画布如何旋转图像和重新绘制而不旋转整个上下文

Canvas how to rotate image and redraw without rotating the entire context

本文关键字：旋转 上下文 绘制 图像 布如何 新绘制      更新时间：2023-09-26

我正试图用帆布和JS重新创建小行星，并且我根据它的角度旋转船有麻烦。我已经测试了计算角度和转换为弧度的方法，并且他们检查，并且在draw()中调用时定义了角度，但旋转没有发生。当我可以进行一些旋转时，它会旋转整个上下文，而不是船。

我已经包含了整个ship类，但是相关的函数是底部的draw函数。

function Ship(posOptions) {
  let options = {game: posOptions['game'], color: 'green', pos: posOptions['pos'], radius: 20, vel: [0,0], wrappable: true, type: 0}
  MovingObject.call(this, options);
  this.facingDir = 0;
  this.H = window.innerHeight; //*0.75,
  this.W = window.innerWidth; //*0.75;
  this.xc = this.W/2; //zeby bylo w centrum :v
  this.yc = this.H/2; //jw.
  this.x =  this.xc;
  this.y =  this.yc;
  this.dv = 0.2;
  this.dt = 1;
  this.vx = 0;
  this.vy = 0;
  this.maxVel = 10;
}
Utils.inherits(Ship, MovingObject);
//Relocate ship if hit by asteroid.
Ship.prototype.relocate = function () {
  this.pos = this.game.randomPosition();
  this.vel = [0,0];
};

Ship.prototype.fireBullet = function () {
  let bulletVel = [(this.vel[0] * 4), (this.vel[1] * 5) - 10];
  let bullet = new Bullet({pos: this.pos, vel: bulletVel, game: this.game});
  this.game.bullets.push(bullet);
};
//Calculate velocity based on keyboard input and angle of rotation
Ship.prototype.power = function (impulse) {
  if (impulse[0] === -2 && this.facingDir !== Math.PI) {
    this.facingDir <= Math.PI ? this.facingDir += 30 / 180 * Math.PI : this.facingDir -= 30 / 180 * Math.PI;
  }
  if (impulse[0] === 2 && this.facingDir !== 0) {
    this.facingDir <= 0 ? this.facingDir += 30 / 180 * Math.PI : this.facingDir -= 30 / 180 * Math.PI;
  }
  if (impulse[1] === 2 && this.facingDir !== (Math.PI / 2 * 3)) {
    this.facingDir <= (3 * Math.PI / 2) ? this.facingDir += 30 / 180 * Math.PI : this.facingDir -= 30 / 180 * Math.PI;
  }
  if (impulse[1] === -2 && this.facingDir !== (Math.PI / 2)) {
    this.facingDir <= (Math.PI / 2) ? this.facingDir += 30 / 180 * Math.PI : this.facingDir -= 30 / 180 * Math.PI;
  }
  this.vel[0] += impulse[0];
  this.vel[1] += impulse[1];
};

Ship.prototype.move = function () {
  if (this.isWrappable) {
    this.pos = this.game.wrap(this.pos);
  }
  this.pos[0] += this.vel[0];
  this.pos[1] += this.vel[1];
  //Attenuate the velocity over time so it doesn't accelerate forever. 
  this.vel[0] *= .98;
  this.vel[1] *= .98;
};
Ship.prototype.convertToRadians = function(degree) {
  return degree*(Math.PI/180);
}
Ship.prototype.draw = function (ctx) {
  const img = new Image();
  img.onload = function () {
    ctx.drawImage(img, this.pos[0]-this.radius, this.pos[1]-this.radius)
  };
  img.src = 'galaga_ship.png';
  ctx.drawImage(img, this.pos[0]-this.radius, this.pos[1]-this.radius);
  **//This is where I am stuck.**
  ctx.save();
  ctx.translate(this.x,this.y);
  ctx.rotate(this.facingDir);
  ctx.translate(-7,-10);
  ctx.restore();
};
module.exports = Ship;

在还原上下文之前没有绘制任何内容。ctx.save()和ctx.restore()的使用方式应该是，你保存它，用原始的旋转和原点的位置，你变换它，做绘图，然后恢复到它原来的位置。因此，只有绘图使用转换后的坐标系，然后其他一切都恢复正常。

  **//This is where I am stuck.**
  ctx.save();
  ctx.translate(this.x,this.y);
  ctx.rotate(this.facingDir);
  ctx.translate(-7,-10);
// PUT DRAWING YOUR SHIP HERE
  ctx.restore();
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