javascript - JS:如何获取受支持的HTML canvas globalCompositeOperation类型的列表

我想制作一个HTML select列表，通过它可以选择在混合两个globalCompositeOperation元素时将应用哪种类型的canvas，如下所示:

<select name="blending-modes" id="blending-modes">
    <option value="source-over">source-over</option>
    <option value="source-in">source-in</option>
    <option value="source-out">source-out</option>
    ...
</select>

有没有一种方法可以以编程方式获取可用globalCompositeOperation类型的列表作为Javascript对象或数组，因此可以用来用数据填充select元素，而不是手动填充它？此信息是否存储在某些本机变量中？

我不想只是验证用户浏览器是否支持某些混合模式，如here所述。我想获得受支持的globalCompositeOperation类型的完整列表，以便在浏览器中选择混合模式。

最佳答案

没有，没有原生属性可以告诉我们浏览器支持哪些globalCompositeOperation模式。
您必须通过遍历所有规范定义的对象进行测试，并检查它是否仍然是您刚刚设置的:

function getGCOModes() {
  var gCO = ["source-over", "source-in", "source-out", "source-atop", "destination-over", "destination-in", "destination-out", "destination-atop", "lighter", "copy", "xor", "multiply", "screen", "overlay", "darken", "color-dodge", "color-burn", "hard-light", "soft-light", "difference", "exclusion", "hue", "saturation", "color", "luminosity"];
  var ctx = document.createElement('canvas').getContext('2d');
  return gCO.filter(function(g) {
    ctx.globalCompositeOperation = g;
    return ctx.globalCompositeOperation === g;
  });
}

var supportedGCO = getGCOModes();

log.innerHTML = supportedGCO.join(' ');

<p id="log"></p>

但是有一个警告/错误，因为Safari(至少9.0.1)确实接受"hue"，"saturation"，"color"和"luminosity""模式，但实际上并不支持它。

所以在这里，我做了一个功能来测试不同的模式。
这个想法是在第三个 Canvas 上绘制两个用纯色填充的3x3px Canvas 。第一个画在左上角，第二个画在左下角，它们每个都在第三个 Canvas 的中央像素中共享一个像素。

显然，这比属性检查要慢，但是每页只需要一次，因此性能可能不会成为问题。

function testGCOModes() {
  // In this object are stored the pixels as they should appear at the 3 positions we'll look :
  // 0 is an empty pixel
  // 1 is the first pixel drawn
  // 2 is the second pixel drawn
  // 3 is none of the above (blending)
  // We'll look to the central pixel first since it is the most likely to change
  var gCO = {
    "source-over": [2, 1, 2],
    "source-in": [2, 0, 0],
    "source-out": [0, 0, 2],
    "source-atop": [2, 1, 0],
    "destination-over": [1, 1, 2],
    "destination-in": [1, 0, 0],
    "destination-out": [0, 1, 0],
    "destination-atop": [1, 0, 2],
    "lighter": [3, 1, 2],
    "copy": [2, 0, 2],
    "xor": [0, 1, 2],
    "multiply": [3, 1, 2],
    "screen": [3, 1, 2],
    "overlay": [3, 1, 2],
    "darken": [1, 1, 2],
    "color-dodge": [3, 1, 2],
    "color-burn": [3, 1, 2],
    "hard-light": [3, 1, 2],
    "soft-light": [3, 1, 2],
    "difference": [3, 1, 2],
    "exclusion": [3, 1, 2],
    "hue": [3, 1, 2],
    "saturation": [3, 1, 2],
    "color": [3, 1, 2],
    "luminosity": [3, 1, 2]
  };
  // create two 3*3 canvases that will be used as layers
  var c1 = document.createElement('canvas');
  c1.width = c1.height = 3;
  var c2 = c1.cloneNode(true),
    // the third one will be the tester
    c3 = c1.cloneNode(true),

    ctx1 = c1.getContext('2d'),
    ctx2 = c2.getContext('2d'),
    ctx3 = c3.getContext('2d');
  // fill our canvases with solid colors
  ctx1.fillStyle = 'green';
  ctx1.fillRect(0, 0, 3, 3);
  ctx2.fillStyle = 'pink';
  ctx2.fillRect(0, 0, 3, 3);
  // get the image data of one pixel that will corresponds to the values in gCO's arrays
  var em = [0, 0, 0, 0], // 0 or empty
    d1 = ctx1.getImageData(0, 0, 1, 1).data, // 1
    d2 = ctx2.getImageData(0, 0, 1, 1).data; // 2
  // the positions of the pixels in our imageData
  // again, start with the central one
  var pos = [16, 0, 32];

  // make an array of all our gCOs
  var keys = Object.keys(gCO);
  return keys.filter(function(g) {
    var i;
    // get the array corresponding to the actual key
    var arr = gCO[g];

    var layer = [];
    // get the correct imageData for each layer we should find
    for (i = 0; i < 3; i++) {
      switch (arr[i]) {
        case 0:
          layer[i] = em;
          break;
        case 1:
          layer[i] = d1;
          break;
        case 2:
          layer[i] = d2;
          break;
        case 3:
          layer[i] = null;
          break;
      }
    }
    // first reset the canvas
    ctx3.globalCompositeOperation = 'source-over';
    ctx3.clearRect(0, 0, 3, 3);
    // draw the first layer in the top-left corner
    ctx3.drawImage(c1, -1, -1);
    // set the current gCO
    ctx3.globalCompositeOperation = g;
    // draw the second layer in the top-right corner so it comes over it
    ctx3.drawImage(c2, 1, 1);
    // get the image data of our test canvas
    var d3 = ctx3.getImageData(0, 0, 3, 3).data;
    // we will first admit that it is supported;
    var tempResult = true;
    // iterate through the 3 positions (center, top-left, bottom-right)
    for (i = 0; i < pos.length; i++) {
      // we know what it should return
      if (layer[i] !== null) {
        // is it the same pixel as expected ?
        tempResult = d3[pos[i]] === layer[i][0] &&
          d3[pos[i] + 1] === layer[i][1] &&
          d3[pos[i] + 2] === layer[i][2] &&
          d3[pos[i] + 3] === layer[i][3];
      }
      // some blending operation
      else {
        // is it different than the last drawn layer ?
        //(if the mode is not supported, the default gCO "source-over" will be used)
        tempResult = d3[pos[i]] !== d2[0] || d3[pos[i] + 1] !== d2[1] || d3[pos[i] + 2] !== d2[2] || d3[pos[i] + 3] !== d2[3];
      }
      // our flag switched to false
      if (!tempResult)
      // no need to go to the other pixels, it's not supported
        return false;
    }
    // this mode is supported
    return true;
  });
}
var supportedGCO = testGCOModes();
log.innerHTML = supportedGCO.join(' ');

<p id="log"></p>

关于javascript - JS:如何获取受支持的HTML canvas globalCompositeOperation类型的列表，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/33955992/