---

Because why just throw powdered colors when you can splash pixels with code? Let’s turn this festival of India into a coding fiesta!

Introduction

I am designing the festival to a whole new level — a digital Holi Celebration filled with CSS and JS colors and virtual sparkles!

The below demo consist of the following features

• Change color with mouse click

• Sparkle color as you move the mouse

Visual Demonstration

A codepen to play with https://codepen.io/ssukhpinder/pen/ExJXggX?editors=0010

Complete Code

Please find below the complete or refer to the Github gist

Html code

<canvas></canvas>
<div>Happy Holi 2024</div>

CSS

html, body {
    overflow: hidden;
    padding:50px;
    color:#fff;
}

canvas {
 position: fixed;
 top:0;
 left:0;
 width:100%;
 height:100%;
 z-index:0.5;
 opacity:1;
}
div{
    position:relative;
    z-index:1;
    font-size:70px;
    padding:0;
    display: table;
    margin:auto;
    letter-spacing:2px;
}

Javascript

/*
ORIGINAL WORK
https://codepen.io/PavelDoGreat/pen/zdWzEL
*/

'use strict';
function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }
var canvas = document.getElementsByTagName('canvas')[0];
canvas.width = canvas.clientWidth;
canvas.height = canvas.clientHeight;
var params = { alpha: false, depth: false, stencil: false, antialias: false };
var gl = canvas.getContext('webgl2', params);
var isWebGL2 = !!gl;
if (!isWebGL2) {
    gl = canvas.getContext('webgl', params) || canvas.getContext('experimental-webgl', params);
}
gl.clearColor(0.0, 0.0, 0.0, 1.0);
var halfFloat = gl.getExtension('OES_texture_half_float');
var support_linear_float = gl.getExtension('OES_texture_half_float_linear');
if (isWebGL2) {
    gl.getExtension('EXT_color_buffer_float');
    support_linear_float = gl.getExtension('OES_texture_float_linear');
}
var TEXTURE_DOWNSAMPLE = 1;
var DENSITY_DISSIPATION = 0.98;
var VELOCITY_DISSIPATION = 0.99;
var SPLAT_RADIUS = 0.005;
var CURL = 30;
var PRESSURE_ITERATIONS = 25;
var GLProgram = function () {
    function GLProgram(vertexShader, fragmentShader) {
        _classCallCheck(this, GLProgram);
        this.uniforms = {};
        this.program = gl.createProgram();
        gl.attachShader(this.program, vertexShader);
        gl.attachShader(this.program, fragmentShader);
        gl.linkProgram(this.program);
        if (!gl.getProgramParameter(this.program, gl.LINK_STATUS)) throw gl.getProgramInfoLog(this.program);
        var uniformCount = gl.getProgramParameter(this.program, gl.ACTIVE_UNIFORMS);
        for (var i = 0; i < uniformCount; i++) {
            var uniformName = gl.getActiveUniform(this.program, i).name;
            this.uniforms[uniformName] = gl.getUniformLocation(this.program, uniformName);
        }
    }
    GLProgram.prototype.bind = function bind() {
        gl.useProgram(this.program);
    };
    return GLProgram;
}();
function compileShader(type, source) {
    var shader = gl.createShader(type);
    gl.shaderSource(shader, source);
    gl.compileShader(shader);
    if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) throw gl.getShaderInfoLog(shader);
    return shader;
};
var baseVertexShader = compileShader(gl.VERTEX_SHADER, '\n    precision highp float;\n    precision mediump sampler2D;\n\n    attribute vec2 aPosition;\n    varying vec2 vUv;\n    varying vec2 vL;\n    varying vec2 vR;\n    varying vec2 vT;\n    varying vec2 vB;\n    uniform vec2 texelSize;\n\n    void main () {\n        vUv = aPosition * 0.5 + 0.5;\n        vL = vUv - vec2(texelSize.x, 0.0);\n        vR = vUv + vec2(texelSize.x, 0.0);\n        vT = vUv + vec2(0.0, texelSize.y);\n        vB = vUv - vec2(0.0, texelSize.y);\n        gl_Position = vec4(aPosition, 0.0, 1.0);\n    }\n');
var displayShader = compileShader(gl.FRAGMENT_SHADER, '\n    precision highp float;\n    precision mediump sampler2D;\n\n    varying vec2 vUv;\n    varying vec2 vL;\n    varying vec2 vR;\n    varying vec2 vT;\n    varying vec2 vB;\n    uniform sampler2D uTexture;\n\n    void main () {\n        gl_FragColor = texture2D(uTexture, vUv);\n    }\n');
