WebGLFilters.js

// Thanks https://gamemechanicexplorer.com/#lightning-3
// Fragment shaders are small programs that run on the graphics card and alter
// the pixels of a texture. Every framework implements shaders differently but
// the concept is the same. This shader takes the lightning texture and alters
// the pixels so that it appears to be glowing. Shader programming itself is
// beyond the scope of this tutorial.
//
// There are a ton of good resources out there to learn it. Odds are that your
// framework already includes many of the most popular shaders out of the box.
//
// This is an OpenGL/WebGL feature. Because it runs in your web browser
// you need a browser that support WebGL for this to work.
Phaser.Filter.Glow = function (game) {
    Phaser.Filter.call(this, game);

    this.fragmentSrc = [
        'precision lowp float;',
        'varying vec2 vTextureCoord;',
        'varying vec4 vColor;',
        'uniform sampler2D uSampler;',

        'void main() {',
            'vec4 sum = vec4(0);',
            'vec2 texcoord = vTextureCoord;',
            'for(int xx = -4; xx <= 4; xx++) {',
                'for(int yy = -3; yy <= 3; yy++) {',
                    'float dist = sqrt(float(xx*xx) + float(yy*yy));',
                    'float factor = 0.0;',
                    'if (dist == 0.0) {',
                        'factor = 2.0;',
                    '} else {',
                        'factor = 2.0/abs(float(dist));',
                    '}',
                    'sum += texture2D(uSampler, texcoord + vec2(xx, yy) * 0.002) * factor;',
                '}',
            '}',
            'gl_FragColor = sum * 0.025 + texture2D(uSampler, texcoord);',
        '}'
    ];
};

Phaser.Filter.Glow.prototype = Object.create(Phaser.Filter.prototype);
Phaser.Filter.Glow.prototype.constructor = Phaser.Filter.Glow;