将物体放在另外两个之间

Question

我是 canvas 的新手，我想将我的 DOM (HTML+JS) 动画移动到 canvas。我试图在 Basic animations - Web API Interfaces | MDN and I managed to do it. Fiddle.

中模仿太阳系动画

我对 globalCompositeOperation 有疑问。在动画中，地球正在投射阴影（低不透明度矩形），而当月亮围绕这个黑暗区域运行时，月亮应该在阴影矩形下方。如果我使用

context.globalCompositeOperation = 'destination-over';

在 draw 函数的顶部，我得到了预期的结果 (fiddle)，但随后月球和地球在轨道线下方旋转。我不想要那个。我希望地球和月球都在轨道线上方，而月球在阴影矩形下方。有没有办法做到这一点？如果没有，我将不得不摆脱阴影。

我没有在我的代码中使用 context.save() 和 context.restore()，尽管它们在原始动画中使用过。我对他们不熟悉。可能跟我的问题有关...

var sun = {
  radius : 80,
  x  : 0,
  y  : 0,
};
var earth = {
  radius : 20,
  x  : sun.radius + 80,
  y  : 0,
  angle : 0,
};
var moon = {
  radius : 8,
  x  : 50,
  y  : 0,
  angle : 0,
};

function draw() {
  var canvas = document.getElementById("canvas");
  var context = canvas.getContext("2d");

  context.resetTransform();
  context.clearRect(0, 0, canvas.width, canvas.height)
  context.translate(canvas.width/2, canvas.height/2);

  // sun
  context.beginPath();
  context.arc(0, 0, sun.radius, 0, 360 * Math.PI/180, false);
  context.fillStyle = "yellow";
  context.fill();

  // sun orbit
  context.beginPath();
  context.arc(0, 0, earth.x, 0, Math.PI*2, false);
  context.strokeStyle = "rgba(180,150,0,.25)";
  context.stroke();

  // earth
  earth.angle += .2;
  if (earth.angle == 360) {
    earth.angle = 0;
  }
  context.globalCompositeOperation = 'source-over'; // earth is above the sun and its orbit
  context.rotate(earth.angle * Math.PI/180);
  context.translate(earth.x, 0);
  context.beginPath();
  context.arc(0, 0, earth.radius, 0, 360*(Math.PI/180), false);
  context.fillStyle = "royalblue";
  context.fill();
  context.globalCompositeOperation = 'destination-over'; // earth's orbit and shadow are below the earth

  // earth shadow
  context.fillStyle = "rgba(0,0,0,0.1)";
  context.fillRect(0, -earth.radius, earth.radius*16, earth.radius*2);

  // earth orbit
  context.beginPath();
  context.arc(0, 0, moon.x, 0, Math.PI*2, false);
  context.strokeStyle = "rgba(0,0,200,.2)";
  context.stroke();

  // moon
  moon.angle += 1;
  if (moon.angle == 360) {
    moon.angle = 0;
  }
  context.globalCompositeOperation = 'source-over'; // moon is above the earth's orbit and shadow
  context.rotate(moon.angle * Math.PI/180);
  context.translate(moon.x, 0);
  context.beginPath();
  context.arc(0, 0, moon.radius, 0, 360 * Math.PI/180, false);
  context.fillStyle = "silver";
  context.fill();

  requestAnimationFrame(draw);
}

draw();

html {
  background: rgb(230,230,230);
}
canvas {
  background: white;
  border: 1px solid rgba(0,0,0,.2);
  box-shadow: 0 0 5px rgba(0,0,0,.1);
}

<canvas id="canvas" width="640" height="480"></canvas>

Answer 1

您只需要稍微更改一下代码中的顺序 - 在这种情况下确实不需要复合模式：

所有静态元素，如太阳和地球轨道，都可以渲染一次并设置为元素的背景。
先画出月球轨道
画月亮秒
第三次画阴影
终于画出地球了。

在这次更新中我没有重新计算旋转等，而是使用了save/restore。

var sun = {
    radius : 80,
    x  : 0,
    y  : 0,
};
var earth = {
    radius : 20,
    x  : sun.radius + 80,
    y  : 0,
    angle : 0,
};
var moon = {
    radius : 8,
    x  : 50,
    y  : 0,
    angle : 0,
};

var canvas = document.getElementById("canvas");
var context = canvas.getContext("2d");

context.setTransform(1,0,0,1,canvas.width/2, canvas.height/2);

// sun
context.beginPath();
context.arc(0, 0, sun.radius, 0, 360 * Math.PI/180, false);
context.fillStyle = "yellow";
context.fill();

// sun orbit
context.beginPath();
context.arc(0, 0, earth.x, 0, Math.PI*2, false);
context.strokeStyle = "rgba(180,150,0,.25)";
context.stroke();

canvas.style.backgroundImage = "url(" + canvas.toDataURL() + ")";

function draw() {

    context.setTransform(1,0,0,1,0,0);    
    context.clearRect(0, 0, canvas.width, canvas.height)
    context.setTransform(1,0,0,1,canvas.width/2, canvas.height/2);
    
    // earth
    earth.angle += .2;
    if (earth.angle == 360) {
        earth.angle = 0;
    }
    context.rotate(earth.angle * Math.PI/180);
    context.translate(earth.x, 0);

        // earth orbit
    context.beginPath();
    context.arc(0, 0, moon.x, 0, Math.PI*2, false);
    context.strokeStyle = "rgba(0,0,200,.2)";
    context.stroke();
    
    // moon
    context.save();
    moon.angle += 1;
    if (moon.angle == 360) {
        moon.angle = 0;
    }
    context.rotate(moon.angle * Math.PI/180);
    context.translate(moon.x, 0);
    context.beginPath();
    context.arc(0, 0, moon.radius, 0, 360 * Math.PI/180, false);
    context.fillStyle = "silver";
    context.fill();
    context.restore();

    // earth shadow
    context.fillStyle = "rgba(0,0,0,0.1)";
    context.fillRect(0, -earth.radius, earth.radius*16, earth.radius*2);

    context.beginPath();
    context.arc(0, 0, earth.radius, 0, 360*(Math.PI/180), false);
    context.fillStyle = "royalblue";
    context.fill();
    
    
    requestAnimationFrame(draw);
}

draw();

html {
    background: rgb(230,230,230);
}
canvas {
    background: white;
    border: 1px solid rgba(0,0,0,.2);
    box-shadow: 0 0 5px rgba(0,0,0,.1);
}

    <canvas id="canvas" width="640" height="480"></canvas>

将物体放在另外两个之间

Place object between another two

html

canvas

globalcompositeoperation