var splatShader = compileShader(gl.FRAGMENT_SHADER, '\n    precision highp float;\n    precision mediump sampler2D;\n\n    varying vec2 vUv;\n    uniform sampler2D uTarget;\n    uniform float aspectRatio;\n    uniform vec3 color;\n    uniform vec2 point;\n    uniform float radius;\n\n    void main () {\n        vec2 p = vUv - point.xy;\n        p.x *= aspectRatio;\n        vec3 splat = exp(-dot(p, p) / radius) * color;\n        vec3 base = texture2D(uTarget, vUv).xyz;\n        gl_FragColor = vec4(base + splat, 1.0);\n    }\n');
var advectionManualFilteringShader = compileShader(gl.FRAGMENT_SHADER, '\n    precision highp float;\n    precision mediump sampler2D;\n\n    varying vec2 vUv;\n    uniform sampler2D uVelocity;\n    uniform sampler2D uSource;\n    uniform vec2 texelSize;\n    uniform float dt;\n    uniform float dissipation;\n\n    vec4 bilerp (in sampler2D sam, in vec2 p) {\n        vec4 st;\n        st.xy = floor(p - 0.5) + 0.5;\n        st.zw = st.xy + 1.0;\n        vec4 uv = st * texelSize.xyxy;\n        vec4 a = texture2D(sam, uv.xy);\n        vec4 b = texture2D(sam, uv.zy);\n        vec4 c = texture2D(sam, uv.xw);\n        vec4 d = texture2D(sam, uv.zw);\n        vec2 f = p - st.xy;\n        return mix(mix(a, b, f.x), mix(c, d, f.x), f.y);\n    }\n\n    void main () {\n        vec2 coord = gl_FragCoord.xy - dt * texture2D(uVelocity, vUv).xy;\n        gl_FragColor = dissipation * bilerp(uSource, coord);\n        gl_FragColor.a = 1.0;\n    }\n');
var advectionShader = compileShader(gl.FRAGMENT_SHADER, '\n    precision highp float;\n    precision mediump sampler2D;\n\n    varying vec2 vUv;\n    uniform sampler2D uVelocity;\n    uniform sampler2D uSource;\n    uniform vec2 texelSize;\n    uniform float dt;\n    uniform float dissipation;\n\n    void main () {\n        vec2 coord = vUv - dt * texture2D(uVelocity, vUv).xy * texelSize;\n        gl_FragColor = dissipation * texture2D(uSource, coord);\n    }\n');
var divergenceShader = compileShader(gl.FRAGMENT_SHADER, '\n    precision highp float;\n    precision mediump sampler2D;\n\n    varying vec2 vUv;\n    varying vec2 vL;\n    varying vec2 vR;\n    varying vec2 vT;\n    varying vec2 vB;\n    uniform sampler2D uVelocity;\n\n    vec2 sampleVelocity (in vec2 uv) {\n        vec2 multiplier = vec2(1.0, 1.0);\n        if (uv.x < 0.0) { uv.x = 0.0; multiplier.x = -1.0; }\n        if (uv.x > 1.0) { uv.x = 1.0; multiplier.x = -1.0; }\n        if (uv.y < 0.0) { uv.y = 0.0; multiplier.y = -1.0; }\n        if (uv.y > 1.0) { uv.y = 1.0; multiplier.y = -1.0; }\n        return multiplier * texture2D(uVelocity, uv).xy;\n    }\n\n    void main () {\n        float L = sampleVelocity(vL).x;\n        float R = sampleVelocity(vR).x;\n        float T = sampleVelocity(vT).y;\n        float B = sampleVelocity(vB).y;\n        float div = 0.5 * (R - L + T - B);\n        gl_FragColor = vec4(div, 0.0, 0.0, 1.0);\n    }\n');
var curlShader = compileShader(gl.FRAGMENT_SHADER, '\n    precision highp float;\n    precision mediump sampler2D;\n\n    varying vec2 vUv;\n    varying vec2 vL;\n    varying vec2 vR;\n    varying vec2 vT;\n    varying vec2 vB;\n    uniform sampler2D uVelocity;\n\n    void main () {\n        float L = texture2D(uVelocity, vL).y;\n        float R = texture2D(uVelocity, vR).y;\n        float T = texture2D(uVelocity, vT).x;\n        float B = texture2D(uVelocity, vB).x;\n        float vorticity = R - L - T + B;\n        gl_FragColor = vec4(vorticity, 0.0, 0.0, 1.0);\n    }\n');
var vorticityShader = compileShader(gl.FRAGMENT_SHADER, '\n    precision highp float;\n    precision mediump sampler2D;\n\n    varying vec2 vUv;\n    varying vec2 vL;\n    varying vec2 vR;\n    varying vec2 vT;\n    varying vec2 vB;\n    uniform sampler2D uVelocity;\n    uniform sampler2D uCurl;\n    uniform float curl;\n    uniform float dt;\n\n    void main () {\n        float L = texture2D(uCurl, vL).y;\n        float R = texture2D(uCurl, vR).y;\n        float T = texture2D(uCurl, vT).x;\n        float B = texture2D(uCurl, vB).x;\n        float C = texture2D(uCurl, vUv).x;\n        vec2 force = vec2(abs(T) - abs(B), abs(R) - abs(L));\n        force *= 1.0 / length(force + 0.00001) * curl * C;\n        vec2 vel = texture2D(uVelocity, vUv).xy;\n        gl_FragColor = vec4(vel + force * dt, 0.0, 1.0);\n    }\n');
