Reveal a blurred image with your mouse (demo at https://test.bendem.be/blurpath/).

app.js

'use strict';

var htmlLoaded = false;
var imgLoaded = false;

var pic = new Image;
pic.onload = function() {
    imgLoaded = true;
    load();
};
pic.src = '3840x2160_80_2.jpg';

var blurCanvas;
var imgCanvas;
var spareCanvas;
var blur;
var img;
var spare;
var w;
var h;

var lastPoint;
var pointsToDraw;
var scheduled;

window.addEventListener('load', function() {
    htmlLoaded = true;

    load();

    document.getElementById('reset').addEventListener('click', function(e) {
        e.preventDefault();
        drawSpare();
    });
});

window.addEventListener('deviceorientation', load);
window.addEventListener('resize', load);

function load() {
    if(!htmlLoaded || !imgLoaded) {
        return;
    }

    blurCanvas = document.getElementById('blur');
    imgCanvas = document.getElementById('img');
    spareCanvas = document.createElement('canvas');
    blur = blurCanvas.getContext('2d');
    img = imgCanvas.getContext('2d');
    spare = spareCanvas.getContext('2d');

    w = spareCanvas.width = imgCanvas.width = blurCanvas.width = window.innerWidth;
    h = spareCanvas.height = imgCanvas.height = blurCanvas.height = window.innerHeight;

    drawInitial();

    var movedTo = function(point) {
        if(!lastPoint) {
            blur.beginPath();
            blur.lineWidth = 120;
            blur.globalCompositeOperation = 'destination-out';
            blur.lineCap = "round";
            blur.lineJoin = "round";
        } else if(Math.abs(lastPoint.x - point.x) + Math.abs(lastPoint.y - point.y) < 1) {
            return;
        }

        lastPoint = point;
        pointsToDraw.push(point);
        schedule();
    };

    window.addEventListener('mousemove', function(e) {
        movedTo({
            x: e.x || e.clientX,
            y: e.y || e.clientY,
        });
    });
    document.addEventListener('touchmove', function(e) {
        movedTo({
            x: e.touches[0].clientX,
            y: e.touches[0].clientY,
        })
    });
}

function drawInitial() {
    lastPoint = null;
    pointsToDraw = [];
    scheduled = false;

    // Compute best size / coords
    var ratio = pic.width / pic.height;

    var drawW;
    var drawH;

    if(w / ratio < h) {
        drawH = h;
        drawW = h * ratio;
    } else {
        drawW = w;
        drawH = w / ratio;
    }

    var drawX = -drawW / 2 + w / 2;
    var drawY = -drawH / 2 + h / 2;

    img.drawImage(
        pic,
        // image coord
        0, 0,
        // image size
        pic.width, pic.height,
        // canvas coord
        drawX, drawY,
        // canvas size
        drawW, drawH
    );

    blur.putImageData(
        stackBlurCanvasRGBA(
            img.getImageData(0, 0, w, h),
            0, 0, w, h, 100
        ),
        0, 0
    );
    spare.drawImage(blurCanvas, 0, 0, w, h);
}

function drawSpare() {
    blur.globalCompositeOperation = 'source-over';
    blur.drawImage(spareCanvas, 0, 0, w, h);
    lastPoint = null;
    blur.beginPath();
}

function schedule() {
    if(!scheduled) {
        scheduled = true;
        refresh();
    }
}

function refresh() {
    requestAnimationFrame(function() {
        scheduled = false;
        var point = pointsToDraw.pop();
        if(!point) return;

        blur.lineTo(point.x, point.y);
        blur.stroke();
    });

    if(pointsToDraw.length) {
        schedule();
    }
}

blur.js

/*

Note from bendem: I totally stripped this down to only get what I needed. See below for links to
the original code.

