Checking a retina display map

This is an example from the post Canvas mapping with a retina display.

The upper part of the map is not corrected for retins displays and will appear blurred in them, and the lower part is corrected and should appear with sharp edges.

See the post to learn how to simulate the retina display if your device doesn't have it.

<!DOCTYPE html>
<meta charset="utf-8">
<style>
#canvasWrapper {
    position: relative;
    width: 680;
    height: 500;
}

#canvasWrapper canvas {
    position: absolute;
    top: 0;
    left: 0;
}
</style>
<body>
<div id="canvasWrapper"></div>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script src="https://d3js.org/topojson.v1.min.js"></script>

var width = 680,
    height = 500;

var projection = d3.geoAzimuthalEqualArea()
    .rotate([-45, -22])
    .scale(1700);

var canvas = d3.select("#canvasWrapper").append("canvas")
    .attr("width", width)
    .attr("height", height);

var context = canvas.node().getContext("2d");

var canvasRetina = d3.select("#canvasWrapper").append("canvas")
    .attr("width", width)
    .attr("height", height);

var contextRetina = canvasRetina.node().getContext("2d");

if (window.devicePixelRatio){
    canvasRetina
        .attr('width', width * window.devicePixelRatio)
        .attr('height', height * window.devicePixelRatio)
        .style('width', width + 'px')
        .style('height', height + 'px');

contextRetina.scale(window.devicePixelRatio, window.devicePixelRatio);
}

d3.json("world-110m.json", function(error, topojsonData) {
    /*Clip path for canvasRetina*/
    contextRetina.beginPath();
    contextRetina.moveTo(0,0);
    contextRetina.lineTo(0, height);
    contextRetina.lineTo(width, height-20);
    contextRetina.closePath();
    contextRetina.clip();
    
    var countries = topojson.feature(topojsonData, topojsonData.objects.countries);
    context.beginPath();
    context.strokeStyle = "#777";
    context.fillStyle = "#ccc";
    var path = d3.geoPath()
        .projection(projection).context(context);
    path(countries);
    context.fill();
    context.stroke();

contextRetina.beginPath();
    contextRetina.strokeStyle = "#777";
    contextRetina.fillStyle = "#ccc";
    var pathRetina = d3.geoPath()
        .projection(projection).context(contextRetina);
    pathRetina(countries);
    contextRetina.fill();
    contextRetina.stroke();

var points = [{"name": "Mecca", "nameAr":"مكة‎‎", "lon": 39.826, "lat": 21.422},
    {"name": "Medina", "nameAr":"المدينة المنورة‎‎", "lon": 39.611, "lat": 24.467}];

context.font="15px Georgia";
    contextRetina.font="15px Georgia";
    points.forEach(function(d){
        var coords = projection([d.lon, d.lat]);
        context.beginPath();
        context.arc(coords[0], coords[1], 4, 0, 2 * Math.PI, false);
        context.fillStyle = 'green';
        context.fill();
        context.lineWidth = 1;
        context.strokeStyle = '#003300';
        context.stroke();

context.fillStyle = '#003300';
        context.fillText(d.name, coords[0] + 7,  coords[1] - 3);
        context.fillText(d.nameAr, coords[0] + 7,  coords[1] + 12);

contextRetina.beginPath();
        contextRetina.arc(coords[0], coords[1], 4, 0, 2 * Math.PI, false);
        contextRetina.fillStyle = 'green';
        contextRetina.fill();
        contextRetina.lineWidth = 1;
        contextRetina.strokeStyle = '#003300';
        contextRetina.stroke();

contextRetina.fillStyle = '#003300';
        contextRetina.fillText(d.name, coords[0] + 7,  coords[1] - 3);
        contextRetina.fillText(d.nameAr, coords[0] + 7,  coords[1] + 12);
    });

contextRetina.beginPath();
    contextRetina.moveTo(0,0);
    contextRetina.lineTo(width, height-20);
    contextRetina.stroke();
});
</script>

Modified http://d3js.org/topojson.v1.min.js to a secure url

https://d3js.org/d3.v4.min.js

https://d3js.org/topojson.v1.min.js