var pressureShader = compileShader(gl.FRAGMENT_SHADER, '\n    precision highp float;\n    precision mediump sampler2D;\n\n    varying vec2 vUv;\n    varying vec2 vL;\n    varying vec2 vR;\n    varying vec2 vT;\n    varying vec2 vB;\n    uniform sampler2D uPressure;\n    uniform sampler2D uDivergence;\n\n    vec2 boundary (in vec2 uv) {\n        uv = min(max(uv, 0.0), 1.0);\n        return uv;\n    }\n\n    void main () {\n        float L = texture2D(uPressure, boundary(vL)).x;\n        float R = texture2D(uPressure, boundary(vR)).x;\n        float T = texture2D(uPressure, boundary(vT)).x;\n        float B = texture2D(uPressure, boundary(vB)).x;\n        float C = texture2D(uPressure, vUv).x;\n        float divergence = texture2D(uDivergence, vUv).x;\n        float pressure = (L + R + B + T - divergence) * 0.25;\n        gl_FragColor = vec4(pressure, 0.0, 0.0, 1.0);\n    }\n');
var gradientSubtractShader = compileShader(gl.FRAGMENT_SHADER, '\n    precision highp float;\n    precision mediump sampler2D;\n\n    varying vec2 vUv;\n    varying vec2 vL;\n    varying vec2 vR;\n    varying vec2 vT;\n    varying vec2 vB;\n    uniform sampler2D uPressure;\n    uniform sampler2D uVelocity;\n\n    vec2 boundary (in vec2 uv) {\n        uv = min(max(uv, 0.0), 1.0);\n        return uv;\n    }\n\n    void main () {\n        float L = texture2D(uPressure, boundary(vL)).x;\n        float R = texture2D(uPressure, boundary(vR)).x;\n        float T = texture2D(uPressure, boundary(vT)).x;\n        float B = texture2D(uPressure, boundary(vB)).x;\n        vec2 velocity = texture2D(uVelocity, vUv).xy;\n        velocity.xy -= vec2(R - L, T - B);\n        gl_FragColor = vec4(velocity, 0.0, 1.0);\n    }\n');
var blit = function () {
    gl.bindBuffer(gl.ARRAY_BUFFER, gl.createBuffer());
    gl.bufferData(gl.ARRAY_BUFFER, new Float32Array([-1, -1, -1, 1, 1, 1, 1, -1]), gl.STATIC_DRAW);
    gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, gl.createBuffer());
    gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, new Uint16Array([0, 1, 2, 0, 2, 3]), gl.STATIC_DRAW);
    gl.vertexAttribPointer(0, 2, gl.FLOAT, false, 0, 0);
    gl.enableVertexAttribArray(0);
    return function (destination) {
        gl.bindFramebuffer(gl.FRAMEBUFFER, destination);
        gl.drawElements(gl.TRIANGLES, 6, gl.UNSIGNED_SHORT, 0);
    };
}();
function clear(target) {
    gl.bindFramebuffer(gl.FRAMEBUFFER, target);
    gl.clear(gl.COLOR_BUFFER_BIT);
}
function createFBO(texId, w, h, internalFormat, format, type, param) {
    gl.activeTexture(gl.TEXTURE0 + texId);
    var texture = gl.createTexture();
    gl.bindTexture(gl.TEXTURE_2D, texture);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, param);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, param);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE);
    gl.texImage2D(gl.TEXTURE_2D, 0, internalFormat, w, h, 0, format, type, null);
    var fbo = gl.createFramebuffer();
    gl.bindFramebuffer(gl.FRAMEBUFFER, fbo);
    gl.framebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.TEXTURE_2D, texture, 0);
    gl.viewport(0, 0, w, h);
    gl.clear(gl.COLOR_BUFFER_BIT);
    return [texture, fbo, texId];
}
function createDoubleFBO(texId, w, h, internalFormat, format, type, param) {
    var fbo1 = createFBO(texId, w, h, internalFormat, format, type, param);
    var fbo2 = createFBO(texId + 1, w, h, internalFormat, format, type, param);
    return {
        get first() {
            return fbo1;
        },
        get second() {
            return fbo2;
        },
        swap: function swap() {
            var temp = fbo1;
            fbo1 = fbo2;
            fbo2 = temp;
        }
    };
}
var textureWidth = undefined;
var textureHeight = undefined;
