Question

我在我的网站上使用this coverflow脚本，我不知道如何输出带圆角的画布。

这是绘制图像的代码

ctx.drawImage(image, cropLeft, cropTop, wid-2*cropLeft, hei-2*cropTop, 0, 0, newWidth, newHeight);

我使用arc（）或arcTo（）函数阅读了一些教程，但我们都没有使用图像作为对象。

UPDATE1：我看到drawimage（）只有以下绘图参数： •图像尺寸和成分与原稿相同 •从原始尺寸调整大小的图像 •从原始

裁剪的图像

所以，我想，通过画布画圆角的图像是不可能的。

Answer 1

您可以使用context.clip()绘制在圆角矩形内剪裁的图像

enter image description here

首先绘制一个带圆角的矩形（无需笔触或填充）：

  // draw a rounded rectangle

  ctx.beginPath();
  ctx.moveTo(x + radius, y);
  ctx.lineTo(x + width - radius, y);
  ctx.quadraticCurveTo(x + width, y, x + width, y + radius);
  ctx.lineTo(x + width, y + height - radius);
  ctx.quadraticCurveTo(x + width, y + height, x + width - radius, y + height);
  ctx.lineTo(x + radius, y + height);
  ctx.quadraticCurveTo(x, y + height, x, y + height - radius);
  ctx.lineTo(x, y + radius);
  ctx.quadraticCurveTo(x, y, x + radius, y);
  ctx.closePath();

然后调用context.clip，这将导致所有未来的绘图在rect

中被剪切

  ctx.clip();

最后，在该矩形内绘制图像，图像将被剪裁成圆形。

  ctx.drawImage(img,10,10,102,77);

以下是示例代码和小提琴：http://jsfiddle.net/m1erickson/FLaee/

<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>

<style>
    body{ background-color: ivory; }
    #canvas{border:1px solid red;}
</style>

<script>
$(function(){

    var canvas=document.getElementById("canvas");
    var ctx=canvas.getContext("2d");

    var img=new Image();
    img.onload=function(){
        ctx.save();
        roundedImage(10,10,102,77,10);
        ctx.clip();
        ctx.drawImage(img,10,10,102,77);
        ctx.restore();
    }
    img.src="https://dl.dropboxusercontent.com/u/139992952/stackoverflow/sky-bg2.jpg";


    function roundedImage(x,y,width,height,radius){
      ctx.beginPath();
      ctx.moveTo(x + radius, y);
      ctx.lineTo(x + width - radius, y);
      ctx.quadraticCurveTo(x + width, y, x + width, y + radius);
      ctx.lineTo(x + width, y + height - radius);
      ctx.quadraticCurveTo(x + width, y + height, x + width - radius, y + height);
      ctx.lineTo(x + radius, y + height);
      ctx.quadraticCurveTo(x, y + height, x, y + height - radius);
      ctx.lineTo(x, y + radius);
      ctx.quadraticCurveTo(x, y, x + radius, y);
      ctx.closePath();
    }

}); // end $(function(){});
</script>

</head>

<body>
    <canvas id="canvas" width=300 height=300></canvas>
</body>
</html>

Answer 2

clip（）解决方案的问题在于Chrome会使用非抗锯齿边框渲染它，如此问题所示。

一个解决方案是使用globalCompositeOperation来完成，正如Daniel在他的回答中所说：

//set-up - probably only needs to be done once
var scratchCanvas = document.createElement('canvas');
scratchCanvas.width = 100;
scratchCanvas.height = 100;
var scratchCtx = scratchCanvas.getContext('2d');


//drawing code
scratchCtx.clearRect(0, 0, scratchCanvas.width, scratchCanvas.height);

scratchCtx.globalCompositeOperation = 'source-over'; //default

//Do whatever drawing you want. In your case, draw your image.
scratchCtx.drawImage(imageToCrop, ...);


//As long as we can represent our clipping region as a single path, 
//we can perform our clipping by using a non-default composite operation.
//You can think of destination-in as "write alpha". It will not touch
//the color channel of the canvas, but will replace the alpha channel.
//(Actually, it will multiply the already drawn alpha with the alpha
//currently being drawn - meaning that things look good where two anti-
//aliased pixels overlap.)
//
//If you can't represent the clipping region as a single path, you can
//always draw your clip shape into yet another scratch canvas.

scratchCtx.fillStyle = '#fff'; //color doesn't matter, but we want full opacity
scratchCtx.globalCompositeOperation = 'destination-in';
scratchCtx.beginPath();
scratchCtx.arc(50, 50, 50, 0, 2 * Math.PI, true);
scratchCtx.closePath();
scratchCtx.fill();


//Now that we have a nice, cropped image, we can draw it in our
//actual canvas. We can even draw it over top existing pixels, and
//everything will look great!

ctx.drawImage(scratchCanves, ...);

带圆角的帆布drawimage

2 个答案: