Fog of War SVG Filter Mask Example

<!DOCTYPE html>
<html>
<head>
<meta charset='utf-8'>
<meta http-equiv='X-UA-Compatible' content='IE=edge'>
<style>
    html, body {
        background: #ffffff;
        font-family: Helvetica, Arial, Tahoma, sans-serif;
        margin: 0;
        padding: 0;
    }

path {
        fill: none;
        stroke: #343434;
    }
    .result {
        fill: none;
        stroke: #343434;
        stroke-width: 4px;
    }

h1,h2,h3 {
        padding-left: 140px;
        padding-top: 0px;
    }
</style>
</head>
<body>
<svg xmlns='https://www.w3.org/2000/svg' xmlns:xlink= 'https://www.w3.org/1999/xlink' height=1500 width=1050>
    <defs>
        <filter id="filter-map" x="-0.15000001" y="-0.15000001" width="1.3" height="1.3" color-interpolation-filters="sRGB" >
            <feGaussianBlur stdDeviation="4" in="SourceGraphic" result="result1" id="feGaussianBlur4202" />
            <feTurbulence type="fractalNoise" baseFrequency="0.05" numOctaves="4" result="result0" id="feTurbulence4204" />
            <feDisplacementMap in2="result0" scale="20" xChannelSelector="R" yChannelSelector="G" in="result1" result="result2" id="feDisplacementMap4206" />
            <feGaussianBlur stdDeviation="3" in="SourceGraphic" result="result4" id="feGaussianBlur4208" />
            <feComposite in2="result2" operator="arithmetic" k1="1.5" k2="-0.25" k3="0.5" k4="0" in="result4" result="result5" id="feComposite4210" />
        </filter>
    </defs>
</svg>

<h3>Fog of War - Click to remove fog</h3>
<em>Note: this is not designed to be performant; this is just an example of how filters and masks can be used</em>

(function setupMap(){
    // 0. setup groups
    // ----------------------------------
    var svg = d3.select('svg');
    var wrapper = svg.append('g').attr({ 'class': 'map-wrapper'});
    var background = wrapper.append('g').attr({ 'class': 'background'});
    var characters = wrapper.append('g').attr({ 'class': 'characters'});

// 1. Setup the mask
    // ----------------------------------
    var maskGroup = svg.select('defs').append('mask')
        .attr({ id: 'map-mask' });

// 2. Add map images
    // ----------------------------------
    // fog version - grayscaled and dark
    var fogMap = background.append("image")
        .attr({ 
            // TODO: use difference background image
            'xlink:href': 'https://vasir-assets.s3.amazonaws.com/fog-of-war/map-dark.jpg',
            'preserveAspectRatio': 'none',
            'class': 'fog', x: 0, y: 0,
            height: '100%', width: '100%'
        });

// full version
    var visibleMap = background.append("image")
        .attr({ 
            // TODO: use difference background image
            'xlink:href': 'https://vasir-assets.s3.amazonaws.com/fog-of-war/map.jpg',
            'preserveAspectRatio': 'none',
            'class': 'visibleArea', x: 0, y: 0,
            height: '100%', width: '100%'
        })
        .style({ 
            // fill with full version of map
            fill: '#336699', mask: 'url(#map-mask)'
        });

// 3. Add a starting circle to break the fog
    // ----------------------------------
    // create a masked path to show visible nodes   
    var vertices = [];

function updateFog(){
        var masks = maskGroup.selectAll('.breakFog')
            .data(vertices);

var newMasks = masks.enter()
            .append('circle')
            .style({
                fill: '#000000',
                opacity: 1
            });

// update existing masks
        masks
            .attr({
                'class': 'breakFog',
                cx: function(d){ 
                
                    console.log(d);
                return d.x; },
                cy: function(d){ return d.y; },
                filter: 'url(#filter-map)',
                r: function(d){ 
                    var r = 43;
                    return r;
                }
            });
            
        var newStyle = { fill: '#ffffff', opacity: 1 };

masks.transition().style(newStyle);
        masks.exit().remove();

// add sprites
        // ------------------------------
        var sprites = characters.selectAll('.character')
            .data(vertices);

var races = ['darkelf.gif', 'human.gif', 'elf.gif', 'mimirian.gif'];

sprites.enter()
            .append("image")
                .attr({ 
                    'class': 'character',
                    'xlink:href': 'https://vasir-assets.s3.amazonaws.com/fog-of-war/' +
                        races[vertices.length % races.length],
                    'preserveAspectRatio': 'none',
                    x: function(d){ return d.x - 53/2; },
                    y: function(d){ return d.y - 53/2; },
                    height: 53, width: 53
                });
      sprites.exit().remove();

};

// Call it immediately 
    updateFog();

// then, call whenever a point is added
    wrapper.on('click', function(){
        vertices.push({
            x: d3.event.clientX,
            y: d3.event.clientY
        });
        updateFog();
    });
  
  wrapper.append('rect').attr({ x:0, y: 0, width: 20, height: 20 }).style({ fill: '#00ff00' }).on('click', function(){
    setTimeout(function(){
  	vertices.pop(); vertices.pop();
    updateFog()
    }, 200);
  })

})();

</script>

</body>
</html>

Modified http://cdnjs.cloudflare.com/ajax/libs/d3/3.4.11/d3.min.js to a secure url

https://cdnjs.cloudflare.com/ajax/libs/d3/3.4.11/d3.min.js