var density = undefined;
var velocity = undefined;
var divergence = undefined;
var curl = undefined;
var pressure = undefined;
function initFramebuffers() {
    textureWidth = gl.drawingBufferWidth >> TEXTURE_DOWNSAMPLE;
    textureHeight = gl.drawingBufferHeight >> TEXTURE_DOWNSAMPLE;
    var internalFormat = isWebGL2 ? gl.RGBA16F : gl.RGBA;
    var internalFormatRG = isWebGL2 ? gl.RG16F : gl.RGBA;
    var formatRG = isWebGL2 ? gl.RG : gl.RGBA;
    var texType = isWebGL2 ? gl.HALF_FLOAT : halfFloat.HALF_FLOAT_OES;
    density = createDoubleFBO(0, textureWidth, textureHeight, internalFormat, gl.RGBA, texType, support_linear_float ? gl.LINEAR : gl.NEAREST);
    velocity = createDoubleFBO(2, textureWidth, textureHeight, internalFormatRG, formatRG, texType, support_linear_float ? gl.LINEAR : gl.NEAREST);
    divergence = createFBO(4, textureWidth, textureHeight, internalFormatRG, formatRG, texType, gl.NEAREST);
    curl = createFBO(5, textureWidth, textureHeight, internalFormatRG, formatRG, texType, gl.NEAREST);
    pressure = createDoubleFBO(6, textureWidth, textureHeight, internalFormatRG, formatRG, texType, gl.NEAREST);
}
initFramebuffers();
var displayProgram = new GLProgram(baseVertexShader, displayShader);
var splatProgram = new GLProgram(baseVertexShader, splatShader);
var advectionProgram = new GLProgram(baseVertexShader, support_linear_float ? advectionShader : advectionManualFilteringShader);
var divergenceProgram = new GLProgram(baseVertexShader, divergenceShader);
var curlProgram = new GLProgram(baseVertexShader, curlShader);
var vorticityProgram = new GLProgram(baseVertexShader, vorticityShader);
var pressureProgram = new GLProgram(baseVertexShader, pressureShader);
var gradienSubtractProgram = new GLProgram(baseVertexShader, gradientSubtractShader);
function pointerPrototype() {
    this.id = -1;
    this.x = 0;
    this.y = 0;
    this.dx = 0;
    this.dy = 0;
    this.down = false;
    this.moved = false;
    this.color = [30, 0, 300];
}
var pointers = [];
pointers.push(new pointerPrototype());
for (var i = 0; i < 10; i++) {
    var color = [Math.random() * 10, Math.random() * 10, Math.random() * 10];
    var x = canvas.width * Math.random();
    var y = canvas.height * Math.random();
    var dx = 1000 * (Math.random() - 0.5);
    var dy = 1000 * (Math.random() - 0.5);
    splat(x, y, dx, dy, color);
}
var lastTime = Date.now();
Update();
function Update() {
    resizeCanvas();
    var dt = Math.min((Date.now() - lastTime) / 1000, 0.016);
    lastTime = Date.now();
    gl.viewport(0, 0, textureWidth, textureHeight);
    advectionProgram.bind();
    gl.uniform2f(advectionProgram.uniforms.texelSize, 1.0 / textureWidth, 1.0 / textureHeight);
    gl.uniform1i(advectionProgram.uniforms.uVelocity, velocity.first[2]);
    gl.uniform1i(advectionProgram.uniforms.uSource, velocity.first[2]);
    gl.uniform1f(advectionProgram.uniforms.dt, dt);
    gl.uniform1f(advectionProgram.uniforms.dissipation, VELOCITY_DISSIPATION);
    blit(velocity.second[1]);
    velocity.swap();
    gl.uniform1i(advectionProgram.uniforms.uVelocity, velocity.first[2]);
    gl.uniform1i(advectionProgram.uniforms.uSource, density.first[2]);
    gl.uniform1f(advectionProgram.uniforms.dissipation, DENSITY_DISSIPATION);
    blit(density.second[1]);
    density.swap();
    for (var i = 0; i < pointers.length; i++) {
        var pointer = pointers[i];
        if (pointer.moved) {
            splat(pointer.x, pointer.y, pointer.dx, pointer.dy, pointer.color);
            pointer.moved = false;
        }
    }
    curlProgram.bind();
    gl.uniform2f(curlProgram.uniforms.texelSize, 1.0 / textureWidth, 1.0 / textureHeight);
    gl.uniform1i(curlProgram.uniforms.uVelocity, velocity.first[2]);
    blit(curl[1]);
    vorticityProgram.bind();
    gl.uniform2f(vorticityProgram.uniforms.texelSize, 1.0 / textureWidth, 1.0 / textureHeight);
    gl.uniform1i(vorticityProgram.uniforms.uVelocity, velocity.first[2]);
    gl.uniform1i(vorticityProgram.uniforms.uCurl, curl[2]);
