HTML5画布 - 用图像填充圆圈

Question

如何在圆圈内绘制图像？如果我做:

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

然后我如何使用fill()用我绘制的图像填充它？

Answer 1

前几天我做了一件我做的大事;

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();
};

工作对我来说很完美.

这是我制作的更复杂版本的图像缓存,https://jsfiddle.net/jaredwilli/ex5n5/

Answer 2

不确定你是否还在寻找答案,但这里是如何:

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";

您也可以使用模式执行此操作,但您可以获得较少的图像放置灵活性

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

考虑使用以下一些替代方法：

• <img>与CSS搭配使用border-radius：http : //jsfiddle.net/ChrisMorgan/BQGxA/

• 而不是使用SVG <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，我不知道这些条件是否可以满足您的要求，但我认为值得考虑。<canvas>并非解决所有问题的方法-在许多情况下，普通HMTL和/或SVG中的CSS可能是更好的选择。

Answer 4

clip() 方法的问题在于 Chrome 会将边框呈现为非抗锯齿，如本问题所示。

一种解决方案是使用 globalCompositeOperation 如丹尼尔的回答所示：

//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, ...);