StackBlur - a fast almost Gaussian Blur For Canvas

Version: 0.5
Author:  Mario Klingemann
Contact: mario@quasimondo.com
Website: http://www.quasimondo.com/StackBlurForCanvas
Twitter: @quasimondo

In case you find this class useful - especially in commercial projects -
I am not totally unhappy for a small donation to my PayPal account
mario@quasimondo.de

Or support me on flattr:
https://flattr.com/thing/72791/StackBlur-a-fast-almost-Gaussian-Blur-Effect-for-CanvasJavascript

Copyright (c) 2010 Mario Klingemann

Permission is hereby granted, free of charge, to any person
obtaining a copy of this software and associated documentation
files (the "Software"), to deal in the Software without
restriction, including without limitation the rights to use,
copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the
Software is furnished to do so, subject to the following
conditions:

The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
OTHER DEALINGS IN THE SOFTWARE.
*/
var mul_table = [
    512,512,456,512,328,456,335,512,405,328,271,456,388,335,292,512,
    454,405,364,328,298,271,496,456,420,388,360,335,312,292,273,512,
    482,454,428,405,383,364,345,328,312,298,284,271,259,496,475,456,
    437,420,404,388,374,360,347,335,323,312,302,292,282,273,265,512,
    497,482,468,454,441,428,417,405,394,383,373,364,354,345,337,328,
    320,312,305,298,291,284,278,271,265,259,507,496,485,475,465,456,
    446,437,428,420,412,404,396,388,381,374,367,360,354,347,341,335,
    329,323,318,312,307,302,297,292,287,282,278,273,269,265,261,512,
    505,497,489,482,475,468,461,454,447,441,435,428,422,417,411,405,
    399,394,389,383,378,373,368,364,359,354,350,345,341,337,332,328,
    324,320,316,312,309,305,301,298,294,291,287,284,281,278,274,271,
    268,265,262,259,257,507,501,496,491,485,480,475,470,465,460,456,
    451,446,442,437,433,428,424,420,416,412,408,404,400,396,392,388,
    385,381,377,374,370,367,363,360,357,354,350,347,344,341,338,335,
    332,329,326,323,320,318,315,312,310,307,304,302,299,297,294,292,
    289,287,285,282,280,278,275,273,271,269,267,265,263,261,259];

var shg_table = [
    9, 11, 12, 13, 13, 14, 14, 15, 15, 15, 15, 16, 16, 16, 16, 17,
    17, 17, 17, 17, 17, 17, 18, 18, 18, 18, 18, 18, 18, 18, 18, 19,
    19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 20, 20, 20,
    20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 21,
    21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21,
    21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 22, 22, 22, 22, 22, 22,
    22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22,
    22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 23,
    23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23,
    23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23,
    23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23,
    23, 23, 23, 23, 23, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24 ];

function stackBlurCanvasRGBA(imageData, top_x, top_y, width, height, radius) {
    if (isNaN(radius) || radius < 1) {
        throw new Error('Invalid radius');
    }
    radius |= 0;

    var pixels = imageData.data;

    var x, y, i, p, yp, yi, yw, r_sum, g_sum, b_sum, a_sum,
    r_out_sum, g_out_sum, b_out_sum, a_out_sum,
    r_in_sum, g_in_sum, b_in_sum, a_in_sum,
    pr, pg, pb, pa, rbs;

    var div = radius + radius + 1;
    var w4 = width << 2;
    var widthMinus1  = width - 1;
    var heightMinus1 = height - 1;
    var radiusPlus1  = radius + 1;
    var sumFactor = radiusPlus1 * (radiusPlus1 + 1) / 2;

    var stackStart = new BlurStack();
    var stack = stackStart;
    for (i = 1; i < div; i++) {
        stack = stack.next = new BlurStack();
        if (i == radiusPlus1) var stackEnd = stack;
    }
    stack.next = stackStart;
    var stackIn = null;
    var stackOut = null;

    yw = yi = 0;

    var mul_sum = mul_table[radius];
    var shg_sum = shg_table[radius];

    for (y = 0; y < height; y++) {
        r_in_sum = g_in_sum = b_in_sum = a_in_sum = r_sum = g_sum = b_sum = a_sum = 0;

        r_out_sum = radiusPlus1 * (pr = pixels[yi]);
        g_out_sum = radiusPlus1 * (pg = pixels[yi+1]);
        b_out_sum = radiusPlus1 * (pb = pixels[yi+2]);
        a_out_sum = radiusPlus1 * (pa = pixels[yi+3]);

