graphics.js

function process(canvas, func) {
    function setPixel(imageData, x, y, rgba) {
        var index = (x + y * imageData.width) * 4;

        for (var i = 0; i < 4; i++) {
            imageData.data[index + i] = rgba[i] * 255;
        }
    }

    var ctx = canvas.getContext("2d");
    var imageData = ctx.createImageData(canvas.width,
        canvas.height);

    for (var y = 0; y < canvas.height; y++) {
        for (var x = 0; x < canvas.width; x++) {
            var result = func(new Point(x, y));

            setPixel(imageData, x, y, result);
        }
    }

    ctx.putImageData(imageData, 0, 0);
}

math.js

function lerp(a, b, fac) {
    var ret = [];

    for (var i = 0; i < Math.min(a.length, b.length); i++) {
        ret[i] = a[i] * (1 - fac) + b[i] * fac;
    }

    return ret;
}

function project(target, initial, current) {
    var delta = initial.sub(target);

    if( (delta.x == 0) && (delta.y == 0) ) {
        return target;
    }

    var t = current.sub(target).mul(delta).div(delta.toDistSquared());

    return delta.mul(t.x + t.y).add(target);
}

function dot(a, b) {
    return a.x * b.x + a.y * b.y;
}

function Circle(location, radius) {
    this.init(location, radius);
}
Circle.prototype = {
    init: function(location, radius) {
        this.location = location? new Point(location[0], location[1]): new Point();
        this.radius = radius? radius: 1;
    },
    contains: function(p) {
        return p.sub(this.location).toDist() <= this.radius;
    },
    intersect: function(p1, p2) {
        // http://local.wasp.uwa.edu.au/~pbourke/geometry/sphereline/
        var dp = p2.sub(p1);
        var a = dp.toDistSquared();
        var b = 2 * (dp.x * (p1.x - this.location.x) +
            dp.y * (p1.y - this.location.y));
        var c = this.location.toDistSquared() + p1.toDistSquared() - 2 *
            (this.location.x * p1.x + this.location.y * p1.y) -
            this.radius * this.radius;

        var bb4ac = b * b - 4 * a * c;

        var epsilon = 0.0001;
        if (Math.abs(a) < epsilon || bb4ac < 0) {
            return [];
        }

        if (bb4ac == 0) {
            return [p2.sub(p1).mul(-b / (2 * a)).add(p1)];
        }

        var mu1 = (-b + Math.sqrt(bb4ac)) / (2 * a);
        var mu2 = (-b - Math.sqrt(bb4ac)) / (2 * a);

        return [p2.sub(p1).mul(mu1).add(p1), p2.sub(p1).mul(mu2).add(p1)]
    }
}

function Point(x, y) {
    this.init(x, y);
}
Point.prototype = {
    init: function(x, y) {
        this.x = x? x: 0;
        this.y = y? y: 0;
    },
    add: function(other) {
        return this._operationTemplate(other, function(a, b) {return a + b});
    },
    sub: function(other) {
        return this._operationTemplate(other, function(a, b) {return a - b});
    },
    mul: function(other) {
        return this._operationTemplate(other, function(a, b) {return a * b});
    },
    div: function(other) {
        return this._operationTemplate(other, function(a, b) {return a / b});
    },
    ceil: function() {
        return this._operationTemplate(null, function(a) {return Math.ceil(a)});
    },
    floor: function() {
        return this._operationTemplate(null, function(a) {return Math.floor(a)});
    },
    round: function() {
        return this._operationTemplate(null, function(a) {return Math.round(a)});
    },
    _operationTemplate: function(other, op) {
        if(isNumber(other)) {
            return new Point(op(this.x, other), op(this.y, other));
        }

        if(other == null) {
            return new Point(op(this.x), op(this.y));
        }

        return new Point(op(this.x, other.x), op(this.y, other.y));
    },
    toDist: function() {
        return Math.sqrt(this.toDistSquared());
    },
    toDistSquared: function() {
        return this.x * this.x + this.y * this.y;
    },
    normalize: function() {
        return this.div(this.toDist());
    },
    invert: function() {
        return new Point(-this.x, -this.y);
    },
    closest: function(points) {
        return this._findTemplate(points,
            function() {
                return Number.MAX_VALUE;
            },
            function(dist, recordDist) {
                return dist < recordDist;
            }
        );
    },
    farthest: function(points) {
        return this._findTemplate(points,
            function() {
                return 0;
            },
            function(dist, recordDist) {
                return dist > recordDist;
            }
        );
    },
    _findTemplate: function(points, init, compare) {
        var record = init();
        var recordPoint = points[0];

        for (var i = 1; i < points.length; i++) {
            var point = points[i];
            var dist = this.sub(point).toDist();

            if (compare(dist, record)) {
                record = dist;
                recordPoint = point;
            }
        }

        return recordPoint;
    }
}

function isNumber(n) {
    return !isNaN(parseFloat(n)) && isFinite(n);
}

rad2.html

<html>
    <head>
        <title>Radial Gradient Test</title>
        <script type="text/javascript" src="graphics.js"></script>
        <script type="text/javascript" src="math.js"></script>
        <script type="text/javascript">
            function draw(x, y) {
                var canvas = document.getElementById("canvas");

                var halfWidth = canvas.width / 2;
                var halfHeight = canvas.height / 2;

                var gradientCenterX = x != undefined? x: halfWidth;
                var gradientCenterY = y != undefined? y: halfHeight;

                var outerCircle = new Circle([halfWidth, halfHeight],
                    halfWidth);
                outerCircle.color = [0.5, 0.5, 1, 0.5]; // semi-opaque light blue

                var innerCircle = new Circle([gradientCenterX,
                    gradientCenterY], 5);
                innerCircle.color = [0, 0, 0.2, 1]; // opaque dark blue

                var radialGradient = function(point) {
                    var lerpFac = 1;

                    if (!outerCircle.contains(point)) {
                        lerpFac = 1;
                    }
                    else if (innerCircle.contains(point)) {
                        lerpFac = 0;
                    }
                    else {
                        var outerIntersections = outerCircle.intersect(point,
                            innerCircle.location);
                        var innerIntersections = innerCircle.intersect(point,
                            innerCircle.location);

                        var closestInner = point.closest(innerIntersections);
                        var farthestOuter = closestInner.farthest(outerIntersections);
                        var innerOuterDist = closestInner.sub(farthestOuter).
                            toDist();
                        var innerPointDist = closestInner.sub(point).toDist();

                        lerpFac = innerPointDist / innerOuterDist;
                    }

                    return lerp(innerCircle.color, outerCircle.color, lerpFac);
                }

                process(canvas, radialGradient);
            }

            function move(event) {
                draw(event.x - event.srcElement.offsetLeft,
                    event.y - event.srcElement.offsetTop);
            }
        </script>
    </head>
    <body onload="draw()">
        <canvas onmousemove="move(event)" id="canvas" width="100" height="100"></canvas>
    </body>
</html>