JS：如何获取受支持的HTML画布globalCompositeOperation类型的列表

Question

我想创建一个HTML select列表，我可以选择在混合两个canvas元素时应用哪种类型的globalCompositeOperation ，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>

有没有办法以编程方式获取可用的globalCompositeOperation类型列表作为Javascript对象或数组，因此它可以用于用数据填充select元素，而不是手动填充它？ 这些信息是否存储在一些原生变量中？

我不想只是验证一些混合模式是否被用户的浏览器所支持，所讨论这里 。 我想获得支持的globalCompositeOperation类型的完整列表，以便在浏览器中选择混合模式。

Answer 1

没有没有本地属性告诉我们浏览器支持哪些globalCompositeOperation模式。
你必须通过循环遍历所有规范定义的那些来测试它，并检查它是否仍然是你刚设置的那个：

 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。
这个想法是将两个填充了纯色的3x3px画布绘制到第三个画布上。 第一个绘制在左上角，第二个绘制在左下角，每个都在第三个画布的中心像素中共享一个像素。

显然这比属性检查慢，但每页只需要一次，因此性能可能不是问题。

 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> 

Answer 2

我刚刚将Kaiido的解决方案转换为带有公共测试（blendModeName）方法的js对象。 也许这对某人有用。

// based on http://stackoverflow.com/questions/33955992/js-how-to-get-list-of-supported-html-canvas-globalcompositeoperation-types
function BlendModeTester () {
  var ctx1, c1, ctx2, c2, ctx3, c3;
  var pos;
  var em, d1, d2;
  var blendModeDefinition = {
      "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]
    };

  this.initialize = function () {
    // create two 3*3 canvases that will be used as layers
    c1 = document.createElement('canvas');
    c1.width = c1.height = 3;
    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 correspond to the values in the blendModeDefinition array
    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
    pos = [16, 0, 32];
  }

  this.test = function(blendModeName) {
    var i;
    // get the array corresponding to the actual key
    var arr = blendModeDefinition[blendModeName];

    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 blend mode
    ctx3.globalCompositeOperation = blendModeName;
    // 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 blend mode "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;
  }
}

有了这个，你可以测试特定的混合模式，而不是一次测试所有。

var blendModeTester = new BlendModeTester();
blendModeTester.initialize();

if(blendModeTester.test('hue')) { 
  // do stuff 
};