在画布中旋转精灵 [英] Rotating a sprite in a canvas

问题描述

我在一幅图像中有几个精灵，我想将单个精灵绘制到画布上.我无法让它们旋转.

I have several sprites in one image and I want to draw individual sprites to a canvas. I am having trouble getting them to rotate.

当我尝试旋转精灵时，似乎它从未绕着应该旋转的点旋转.

When I attempt to rotate a sprite, it seems that it never rotates around the point that it should be.

我正在使用以下功能.角度是在加载时设置的.

I'm using the following function. The angle is set at loading time.

Sprite.prototype.draw = function (ctx, x ,y) {
    if (this.angle == 0) {
        ctx.drawImage(this.img, this.x, this.y, this.width, this.height, x, y, this.width, this.height);
    }
    else {
        ctx.save();
        ctx.translate(x, y);
        ctx.rotate(this.angle * this._TORADIANS);
        ctx.drawImage(this.img, this.x, this.y, this.width, this.height, x, y, this.width, this.height);
        ctx.restore();
    }
}

我浏览了一些教程，在这种情况下似乎无济于事.

I've looked through several tutorials and nothing seems to work in this case.

推荐答案

首先将上下文翻译成 x，y ，再加上图像尺寸的一半，以便上下文的原点位于图片的所需位置:

Start by translating the context to x, y plus half the dimensions of the image so that the context origin is at the center of the image's desired location:

ctx.translate(x + this.width / 2, y + this.height / 2);

将上下文旋转所需的度数:

Rotate the context by the desired number of degrees:

ctx.rotate(this.angle * Math.PI / 180);

绘制图像，将其尺寸偏移一半以说明原点的位置:

Draw the image, offset by half its dimensions to account for the position of the origin:

ctx.drawImage(this.img, this.x, this.y, this.width, this.height,
                        -this.width / 2, -this.height / 2, this.width, this.height);

现在撤消旋转和平移:

ctx.rotate(-this.angle * Math.PI / 180);
ctx.translate(-x - this.width / 2, -y - this.height / 2);

根据您的情况，您可以按以下方式更改 Sprite.draw().

In your case, you can change Sprite.draw() as follows.

Sprite.prototype.draw = function (ctx, x, y) {
    ctx.save();
    ctx.translate(x + this.width / 2, y + this.height / 2);
    ctx.rotate(this.angle * Math.PI / 180);
    ctx.drawImage(this.img, this.x, this.y, this.width, this.height,
                            -this.width / 2, -this.height / 2, this.width, this.height);
    ctx.restore();
};

以下代码段大体上演示了这种方法.

The following snippet demonstrates this approach in general.

window.onload = function () {
  var width = 600,
      height = 600,
      canvas = document.getElementsByTagName('canvas')[0],
      context = canvas.getContext('2d');
  canvas.width = width;
  canvas.height = height;
  var image = new Image();
  image.src = 'https://cdn.sstatic.net/Sites/stackoverflow/img/apple-touch-icon.png?v=c78bd457575a';
  function draw(x, y, degrees) {
    context.translate(x + image.width / 2, y + image.height / 2);
    context.rotate(degrees * Math.PI / 180);
    context.drawImage(image, 0, 0, image.width, image.height,
        -image.width / 2, -image.height / 2, image.width, image.height);
    context.rotate(-degrees * Math.PI / 180);
    context.translate(-x - image.width / 2, -y - image.height / 2);
  }
  image.onload = function () {
    var degrees = 0;
    function loop() {
      degrees = (degrees + 1) % 360;
      context.fillRect(0, 0, canvas.width, canvas.height);
      draw(0, 0, degrees);
      window.requestAnimationFrame(loop);
    };
    window.requestAnimationFrame(loop);
  };
};

canvas {
  border: 2px solid #ccc;
}

<canvas></canvas>

这篇关于在画布中旋转精灵的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！