javascript - Canvas 动画图像旋转

Question

我想在 Canvas 上为我的脚本创建一个背景，使图像在屏幕上落下并向下旋转。那么有人可以向我解释如何旋转图像，然后使用 <canvas> 将其绘制到屏幕上吗？元素。我有以下代码:

    Equations.prototype.Draw = function() {
        //increases the rotational value every loop
        this.rotate = (this.rotate + 1) % 360;
        //rotates the canvas
        ctx.rotate(this.rotate*Math.PI/180);
        //draw the image using current canvas rotation
        ctx.drawImage(this.img,this.x,this.y);
        //restore canvas to its previous state
        ctx.rotate(-this.rotate*Math.PI/180);
    };

我尝试这样做，发现图像会旋转，但也会在屏幕上围绕点 (0,0) 以圆形移动，我希望它留在原地旋转的同一位置。我该如何解决这个问题，谢谢!

Answer 1

保存上下文，对其进行变换、旋转、绘制、恢复。

const rand = (m, M) => Math.random() * (M - m) + m,
  PI = Math.PI,
  TAU = PI * 2,
  width = window.innerWidth,
  height = window.innerHeight,
  ctx = document.getElementById('cvs').getContext('2d'),
  items = [],
  Item = function() {
    this.h = 32;
    this.w = 32;
    this.IMG = new Image();
    this.IMG.src = 'http://stackoverflow.com/favicon.ico';
    this.start();
    return this;
  };
  
Item.prototype.start = function() {
  this.x = rand(0, width - this.w / 2);
  this.y = rand(0, height);
  this.angle = rand(0, TAU);
  this.speed = rand(0.1, 0.5);
}

Item.prototype.move = function() {
  // Manipulate properties
  if (this.y > height + this.h) { // if is below bottom
    this.start();
    this.y = -this.h; // restart from top
  }
  this.y += this.speed / 0.1;
  this.angle += this.speed;
  this.angle %= TAU;

  // Manipulate context 
  ctx.save(); // save context
  ctx.translate(this.x, this.y); // move to point
  ctx.rotate(this.angle); // rotate around that point
  ctx.drawImage(this.IMG, -this.w/2, -this.h/2);
  ctx.restore(); // restore to initial coordinates 
};

// Setup canvas
ctx.canvas.width = width;
ctx.canvas.height = height;

// Create falling Icons
for (var i = 0; i < 100; i++) items.push(new Item());

// Animation loop
(function loop() {
  ctx.clearRect(0, 0, width, height);
  items.forEach(Item => Item.move());
  requestAnimationFrame(loop);
}());

body {margin: 0;}
canvas {display: block;}

<canvas id="cvs"></canvas>