    gl.uniform1f(vorticityProgram.uniforms.curl, CURL);
    gl.uniform1f(vorticityProgram.uniforms.dt, dt);
    blit(velocity.second[1]);
    velocity.swap();
    divergenceProgram.bind();
    gl.uniform2f(divergenceProgram.uniforms.texelSize, 1.0 / textureWidth, 1.0 / textureHeight);
    gl.uniform1i(divergenceProgram.uniforms.uVelocity, velocity.first[2]);
    blit(divergence[1]);
    clear(pressure.first[1]);
    pressureProgram.bind();
    gl.uniform2f(pressureProgram.uniforms.texelSize, 1.0 / textureWidth, 1.0 / textureHeight);
    gl.uniform1i(pressureProgram.uniforms.uDivergence, divergence[2]);
    for (var i = 0; i < PRESSURE_ITERATIONS; i++) {
        gl.uniform1i(pressureProgram.uniforms.uPressure, pressure.first[2]);
        blit(pressure.second[1]);
        pressure.swap();
    }
    gradienSubtractProgram.bind();
    gl.uniform2f(gradienSubtractProgram.uniforms.texelSize, 1.0 / textureWidth, 1.0 / textureHeight);
    gl.uniform1i(gradienSubtractProgram.uniforms.uPressure, pressure.first[2]);
    gl.uniform1i(gradienSubtractProgram.uniforms.uVelocity, velocity.first[2]);
    blit(velocity.second[1]);
    velocity.swap();
    gl.viewport(0, 0, gl.drawingBufferWidth, gl.drawingBufferHeight);
    displayProgram.bind();
    gl.uniform1i(displayProgram.uniforms.uTexture, density.first[2]);
    blit(null);
    requestAnimationFrame(Update);
}
function splat(x, y, dx, dy, color) {
    splatProgram.bind();
    gl.uniform1i(splatProgram.uniforms.uTarget, velocity.first[2]);
    gl.uniform1f(splatProgram.uniforms.aspectRatio, canvas.width / canvas.height);
    gl.uniform2f(splatProgram.uniforms.point, x / canvas.width, 1.0 - y / canvas.height);
    gl.uniform3f(splatProgram.uniforms.color, dx, -dy, 1.0);
    gl.uniform1f(splatProgram.uniforms.radius, SPLAT_RADIUS);
    blit(velocity.second[1]);
    velocity.swap();
    gl.uniform1i(splatProgram.uniforms.uTarget, density.first[2]);
    gl.uniform3f(splatProgram.uniforms.color, color[0] * 0.3, color[1] * 0.3, color[2] * 0.3);
    blit(density.second[1]);
    density.swap();
}
function resizeCanvas() {
    if (canvas.width != canvas.clientWidth || canvas.height != canvas.clientHeight) {
        canvas.width = canvas.clientWidth;
        canvas.height = canvas.clientHeight;
        initFramebuffers();
    }
}
canvas.addEventListener('mousemove', function (e) {
    pointers[0].moved = pointers[0].down;
    pointers[0].dx = (e.offsetX - pointers[0].x) * 10.0;
    pointers[0].dy = (e.offsetY - pointers[0].y) * 10.0;
    pointers[0].x = e.offsetX;
    pointers[0].y = e.offsetY;
 pointers[0].down = true;
});
canvas.addEventListener('touchmove', function (e) {
    e.preventDefault();
    var touches = e.targetTouches;
    for (var i = 0; i < e.touches.length; i++) {
        var pointer = pointers[i];
        pointer.moved = pointer.down;
        pointer.dx = (touches[i].pageX - pointer.x) * 10.0;
        pointer.dy = (touches[i].pageY - pointer.y) * 10.0;
        pointer.x = touches[i].pageX;
        pointer.y = touches[i].pageY;
    }
}, false);
canvas.addEventListener('mousedown', function () {
    pointers[0].down = true;
    pointers[0].color = [Math.random() + 0.2, Math.random() + 0.2, Math.random() + 0.2];
});
canvas.addEventListener('touchstart', function (e) {
    var touches = e.targetTouches;
    for (var i = 0; i < touches.length; i++) {
        if (i >= pointers.length) pointers.push(new pointerPrototype());
        pointers[i].id = touches[i].identifier;
        pointers[i].down = true;
        pointers[i].x = touches[i].pageX;
        pointers[i].y = touches[i].pageY;
        pointers[i].color = [Math.random() + 0.2, Math.random() + 0.2, Math.random() + 0.2];
    }
});
window.addEventListener('mouseup', function () {
    pointers[0].down = true;
});
window.addEventListener('touchend', function (e) {
    var touches = e.changedTouches;
    for (var i = 0; i < touches.length; i++) {
        for (var j = 0; j < pointers.length; j++) {
            if (touches[i].identifier == pointers[j].id) pointers[j].down = false;
        }
    }
});

