HTML5 Canvas - 用图像填充圆圈

Question

如何在圆圈内绘制图像？如果我这样做：

context.beginPath();
context.arc((e.pageX),(e.pageY),161,0,Math.PI*2,true);
context.closePath();

然后如何使用 fill() 来填充我绘制的图像？

Answer 1

前几天我这样做是为了做一件大事；

var thumbImg = document.createElement('img');

thumbImg.src = 'path_to_image';
thumbImg.onload = function() {
    tmpCtx.save();
    tmpCtx.beginPath();
    tmpCtx.arc(25, 25, 25, 0, Math.PI * 2, true);
    tmpCtx.closePath();
    tmpCtx.clip();

    tmpCtx.drawImage(thumbImg, 0, 0, 50, 50);

    tmpCtx.beginPath();
    tmpCtx.arc(0, 0, 25, 0, Math.PI * 2, true);
    tmpCtx.clip();
    tmpCtx.closePath();
    tmpCtx.restore();
};

非常适合我。

这是我制作的一个更复杂的版本，它也进行图像缓存，https://jsfiddle.net/jaredwilli/ex5n5/

Answer 2

不确定您是否仍在寻找答案，但方法如下：

var ctx = document.getElementById('your_canvas').getContext("2d");
//ctx.lineWidth = 13; 
//ctx.strokeStyle = 'rgba(0,0,0,1)'; 
//ctx.fillStyle="rgba(0,0,0,0)" // if using this, make sure alpha < 1

ctx.arc(100,100, 50, 0, Math.PI*2,true); // you can use any shape
ctx.clip();

var img = new Image();
img.addEventListener('load', function(e) {
    ctx.drawImage(this, 0, 0, 200, 300);
    //ctx.fill();
//ctx.stroke();
}, true);
img.src="/path/to/image.jpg";

您也可以使用图案来做到这一点，但图像放置的灵活性会降低

ctx.arc(100,100, 70, 0, Math.PI*2,true);
ctx.clip();

img = new Image()
img.addEventListener('load', function(e) {
    ctx.fillStyle = ctx.createPattern(this, 'no-repeat') 
    ctx.fill();
}, true);
img.src="/path/to/image.jpg"

Answer 3

考虑使用其中一些替代方案：

• 对border-radius 使用带有CSS 的<img>：http://jsfiddle.net/ChrisMorgan/BQGxA/

• 使用 SVG 而不是 <canvas> 并将椭圆设置为图像的剪切路径。 （更复杂的剪切路径也很容易）

不了解您的要求和情况我不知道这些是否能满足您的要求，但我认为它们值得考虑。 <canvas> 并不能解决您的所有问题 - 对于其中许多情况，普通 HMTL 和/或 SVG 中的 CSS 可能更适合。

Answer 4

clip() 方法的问题是 Chrome 会将边框呈现为非抗锯齿，如this question 所示。

一种解决方案是使用 globalCompositeOperation，如 Daniel 的回答所示：

//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, ...);