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HTML

              
                <div class="page">
    <div class="header">
        How it's done
    </div>
    <div class="content">
        <p>
            The HTML content you're reading right now is overlaid with a full-screen <b>&lt;canvas&gt;</b> element.
            There is a fragment shader that defines opacity and color for each pixel of the <b>&lt;canvas&gt;</b>.
            Shader input values are <b>animation progress</b>, <b>time</b>, and <b>resolution</b>.
        </p>
        <p>
            While <b>time</b> and <b>window size (resolution)</b> are super easy to gather, for <b>animation progress</b> I use <a href="https://gsap.com/docs/v3/Plugins/ScrollTrigger/" target="_blank">GSAP ScrollTrigger</a> plugin.
        </p>
        <p>
            Once the inputs are prepared, we pass them as uniforms to the shader.
            The WebGL part of this demo is a basic JS boilerplate to render a fragment shader on the single full-screen plane. No extra libraries here.
        </p>
        <p>
            The fragment shader is based on <a href="https://thebookofshaders.com/13/" target="_blank">Fractal Brownian Motion (fBm)</a> noise.
        </p>
        <p>
            First, we create a semi-transparent mask to define a contour of burning paper. It is basically a low-scale fBm noise with <b>animation progress</b> value used as a threshold.
            Taking the same fBm noise with different thresholds we can
            <br>
            (a) darken parts of the paper so each pixel gets darker before turning transparent
            <br>
            (b) define the stripe along the paper edge for fire.
        </p>
        <p>
            The fire is done as another two fBm based functions, one for shape and one for color. Both have a higher scale and both are animated with <b>time</b> value instead of <b>animation progress</b>.
        </p>

        <p class="last-line">
            <a href="https://www.linkedin.com/in/ksenia-kondrashova/" target="_blank">linkedIn</a> | <a href="https://codepen.io/ksenia-k" target="_blank">codepen</a> | <a href="https://twitter.com/uuuuuulala" target="_blank">twitter (X)</a>
        </p>
    </div>
</div>

<canvas id="fire-overlay"></canvas>
<div class="scroll-msg">
    <div>Hello 👋</div>
    <div>scroll me</div>
    <div class="arrow-animated">&darr;</div>
</div>

<script type="x-shader/x-fragment" id="vertShader">
    precision mediump float;

    varying vec2 vUv;
    attribute vec2 a_position;

    void main() {
        vUv = a_position;
        gl_Position = vec4(a_position, 0.0, 1.0);
    }
</script>


<script type="x-shader/x-fragment" id="fragShader">
    precision mediump float;

    varying vec2 vUv;
    uniform vec2 u_resolution;
    uniform float u_progress;
    uniform float u_time;

    float rand(vec2 n) {
        return fract(cos(dot(n, vec2(12.9898, 4.1414))) * 43758.5453);
    }
    float noise(vec2 n) {
        const vec2 d = vec2(0., 1.);
        vec2 b = floor(n), f = smoothstep(vec2(0.0), vec2(1.0), fract(n));
        return mix(mix(rand(b), rand(b + d.yx), f.x), mix(rand(b + d.xy), rand(b + d.yy), f.x), f.y);
    }
    float fbm(vec2 n) {
        float total = 0.0, amplitude = .4;
        for (int i = 0; i < 4; i++) {
            total += noise(n) * amplitude;
            n += n;
            amplitude *= 0.6;
        }
        return total;
    }

    void main() {
        vec2 uv = vUv;
        uv.x *= min(1., u_resolution.x / u_resolution.y);
        uv.y *= min(1., u_resolution.y / u_resolution.x);

        float t = u_progress;
        vec3 color = vec3(1., 1., .95);

        float main_noise = 1. - fbm(.75 * uv + 10. - vec2(.3, .9 * t));

        float paper_darkness = smoothstep(main_noise - .1, main_noise, t);
        color -= vec3(.99, .95, .99) * paper_darkness;

        vec3 fire_color = fbm(6. * uv - vec2(0., .005 * u_time)) * vec3(6., 1.4, .0);
        float show_fire = smoothstep(.4, .9, fbm(10. * uv + 2. - vec2(0., .005 * u_time)));
        show_fire += smoothstep(.7, .8, fbm(.5 * uv + 5. - vec2(0., .001 * u_time)));

        float fire_border = .02 * show_fire;
        float fire_edge = smoothstep(main_noise - fire_border, main_noise - .5 * fire_border, t);
        fire_edge *= (1. - smoothstep(main_noise - .5 * fire_border, main_noise, t));
        color += fire_color * fire_edge;

        float opacity = 1. - smoothstep(main_noise - .0005, main_noise, t);

        gl_FragColor = vec4(color, opacity);
    }

</script>

              
            
!