        r_sum += sumFactor * pr;
        g_sum += sumFactor * pg;
        b_sum += sumFactor * pb;
        a_sum += sumFactor * pa;

        stack = stackStart;

        for(i = 0; i < radiusPlus1; i++) {
            stack.r = pr;
            stack.g = pg;
            stack.b = pb;
            stack.a = pa;
            stack = stack.next;
        }

        for(i = 1; i < radiusPlus1; i++) {
            p = yi + ((widthMinus1 < i ? widthMinus1 : i) << 2);
            r_sum += (stack.r = (pr = pixels[p])) * (rbs = radiusPlus1 - i);
            g_sum += (stack.g = (pg = pixels[p+1])) * rbs;
            b_sum += (stack.b = (pb = pixels[p+2])) * rbs;
            a_sum += (stack.a = (pa = pixels[p+3])) * rbs;

            r_in_sum += pr;
            g_in_sum += pg;
            b_in_sum += pb;
            a_in_sum += pa;

            stack = stack.next;
        }


        stackIn = stackStart;
        stackOut = stackEnd;
        for (x = 0; x < width; x++) {
            pixels[yi+3] = pa = (a_sum * mul_sum) >> shg_sum;
            if (pa != 0) {
                pa = 255 / pa;
                pixels[yi]   = ((r_sum * mul_sum) >> shg_sum) * pa;
                pixels[yi+1] = ((g_sum * mul_sum) >> shg_sum) * pa;
                pixels[yi+2] = ((b_sum * mul_sum) >> shg_sum) * pa;
            } else {
                pixels[yi] = pixels[yi+1] = pixels[yi+2] = 0;
            }

            r_sum -= r_out_sum;
            g_sum -= g_out_sum;
            b_sum -= b_out_sum;
            a_sum -= a_out_sum;

            r_out_sum -= stackIn.r;
            g_out_sum -= stackIn.g;
            b_out_sum -= stackIn.b;
            a_out_sum -= stackIn.a;

            p =  (yw + ((p = x + radius + 1) < widthMinus1 ? p : widthMinus1)) << 2;

            r_in_sum += (stackIn.r = pixels[p]);
            g_in_sum += (stackIn.g = pixels[p+1]);
            b_in_sum += (stackIn.b = pixels[p+2]);
            a_in_sum += (stackIn.a = pixels[p+3]);

            r_sum += r_in_sum;
            g_sum += g_in_sum;
            b_sum += b_in_sum;
            a_sum += a_in_sum;

            stackIn = stackIn.next;

            r_out_sum += (pr = stackOut.r);
            g_out_sum += (pg = stackOut.g);
            b_out_sum += (pb = stackOut.b);
            a_out_sum += (pa = stackOut.a);

            r_in_sum -= pr;
            g_in_sum -= pg;
            b_in_sum -= pb;
            a_in_sum -= pa;

            stackOut = stackOut.next;

            yi += 4;
        }
        yw += width;
    }


    for (x = 0; x < width; x++) {
        g_in_sum = b_in_sum = a_in_sum = r_in_sum = g_sum = b_sum = a_sum = r_sum = 0;

        yi = x << 2;
        r_out_sum = radiusPlus1 * (pr = pixels[yi]);
        g_out_sum = radiusPlus1 * (pg = pixels[yi+1]);
        b_out_sum = radiusPlus1 * (pb = pixels[yi+2]);
        a_out_sum = radiusPlus1 * (pa = pixels[yi+3]);

        r_sum += sumFactor * pr;
        g_sum += sumFactor * pg;
        b_sum += sumFactor * pb;
        a_sum += sumFactor * pa;

        stack = stackStart;

        for(i = 0; i < radiusPlus1; i++) {
            stack.r = pr;
            stack.g = pg;
            stack.b = pb;
            stack.a = pa;
            stack = stack.next;
        }

        yp = width;

        for(i = 1; i <= radius; i++) {
            yi = (yp + x) << 2;

