zig.js

var canvas = document.querySelector('canvas')
var width = canvas.width
var height= canvas.height
var center = [width / 2, height / 2]
var maxd = 100//distance([0,0], center)

return function (r, w) {
for (var y = 0; y < height; ++y) {
    for (var x = 0; x < width; ++x) {
        var index = (y * width + x) * 4;

        var value = x * y / Math.floor(Date.now() % 256/2) & 0xff;
        var a = r.data[index] //  = value;    // red
        var aa = r.data[++index]// = value;    // green
        var aaa = r.data[++index]// = value;
        var d = distance([x, y], center) / maxd / 2 / 2
        //value = value & 0xff
        w.data[index]   = Math.floor(a * d );    // red
        w.data[++index] = Math.floor(aa * d);    // green
        w.data[++index] = Math.floor(aaa * d);    // blue
        w.data[++index] = 255;      // alpha
    }
}
}


function test(c){
  if(c[0] < width / 2 && c[1] < height / 2) return 1
  else return 0
}

  function angle ( point1, point2 ) {
    var dx = point2[0] - point1[0];
    var dy = point2[1] - point1[1];
    return Math.atan2( dy, dx );
  }


function distance(a, b){
  return Math.sqrt(Math.pow(b[0] - a[0], 2) + Math.pow(b[1] - a[1], 2)) 
}