CSS

              
                body, html {
    margin: 0;
    padding: 0;
    font-family: sans-serif;
    font-size: 20px;
    color: #3d3d3d;
}

a {
    color: inherit;
}

.page {
    width: 100%;
    min-height: 180vh;
    display: flex;
    flex-direction: column;
    align-items: center;
    opacity: 0;
}

.page .header {
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 40px;
    text-transform: uppercase;
    width: 100vw;
    margin-top: 20vh;
    height: 25vh;
}

.page .content {
    max-width: 800px;
    padding: 10px;
}

.page .last-line {
    text-align: right;
    padding-top: 1em;
}

.page ::-moz-selection {
    background: #F7C02D;
}

.page ::selection {
    background: #F7C02D;
}

.scroll-msg {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100vh;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    pointer-events: none;
	 padding-top: 2em;
}

.scroll-msg > div:nth-child(1) {
    margin-top: -10vh;
    padding-bottom: 1em;
    text-transform: uppercase;
    font-size: 2em;
}

canvas#fire-overlay {
    position: fixed;
    top: 0;
    left: 0;
    display: block;
    width: 100%;
    pointer-events: none;
}

.arrow-animated {
    font-size: 1em;
    animation: arrow-float 1s infinite;
}

@keyframes arrow-float {
    0% {
        transform: translateY(0);
        animation-timing-function: ease-out;
    }
    60% {
        transform: translateY(50%);
        animation-timing-function: ease-in-out;
    }
    100% {
        transform: translateY(0);
        animation-timing-function: ease-out;
    }
}
              
            
!

JS

              
                const canvasEl = document.querySelector("#fire-overlay");
const scrollMsgEl = document.querySelector(".scroll-msg");

const devicePixelRatio = Math.min(window.devicePixelRatio, 2);
// const devicePixelRatio = 1;

const params = {
    fireTime: .35,
    fireTimeAddition: 0
}

let st, uniforms;
const gl = initShader();

st = gsap.timeline({
    scrollTrigger: {
        trigger: ".page",
        start: "0% 0%",
        end: "100% 100%",
        // markers: true,
        scrub: true,
        onLeaveBack: () => {
            params.fireTimeAddition = Math.min(params.fireTimeAddition, .3);
            gsap.to(params, {
                duration: .75,
                fireTimeAddition: 0
            })
        },
    },
})
	 .to(scrollMsgEl, {
	   duration: .1,
   	opacity: 0
    }, 0)
    .to(params, {
        fireTime: .63
    }, 0)


window.addEventListener("resize", resizeCanvas);
resizeCanvas();

gsap.set(".page", {
    opacity: 1
})


function initShader() {
    const vsSource = document.getElementById("vertShader").innerHTML;
    const fsSource = document.getElementById("fragShader").innerHTML;

    const gl = canvasEl.getContext("webgl") || canvasEl.getContext("experimental-webgl");

    if (!gl) {
        alert("WebGL is not supported by your browser.");
    }

    function createShader(gl, sourceCode, type) {
        const shader = gl.createShader(type);
        gl.shaderSource(shader, sourceCode);
        gl.compileShader(shader);

        if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) {
            console.error("An error occurred compiling the shaders: " + gl.getShaderInfoLog(shader));
            gl.deleteShader(shader);
            return null;
        }

        return shader;
    }

    const vertexShader = createShader(gl, vsSource, gl.VERTEX_SHADER);
    const fragmentShader = createShader(gl, fsSource, gl.FRAGMENT_SHADER);

    function createShaderProgram(gl, vertexShader, fragmentShader) {
        const program = gl.createProgram();
        gl.attachShader(program, vertexShader);
        gl.attachShader(program, fragmentShader);
        gl.linkProgram(program);

        if (!gl.getProgramParameter(program, gl.LINK_STATUS)) {
            console.error("Unable to initialize the shader program: " + gl.getProgramInfoLog(program));
            return null;
        }

        return program;
    }

    const shaderProgram = createShaderProgram(gl, vertexShader, fragmentShader);
    uniforms = getUniforms(shaderProgram);

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

    const vertices = new Float32Array([-1., -1., 1., -1., -1., 1., 1., 1.]);

    const vertexBuffer = gl.createBuffer();
    gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
    gl.bufferData(gl.ARRAY_BUFFER, vertices, gl.STATIC_DRAW);

    gl.useProgram(shaderProgram);

    const positionLocation = gl.getAttribLocation(shaderProgram, "a_position");
    gl.enableVertexAttribArray(positionLocation);

    gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
    gl.vertexAttribPointer(positionLocation, 2, gl.FLOAT, false, 0, 0);

    return gl;
}

function render() {
    const currentTime = performance.now();
    gl.uniform1f(uniforms.u_time, currentTime);

    if (st.scrollTrigger.isActive && st.scrollTrigger.direction === 1) {
        params.fireTimeAddition += .001;
    }

    gl.uniform1f(uniforms.u_progress, params.fireTime + params.fireTimeAddition);
    gl.drawArrays(gl.TRIANGLE_STRIP, 0, 4);

    requestAnimationFrame(render);
}

function resizeCanvas() {
    canvasEl.width = window.innerWidth * devicePixelRatio;
    canvasEl.height = window.innerHeight * devicePixelRatio;
    gl.viewport(0, 0, canvasEl.width, canvasEl.height);
    gl.uniform2f(uniforms.u_resolution, canvasEl.width, canvasEl.height);
    render();
}
              
            
!
999px

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