Getting Started

Please find below the definitions I have understood so far about the JavaScript code

Breakdown the code

var canvas = document.getElementsByTagName('canvas')[0];
canvas.width = canvas.clientWidth;
canvas.height = canvas.clientHeight;

var params = { alpha: false, depth: false, stencil: false, antialias: false };
var gl = canvas.getContext('webgl2', params);
var isWebGL2 = !!gl;
if (!isWebGL2) {
    gl = canvas.getContext('webgl', params) || canvas.getContext('experimental-webgl', params);
}
gl.clearColor(0.0, 0.0, 0.0, 1.0);

• var canvas = document.getElementsByTagName('canvas')[0];: Fetches the first canvas element in the HTML document.

• canvas.width = canvas.clientWidth; and canvas.height = canvas.clientHeight;: Sets the canvas dimensions to match the client dimensions, ensuring it fills the available space.

• var params = { alpha: false, depth: false, stencil: false, antialias: false };: Defines parameters for WebGL context creation, including options like alpha, depth, stencil, and antialiasing.

• var gl = canvas.getContext('webgl2', params);: Tries to create a WebGL2 context with the specified parameters. If WebGL2 is not supported, it falls back to WebGL or experimental WebGL.

• gl.clearColor(0.0, 0.0, 0.0, 1.0);: Sets the clear color for the WebGL context to black.

var halfFloat = gl.getExtension('OES_texture_half_float');
var support_linear_float = gl.getExtension('OES_texture_half_float_linear');
if (isWebGL2) {
    gl.getExtension('EXT_color_buffer_float');
    support_linear_float = gl.getExtension('OES_texture_float_linear');
}

var TEXTURE_DOWNSAMPLE = 1;
var DENSITY_DISSIPATION = 0.98;
var VELOCITY_DISSIPATION = 0.99;
var SPLAT_RADIUS = 0.005;
var CURL = 30;
var PRESSURE_ITERATIONS = 25;

var GLProgram = function () {
    function GLProgram(vertexShader, fragmentShader) {
        _classCallCheck(this, GLProgram);

        this.uniforms = {};
        this.program = gl.createProgram();

        gl.attachShader(this.program, vertexShader);
        gl.attachShader(this.program, fragmentShader);
        gl.linkProgram(this.program);

        if (!gl.getProgramParameter(this.program, gl.LINK_STATUS)) throw gl.getProgramInfoLog(this.program);

        var uniformCount = gl.getProgramParameter(this.program, gl.ACTIVE_UNIFORMS);
        for (var i = 0; i < uniformCount; i++) {
            var uniformName = gl.getActiveUniform(this.program, i).name;
            this.uniforms[uniformName] = gl.getUniformLocation(this.program, uniformName);
        }
    }

    GLProgram.prototype.bind = function bind() {
        gl.useProgram(this.program);
    };

    return GLProgram;
}();

• The code initializes WebGL extensions (halfFloat and support_linear_float) to enhance WebGL capabilities related to texture handling.

