javascript - 如何在 Canvas 元素上正确应用 globalCompositeOperation？

Question

我对 globalCompositeOperation 有疑问。

我的目标是让蓝色元素仅显示在红色元素内部，并且根本不应该在红色矩形外部可见(有点溢出隐藏效果)。另外，红色和蓝色都必须具有转换能力(都是可编辑的)。

如您所见，如果我再向 Canvas 中添加一个元素(黄色元素)，蓝色元素将在黄色和蓝色重叠的区域可见。

http://jsfiddle.net/redlive/q4bvu0tb/1/

var canvas = new fabric.Canvas('c');


var yellow = new fabric.Circle({
  top: 200,
  left: 0,
  radius: 100,
  strokeDashArray: [5, 5],
  stroke: 'black',
  strokeWidth: 5,
  fill: 'yellow'
});
canvas.add(yellow);


var red = new fabric.Rect({
  top: 0,
  left: 0,
  width: 300,
  height: 300,
  strokeDashArray: [5, 5],
  stroke: 'black',
  strokeWidth: 5,
  fill: 'red',
  rx: 40
});
canvas.add(red);


var blue = new fabric.Circle({
  top: 150,
  left: 80,
  radius: 100,
  strokeDashArray: [5, 5],
  stroke: 'black',
  strokeWidth: 5,
  fill: 'blue',
    globalCompositeOperation: 'source-atop'
});
canvas.add(blue);



var green = new fabric.Circle({
  top: 0,
  left: 0,
  radius: 100,
  strokeDashArray: [5, 5],
  stroke: 'black',
  strokeWidth: 5,
  fill: 'green'
});
canvas.add(green);

强制条件:

• 保留元素在 Canvas 上的外观。
• 无裁剪(因为裁剪不允许同时添加背景颜色和图像)

Answer 1

可以通过以下步骤完成...

• 在绘制蓝色元素之前移除黄色元素
• 绘制完 blue 元素后，将 yellow 元素的 globalCompositeOperation 设置为 destination-over 并将其添加回去

var canvas = new fabric.Canvas('c');

var yellow = new fabric.Circle({
  top: 200,
  left: 0,
  radius: 100,
  strokeDashArray: [5, 5],
  stroke: 'black',
  strokeWidth: 5,
  fill: 'yellow'
});
canvas.add(yellow);

var red = new fabric.Rect({
  top: 0,
  left: 0,
  width: 300,
  height: 300,
  strokeDashArray: [5, 5],
  stroke: 'black',
  strokeWidth: 5,
  fill: 'red',
  rx: 40
});
canvas.add(red);

canvas.remove(yellow); //remove yellow

var blue = new fabric.Circle({
  top: 150,
  left: 80,
  radius: 100,
  strokeDashArray: [5, 5],
  stroke: 'black',
  strokeWidth: 5,
  fill: 'blue',
  globalCompositeOperation: 'source-atop'
});
canvas.add(blue);

yellow.set({globalCompositeOperation: 'destination-over'}); //set gCO for yellow
canvas.add(yellow); //add yellow back

var green = new fabric.Circle({
  top: 0,
  left: 0,
  radius: 100,
  strokeDashArray: [5, 5],
  stroke: 'black',
  strokeWidth: 5,
  fill: 'green'
});
canvas.add(green);

<script src="https://cdnjs.cloudflare.com/ajax/libs/fabric.js/1.5.0/fabric.min.js"></script>
<canvas id="c" width="800" height="800"></canvas>