public test(blendModeName)メソッドを使用して、Kaiidoのソリューションを js オブジェクトに変換しました。たぶんそれは誰かの役に立ちます。
// 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;
}
}
これにより、すべてを一度にテストする代わりに、特定のブレンド モードをテストできます。
var blendModeTester = new BlendModeTester();
blendModeTester.initialize();
if(blendModeTester.test('hue')) {
// do stuff
};