Ipad not showing canvas lines properly - ios

Question

I have created small script for vision testing. I posted part of it on https://jsfiddle.net/jaka_87/bpadyanh/
The theory behind it: Canvas is filled with pattern.In each pattern there are 3 lines drawn. Two black ones with the white one in the middle. The white line has 2*with of one black line. When width of this lines is so small that our eye cant distinguish between these three we see only gray background (even thou the lines are still there).
I tested it on few computers. On most of them it works well (thou i saw some strange patterns on some of the older ones - one column of vertical white lines then 2-3 dark column then white one....) I was assuming that it has do do with the display/graphic card or something similar. I tested it on some some mobile devices. It works fine on my Nexus 7 and Moto G, but not on my Transformer Prime pad (strange pattern like described before - for which again I blame the tablet).
It looks absolutely horrible (by far the worst from all tested) on my Ipad and my friends Iphone. I was expecting the best result there since they are known for very good screens but the results are horrible. When the lines are wide enough its OK but when they get narrower they are merged together to one either black or white line - not shown separately.
Is there any way to fix that so it would work on iOS ??
var povecava2 = "0.04226744186046511";
var izmerjeno =1;
var razmak =3.5;
var contrast = 1;
var canvas = document.getElementById('canvas1');
var ctx = canvas.getContext('2d');
// širina canvasa
ctx.canvas.width = window.innerWidth-23;
ctx.canvas.height = window.innerHeight-70;
var sirinaopto=Math.round(200*izmerjeno*povecava2*razmak);
if(sirinaopto & 1){sirinaopto=sirinaopto;}else{ sirinaopto=sirinaopto+1;} // če je širina optotipa soda ali liha
var enota4 =((0.19892970392*130*izmerjeno*povecava2)/4).toFixed(2); // 1 kotna minuta
var center= Math.round((ctx.canvas.width-(sirinaopto))/2);
// kolkrat gre v višino
var kolkratgre = Math.floor(ctx.canvas.height/(sirinaopto));
var visina2= sirinaopto*kolkratgre;
// kolkrat gre v širino
var kolkratgrehor = Math.ceil(ctx.canvas.width/sirinaopto); if(kolkratgrehor % 2 == 0) { var kolkratgrehor=kolkratgrehor-1; }
var zacetek = (ctx.canvas.width-(kolkratgrehor*sirinaopto))/2;
ctx.rect(0,0,ctx.canvas.width,ctx.canvas.height);
ctx.fillStyle="rgb(140,140,140)";
ctx.fill();
// 90 stopinj
var canvasPattern0 = document.createElement("canvas");
canvasPattern0.width = sirinaopto;
canvasPattern0.height = sirinaopto;
var contextPattern0 = canvasPattern0.getContext("2d");
contextPattern0.mozImageSmoothingEnabled = false;
contextPattern0.imageSmoothingEnabled = false;
contextPattern0.translate((canvasPattern0.width/2)-(enota4*2),(canvasPattern0.width/2)-(10*enota4));
contextPattern0.beginPath();
contextPattern0.globalAlpha = contrast;
contextPattern0.moveTo(enota4/2,0);
contextPattern0.lineTo(enota4/2,20*enota4);
contextPattern0.lineWidth=enota4;
contextPattern0.strokeStyle = 'black';
contextPattern0.stroke();
contextPattern0.closePath();
contextPattern0.beginPath();
contextPattern0.globalAlpha = contrast;
contextPattern0.moveTo(enota4*2,0);
contextPattern0.lineTo(enota4*2,20*enota4);
contextPattern0.lineWidth=enota4*2;
contextPattern0.strokeStyle = 'white';
contextPattern0.stroke();
contextPattern0.closePath();
contextPattern0.beginPath();
contextPattern0.globalAlpha = contrast;
contextPattern0.moveTo(enota4*3.5,0);
contextPattern0.lineTo(enota4*3.5,20*enota4);
contextPattern0.lineWidth=enota4;
contextPattern0.strokeStyle = 'black';
contextPattern0.stroke();
contextPattern0.closePath();
// 0 stopinj
var canvasPattern1 = document.createElement("canvas");
canvasPattern1.width = sirinaopto;canvasPattern1.height = sirinaopto;
var contextPattern1 = canvasPattern1.getContext("2d");
contextPattern1.translate(sirinaopto/2, sirinaopto/2);
contextPattern1.rotate(90*Math.PI/180);
contextPattern1.drawImage(canvasPattern0, sirinaopto*(-0.5), sirinaopto*(-0.5));
contextPattern1.save();
var imagesLoaded = [];
imagesLoaded.push(canvasPattern0);
imagesLoaded.push(canvasPattern1);
var randomPattern = function(imgWidth, imgHeight, areaWidth, areaHeight) {
// either set a defined width/height for our images, or use the first one's
imgWidth = sirinaopto;
imgHeight = sirinaopto;
// restrict the randmoness size by using an areaWidth/Height
areaWidth = ctx.canvas.width;
areaHeight = visina2;
// create a buffer canvas
var patternCanvas = canvas.cloneNode(true);
var patternCtx = patternCanvas.getContext('2d');
patternCanvas.width = areaWidth;
patternCanvas.height = areaHeight;
// var xloops = Math.ceil(areaWidth / imgWidth);
var xloops = Math.ceil(areaWidth / imgWidth); if(xloops % 2 == 0) { var xloops=xloops-1; }
var yloops = Math.ceil(areaHeight / imgHeight);
//alert(xloops);
for (var xpos = 0; xpos < xloops; xpos++) {
for (var ypos = 0; ypos < yloops; ypos++) {
var img = imagesLoaded[Math.floor(Math.random() * imagesLoaded.length)];
patternCtx.drawImage(img, (xpos * imgWidth)+zacetek, (ypos * imgHeight), imgWidth, imgHeight);
}
}
// create a pattern from this randomly created image
return patternCtx.createPattern(patternCanvas, 'repeat');
}
var draw = function() {
//create the random pattern (should be moved out of the draw)
var patt = randomPattern(sirinaopto,sirinaopto);
ctx.fillStyle = patt;
ctx.fillRect(0,0,ctx.canvas.width, visina2)
};
draw();