jCanvas上的globalCompositeOperation

Question

如何在jCanvas jQuery插件中集成globalCompositeOperation(或任何其他提供“multiply”颜色操作的插件)？

// How do I get this working? //
var ctx = document.getElementById('canvas').getContext('2d');
ctx.globalCompositeOperation = 'darker';

// With this - //
$("canvas").drawArc({
  fillStyle: "#c7302a",
  x: 100, y: 100,
  radius: 50
});

$("canvas").drawArc({
  fillStyle: "#395797",
  x: 170, y: 100,
  radius: 50,
  opacity: 1
});

Answer 1

好的，我解决了。在苦苦挣扎了几个小时后，它太简单了:我使用了context blender插件。

JS代码：

$("#canvasReal").drawArc({ // Draw on the real canvas
  fillStyle: "#c7302a",
  x: 100, y: 100,
  radius: 50
});

$("#canvasOff").drawArc({ // Draw on the off screen canvas
  fillStyle: "#395797",
  x: 150, y: 100,
  radius: 50
});

// Blend off-screen canvas onto the real canvas
    var over = canvasOff.getContext('2d'); 
    var under = canvasReal.getContext('2d');
    over.blendOnto(under,'multiply'); 

HTML代码：

<canvas width="500" height="250" id="canvasReal"></canvas>
<canvas width="500" height="250"id="canvasOff" style="display:none;"></canvas>

Answer 2

对于记录，jCanvas有一个compositing属性，该属性的值映射到ctx.globalCompositeOperation。

$("canvas").drawArc({
  fillStyle: "#c7302a",
  x: 100, y: 100,
  radius: 50,
  compositing: 'darker'
});

-Caleb