onload = function () {
  ctx.drawImage(img,480,0,480,320,0,0,480,320);
}

canvas.drawImage 将裁剪后的图像不成比例

canvas.drawImage blows the cropped imaged out of proportion

html

javascript

css

canvas

drawimage