HTML5 Canvas - 车轮旋转功能很奇怪

Question

In HTML5 canvas, I have a wheel, that I'm trying to spin, however the wheel is acting really crazy. It's being swung around the screen, out of it and into it again.

 var canvas = document.getElementById("my-canvas"); var ctx = canvas.getContext("2d"); var img = new Image(); img.src = "http://www.roulette30.com/wp-content/uploads/americanroulette.png"; ctx.drawImage(img, 70, 70, 200, 200); function spinWheel() { ctx.clearRect(0, 0, canvas.width, canvas.height); ctx.translate(0, 0, canvas.width, canvas.height); ctx.rotate(1 * Math.PI / 180); ctx.drawImage(img, 70, 70, 200, 200); } setInterval(spinWheel, 0); 

 #my-canvas { border: 1px solid black; } 

 <!--<img id="my-image" height="200" width="200" src="http://www.roulette30.com/wp-content/uploads/americanroulette.png"> --> <canvas id="my-canvas" width="400" height="400"></canvas> 

I was certain that if I called the translate method, I could always have the wheel positioned in the center of the screen. Along with rotate , this was sure to work.

Any insights will be welcomed.

Answer 1

You need to translate outside of your spin code, to half width/height for dead center.

 var canvas = document.getElementById("my-canvas"); var ctx = canvas.getContext("2d"); var img = new Image(); img.src = "http://www.roulette30.com/wp-content/uploads/americanroulette.png"; var speed = 1; // adding increases speed var loop = true; ctx.translate(canvas.width/2, canvas.height/2); function spinWheel() { ctx.clearRect(0, 0, canvas.width, canvas.height); ctx.rotate(speed * Math.PI / 180); ctx.drawImage(img, -(img.width/2), -(img.height/2)); if (loop) requestAnimationFrame(spinWheel); } spinWheel(); 

 #my-canvas { border: 1px solid black; } 

 <canvas id="my-canvas" width="400" height="400"></canvas>