10

我在我的网站上使用这个封面流脚本，我不知道如何输出带有圆角的画布。

这是绘制图像的代码

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

我阅读了一些使用 arc() 或 arcTo() 函数的教程，但我们都没有使用图像作为对象。

更新 1：我看到 drawimage() 只有以下绘图参数： • 与原始图像大小和组成相同的图像 • 从原始图像调整大小的图像 • 从原始图像裁剪的图像

所以，我想，不可能通过画布绘制圆角的图像..

4

2 回答 2

28

您可以context.clip()用来绘制在圆角矩形内剪辑的图像

在此处输入图像描述

首先画一个圆角的矩形（不需要描边或填充）：

  // 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都被剪辑在矩形内

  ctx.clip();

最后，在该矩形内绘制您的图像，您的图像将被裁剪。

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

这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>
于 2013-10-25T15:31:31.733 回答
3

clip() 解决方案的问题是 Chrome 将使用非抗锯齿的边框呈现它，如本问题所示。

正如丹尼尔在他的回答中所说，一种解决方案是使用 globalCompositeOperation 来做到这一点：

//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, ...);
于 2014-08-27T15:56:16.540 回答