            r_sum += (stack.r = (pr = pixels[yi])) * (rbs = radiusPlus1 - i);
            g_sum += (stack.g = (pg = pixels[yi+1])) * rbs;
            b_sum += (stack.b = (pb = pixels[yi+2])) * rbs;
            a_sum += (stack.a = (pa = pixels[yi+3])) * rbs;

            r_in_sum += pr;
            g_in_sum += pg;
            b_in_sum += pb;
            a_in_sum += pa;

            stack = stack.next;

            if(i < heightMinus1) {
                yp += width;
            }
        }

        yi = x;
        stackIn = stackStart;
        stackOut = stackEnd;
        for (y = 0; y < height; y++) {
            p = yi << 2;
            pixels[p+3] = pa = (a_sum * mul_sum) >> shg_sum;
            if (pa > 0) {
                pa = 255 / pa;
                pixels[p]   = ((r_sum * mul_sum) >> shg_sum) * pa;
                pixels[p+1] = ((g_sum * mul_sum) >> shg_sum) * pa;
                pixels[p+2] = ((b_sum * mul_sum) >> shg_sum) * pa;
            } else {
                pixels[p] = pixels[p+1] = pixels[p+2] = 0;
            }

            r_sum -= r_out_sum;
            g_sum -= g_out_sum;
            b_sum -= b_out_sum;
            a_sum -= a_out_sum;

            r_out_sum -= stackIn.r;
            g_out_sum -= stackIn.g;
            b_out_sum -= stackIn.b;
            a_out_sum -= stackIn.a;

            p = (x + (((p = y + radiusPlus1) < heightMinus1 ? p : heightMinus1) * width)) << 2;

            r_sum += (r_in_sum += (stackIn.r = pixels[p]));
            g_sum += (g_in_sum += (stackIn.g = pixels[p+1]));
            b_sum += (b_in_sum += (stackIn.b = pixels[p+2]));
            a_sum += (a_in_sum += (stackIn.a = pixels[p+3]));

            stackIn = stackIn.next;

            r_out_sum += (pr = stackOut.r);
            g_out_sum += (pg = stackOut.g);
            b_out_sum += (pb = stackOut.b);
            a_out_sum += (pa = stackOut.a);

            r_in_sum -= pr;
            g_in_sum -= pg;
            b_in_sum -= pb;
            a_in_sum -= pa;

            stackOut = stackOut.next;

            yi += width;
        }
    }

    return imageData;
}

function BlurStack() {
    this.r = 0;
    this.g = 0;
    this.b = 0;
    this.a = 0;
    this.next = null;
}

index.html

<!doctype html>
<html>
    <head>
        <meta charset="utf-8">
        <script src="blur.js"></script>
        <script src="app.js"></script>
        <link rel="stylesheet" href="style.css">
        <title>Blur Path</title>
    </head>
    <body>
        <div class="content">
            <canvas id="img"></canvas>
            <canvas id="blur"></canvas>
            <header>
                <h1 id="reset">The revealing blur</h1>
            </header>
        </div>
    </body>
</html>

style.css

head, body, div, section, canvas, article, header, nav, footer, h1 {
    display: block;
    margin: 0;
    padding: 0;
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
}

html, body, .content {
    width: 100%;
    height: 100%;
}

body {
    background-color: #222;
    color: #eee;
    font-family: monospace;
}

header {
    position: absolute;
    left: 0; right: 0;
    top: 5em;
    font-size: 2em;
    min-width: 20ch;
    width: 50%;
    height: 3em;
    line-height: 3em;
    border-radius: 3px;
    margin: auto;
    background-color: rgba(0, 0, 0, 0.7);
    text-align: center;
}

h1 {
    font-size: inherit;
    cursor: pointer;
    -webkit-touch-callout: none;
    -webkit-user-select: none;
    -khtml-user-select: none;
    -moz-user-select: none;
    -ms-user-select: none;
    user-select: none;
}

canvas {
    position: absolute;
    top: 0; left: 0; bottom: 0; right: 0;
    width: 100%;
    height: 100%;
}