• It checks if WebGL2 is supported and enables additional extensions (EXT_color_buffer_float and OES_texture_float_linear) accordingly.

• Constants like TEXTURE_DOWNSAMPLE, DENSITY_DISSIPATION, etc., are defined to control simulation parameters.

• The GLProgram class is defined to manage WebGL programs. It handles program creation, shader attachment, program linking, and retrieval of uniform locations for setting values during rendering.

Shader

var baseVertexShader = compileShader(gl.VERTEX_SHADER, `
    precision highp float;
    precision mediump sampler2D;

    attribute vec2 aPosition;
    varying vec2 vUv;
    varying vec2 vL;
    varying vec2 vR;
    varying vec2 vT;
    varying vec2 vB;
    uniform vec2 texelSize;

    void main () {
        vUv = aPosition * 0.5 + 0.5;
        vL = vUv - vec2(texelSize.x, 0.0);
        vR = vUv + vec2(texelSize.x, 0.0);
        vT = vUv + vec2(0.0, texelSize.y);
        vB = vUv - vec2(0.0, texelSize.y);
        gl_Position = vec4(aPosition, 0.0, 1.0);
    }
`);

• This code defines a vertex shader written in GLSL.

• It sets precision for floating-point values and texture sampling.

• Declares an attribute aPosition for vertex positions and varying variables (vUv, vL, vR, vT, vB) for texture coordinates and their neighbors.

• Calculates texture coordinates based on texelSize and sets the vertex position in clip space.

And so on for rest of the shaders as each shader serves a specific purpose in WebGL rendering, such as vertex transformations, texture display, and color splatting.

var blit = function () {
    gl.bindBuffer(gl.ARRAY_BUFFER, gl.createBuffer());
    gl.bufferData(gl.ARRAY_BUFFER, new Float32Array([-1, -1, -1, 1, 1, 1, 1, -1]), gl.STATIC_DRAW);
    gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, gl.createBuffer());
    gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, new Uint16Array([0, 1, 2, 0, 2, 3]), gl.STATIC_DRAW);
    gl.vertexAttribPointer(0, 2, gl.FLOAT, false, 0, 0);
    gl.enableVertexAttribArray(0);

    return function (destination) {
        gl.bindFramebuffer(gl.FRAMEBUFFER, destination);
        gl.drawElements(gl.TRIANGLES, 6, gl.UNSIGNED_SHORT, 0);
    };
}();

function clear(target) {
    gl.bindFramebuffer(gl.FRAMEBUFFER, target);
    gl.clear(gl.COLOR_BUFFER_BIT);
}

• blit is a function that initializes and returns another function responsible for blitting (copying) content to a destination framebuffer.

• Inside blit, it sets up buffers for vertex positions and element indices to draw a full-screen quad.

• The clear function is used to clear the color buffer of a specified target framebuffer.

function createDoubleFBO(texId, w, h, internalFormat, format, type, param) {
    var fbo1 = createFBO(texId, w, h, internalFormat, format, type, param);
    var fbo2 = createFBO(texId + 1, w, h, internalFormat, format, type, param);

    return {
        get first() {
            return fbo1;
        },
        get second() {
            return fbo2;
        },
        swap: function swap() {
            var temp = fbo1;
            fbo1 = fbo2;
            fbo2 = temp;
        }
    };
}

var textureWidth = undefined;
var textureHeight = undefined;
var density = undefined;
var velocity = undefined;
var divergence = undefined;
var curl = undefined;
var pressure = undefined;

function initFramebuffers() {
    textureWidth = gl.drawingBufferWidth >> TEXTURE_DOWNSAMPLE;
    textureHeight = gl.drawingBufferHeight >> TEXTURE_DOWNSAMPLE;

    var internalFormat = isWebGL2 ? gl.RGBA16F : gl.RGBA;
    var internalFormatRG = isWebGL2 ? gl.RG16F : gl.RGBA;
    var formatRG = isWebGL2 ? gl.RG : gl.RGBA;
    var texType = isWebGL2 ? gl.HALF_FLOAT : halfFloat.HALF_FLOAT_OES;

    density = createDoubleFBO(0, textureWidth, textureHeight, internalFormat, gl.RGBA, texType, support_linear_float ? gl.LINEAR : gl.NEAREST);
    velocity = createDoubleFBO(2, textureWidth, textureHeight, internalFormatRG, formatRG, texType, support_linear_float ? gl.LINEAR : gl.NEAREST);
    divergence = createFBO(4, textureWidth, textureHeight, internalFormatRG, formatRG, texType, gl.NEAREST);
    curl = createFBO(5, textureWidth, textureHeight, internalFormatRG, formatRG, texType, gl.NEAREST);
    pressure = createDoubleFBO(6, textureWidth, textureHeight, internalFormatRG, formatRG, texType, gl.NEAREST);
}

