JS: How to get list of supported HTML canvas globalCompositeOperation types

Question

I want to make a HTML select list, with which I can choose which type of globalCompositeOperation will be applied when blending two canvas

Answer 1

I've just converted Kaiido's solution into an js object with a public test(blendModeName) method. Maybe it's of use to someone.

// 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;
  }
}

With this you can test for specific blend modes, instead of testing all at once.

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

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