Javascript 在画布中围绕其中心旋转图像

Rotate an image around its center in canvas

提问by Mark Steggles

I'm trying to do my first image animation on canvas. I want the image to rotate but something is not correct in my code. Any ideas? This is all in a jquery document ready:

我正在尝试在画布上制作我的第一个图像动画。我希望图像旋转，但我的代码中有些地方不正确。有任何想法吗？这一切都在准备好的 jquery 文档中：

var canvas = document.getElementById('logobg1');  
    var ctx = canvas.getContext('2d');

    var img = new Image();   // Create new Image object
    img.src = 'images/containerbg.png'; // Set source path // set img src

    img.onload = function(){ // when image loads
        ctx.drawImage(img,0,0);
        setInterval(function() {
            ctx.save();
            ctx.clearRect(-ctx.canvas.width/2, -ctx.canvas.height/2, ctx.canvas.width, ctx.canvas.height);
            ctx.drawImage(img,0,0);
            ctx.translate(ctx.canvas.width/2, ctx.canvas.height/2); // set canvas context to center
            ctx.rotate(Math.PI / 180 * 0.5); // 1/2 a degree
            ctx.restore();
        }, 16);
    }

回答by

just change the order of your code, i.e.,

只需更改代码的顺序，即，

ctx.rotate(...);

ctx.drawImage(...);

See a working example http://jsbin.com/owuyiq/

查看一个工作示例http://jsbin.com/owuyiq/

$(function () {
    var canvas = document.getElementById('logobg1');
    var ctx = canvas.getContext('2d');
    var img = new Image();

    var ang = 0; //angle
    var fps = 1000 / 25; //number of frames per sec
    img.onload = function () { //on image load do the following stuff
        canvas.width = this.width << 1; //double the canvas width
        canvas.height = this.height << 1; //double the canvas height
        var cache = this; //cache the local copy of image element for future reference
        setInterval(function () {
            ctx.save(); //saves the state of canvas
            ctx.clearRect(0, 0, canvas.width, canvas.height); //clear the canvas
            ctx.translate(cache.width, cache.height); //let's translate
            ctx.rotate(Math.PI / 180 * (ang += 5)); //increment the angle and rotate the image 
            ctx.drawImage(img, -cache.width / 2, -cache.height / 2, cache.width, cache.height); //draw the image ;)
            ctx.restore(); //restore the state of canvas
        }, fps);
    };

    img.src = 'http://i.stack.imgur.com/Z97wf.jpg?s=128'; //img
});

回答by jonyB

Based on the comments above, but a bit more simple and in vanilla. This one worked for me perfectly. Of course you should use clearRect in order to erase the canvas on each rendering.

基于上面的评论，但更简单和香草。这个对我来说非常有效。当然，您应该使用 clearRect 以便在每次渲染时擦除画布。

var canvas = document.querySelector('#my-canvas');
var ctx = canvas.getContext('2d')
var ang = 0

function rotateAndRenderImg() {
    var img = document.querySelector('img')
    ctx.save()
    var pos = {x: desiredRenderPosX, y: desiredRenderPosY}
    ctx.translate(pos.x ,pos.y)    
    ctx.rotate(Math.PI / 180 * (ang += 5))
    ctx.drawImage(img, -img.width / 2, -img.height / 2, img.width, img.height)
    ctx.restore()
}

回答by lepe

Based in the accepted answer, this example, allows you to use a fixed canvas size (and not in relation to the image size):

根据已接受的答案，此示例允许您使用固定的画布大小（与图像大小无关）：

 $(function() {
 
        var canvas = document.getElementById('logobg1');
        var ctx = canvas.getContext('2d');
        var img = new Image();
    
        var ang = 0; //angle
        var fps = 1000 / 25; //number of frames per sec
        img.onload = function () { //on image load do the following stuff
            canvas.width = 500; //Any width
            canvas.height = 500; //Any height
            var cache = this; //cache the local copy of image element for future reference
            var iw = cache.width;
            var ih = cache.height;
            setInterval(function () {
                ctx.save(); //saves the state of canvas
                ctx.clearRect(0, 0, canvas.width, canvas.height); //clear the canvas
                ctx.translate(canvas.width/2, canvas.height/2); //let's translate
                ctx.rotate(Math.PI / 180 * (ang += 5)); //increment the angle and rotate the image 
                ctx.translate(-(canvas.width/2), -(canvas.height/2)); //let's translate
                ctx.drawImage(img, canvas.width/2 - iw/2, canvas.height/2 - ih/2, iw, ih); //draw the image ;)
                ctx.restore(); //restore the state of canvas
            }, fps);
        };
        
        img.src = 'https://lh4.ggpht.com/wKrDLLmmxjfRG2-E-k5L5BUuHWpCOe4lWRF7oVs1Gzdn5e5yvr8fj-ORTlBF43U47yI=w300'; //img
 
 })
 

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<canvas id="logobg1" > test canvas <canvas>

Working example: jsbin.com/suwovibove/

工作示例：jsbin.com/suwovibove/

Note: try removing ctx.saveand ctx.restorefor a cool spin.

注意：尝试移除ctx.save并ctx.restore进行凉爽的旋转。