initFramebuffers();

var displayProgram = new GLProgram(baseVertexShader, displayShader);
var splatProgram = new GLProgram(baseVertexShader, splatShader);
var advectionProgram = new GLProgram(baseVertexShader, support_linear_float ? advectionShader : advectionManualFilteringShader);
var divergenceProgram = new GLProgram(baseVertexShader, divergenceShader);
var curlProgram = new GLProgram(baseVertexShader, curlShader);
var vorticityProgram = new GLProgram(baseVertexShader, vorticityShader);
var pressureProgram = new GLProgram(baseVertexShader, pressureShader);
var gradienSubtractProgram = new GLProgram(baseVertexShader, gradientSubtractShader);

function pointerPrototype() {
    this.id = -1;
    this.x = 0;
    this.y = 0;
    this.dx = 0;
    this.dy = 0;
    this.down = false;
    this.moved = false;
    this.color = [30, 0, 300];
}

var pointers = [];
pointers.push(new pointerPrototype());

for (var i = 0; i < 10; i++) {
    var color = [Math.random() * 10, Math.random() * 10, Math.random() * 10];
    var x = canvas.width * Math.random();
    var y = canvas.height * Math.random();
    var dx = 1000 * (Math.random() - 0.5);
    var dy = 1000 * (Math.random() - 0.5);
    splat(x, y, dx, dy, color);
}

• The createDoubleFBO function is used to create double framebuffer objects (FBOs) for rendering, allowing for efficient swapping between two FBOs.

• The initFramebuffers function initializes various framebuffers (FBOs) for simulation, including double FBOs for density, velocity, and pressure, as well as single FBOs for divergence and curl.

• GL programs are created using GLProgram for different shaders used in the simulation.

• Pointer objects are initialized using a prototype for handling user input (e.g., mouse interactions).

• Random splats are generated on the canvas to initialize the simulation with initial conditions.

canvas.addEventListener('mousemove', function (e) {
    pointers[0].moved = pointers[0].down;
    pointers[0].dx = (e.offsetX - pointers[0].x) * 10.0;
    pointers[0].dy = (e.offsetY - pointers[0].y) * 10.0;
    pointers[0].x = e.offsetX;
    pointers[0].y = e.offsetY;
 pointers[0].down = true;
});

canvas.addEventListener('touchmove', function (e) {
    e.preventDefault();
    var touches = e.targetTouches;
    for (var i = 0; i < e.touches.length; i++) {
        var pointer = pointers[i];
        pointer.moved = pointer.down;
        pointer.dx = (touches[i].pageX - pointer.x) * 10.0;
        pointer.dy = (touches[i].pageY - pointer.y) * 10.0;
        pointer.x = touches[i].pageX;
        pointer.y = touches[i].pageY;
    }
}, false);

canvas.addEventListener('mousedown', function () {
    pointers[0].down = true;
    pointers[0].color = [Math.random() + 0.2, Math.random() + 0.2, Math.random() + 0.2];
});

canvas.addEventListener('touchstart', function (e) {
    var touches = e.targetTouches;
    for (var i = 0; i < touches.length; i++) {
        if (i >= pointers.length) pointers.push(new pointerPrototype());

        pointers[i].id = touches[i].identifier;
        pointers[i].down = true;
        pointers[i].x = touches[i].pageX;
        pointers[i].y = touches[i].pageY;
        pointers[i].color = [Math.random() + 0.2, Math.random() + 0.2, Math.random() + 0.2];
    }
});

window.addEventListener('mouseup', function () {
    pointers[0].down = true;
});

window.addEventListener('touchend', function (e) {
    var touches = e.changedTouches;
    for (var i = 0; i < touches.length; i++) {
        for (var j = 0; j < pointers.length; j++) {
            if (touches[i].identifier == pointers[j].id) pointers[j].down = false;
        }
    }
});

• The code sets up event listeners for mouse and touch interactions on the canvas.

• For mouse movement (mousemove), it updates the properties of the first pointer (pointers[0]) based on the mouse's position and movement.

• For touch movement (touchmove), it handles multiple touches by updating properties for each pointer accordingly.

• Mouse button press (mousedown) and touch start (touchstart) events update the pointer's down state and color.

• Mouse button release (mouseup) and touch end (touchend) events update the pointer's down state to false, indicating the release of interaction.

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