<!DOCTYPE html>
<html lang="en-us">
<head>
<meta charset="utf-8">
<meta name="robots" content="noindex">
<meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
<title>Trails</title>
<style>
html,
body,
canvas {
margin: 0;
padding: 0;
width: 100%;
height: 100%;
overflow: hidden;
}
</style>
</head>
<body>
<canvas id="canvas"></canvas>
<script id="vertexShader" type="x-shader/x-vertex">
#version 300 es
in vec4 vertexPosition;
void main() { // no-op vertex shader
gl_Position = vertexPosition;
}
</script>
<!--
4 key things are needed to create the trails effect.
(1) is in the fragment shader. (2), (3), and (4) are in the JavaScript.
-->
<script id="fragmentShader" type="x-shader/x-fragment">
#version 300 es
precision highp float;
uniform vec2 canvasSize;
uniform float time;
out vec4 fragColor;
void main() {
vec2 coord = (2.*gl_FragCoord.xy - canvasSize)/min(canvasSize.x, canvasSize.y); // [-1,1] in smaller dimension
float angle = time;
float radius = 0.8*sin(3.*time);
vec2 center = vec2(radius*cos(angle), radius*sin(angle));
if (length(coord - center) < 0.1) {
fragColor = vec4(0, sin(time/1.7)/4.+.3, .5, 1);
}
else {
fragColor = vec4(1, 1, 1, 0.005); // (1) Draw a semi-transparent background
}
}
</script>
<script id="webgl" type="text/javascript">
const canvas = document.querySelector('canvas');
const vertexShader = document.querySelector('script[type="x-shader/x-vertex"]');
const fragmentShader = document.querySelector('script[type="x-shader/x-fragment"]');
const gl = canvas.getContext("webgl2", {
preserveDrawingBuffer: true, // (2) Don't clear the buffer after every frame
});
if (!gl) {
document.body.innerHTML ='<p>Error: WebGL2 is <a href="https://get.webgl.org/webgl2/">not supported by your browser</a></p>';
throw "WebGL2 not supported";
}
function createShader(shaderType, sourceCode) {
const shader = gl.createShader(shaderType);
gl.shaderSource(shader, sourceCode);
gl.compileShader(shader);
if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) throw gl.getShaderInfoLog(shader);
return shader;
}
const program = gl.createProgram();
gl.attachShader(program, createShader(gl.VERTEX_SHADER, vertexShader.textContent.trim()));
gl.attachShader(program, createShader(gl.FRAGMENT_SHADER, fragmentShader.textContent.trim()));
gl.linkProgram(program);
if (!gl.getProgramParameter(program, gl.LINK_STATUS)) throw gl.getProgramInfoLog(program);
gl.useProgram(program);
const vertices = [[-1, -1], [1, -1], [-1, 1], [1, 1]];
gl.bindBuffer(gl.ARRAY_BUFFER, gl.createBuffer());
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(vertices.flat()), gl.STATIC_DRAW);
const vertexPosition = gl.getAttribLocation(program, "vertexPosition");
gl.enableVertexAttribArray(vertexPosition);
gl.vertexAttribPointer(vertexPosition, 2, gl.FLOAT, false, 0, 0);
const canvasSizeUniform = gl.getUniformLocation(program, 'canvasSize');
const timeUniform = gl.getUniformLocation(program, 'time');
gl.enable(gl.BLEND); // (3) Enable blend mode
gl.blendFunc(gl.SRC_ALPHA, gl.ONE_MINUS_SRC_ALPHA); // (4) Apply the needed blend function
function draw() {
const width = canvas.clientWidth;
const height = canvas.clientHeight;
canvas.width = width;
canvas.height = height;
gl.viewport(0, 0, width, height);
gl.uniform2f(canvasSizeUniform, width, height);
gl.uniform1f(timeUniform, performance.now() / 1000.);
gl.drawArrays(gl.TRIANGLE_STRIP, 0, vertices.length);
requestAnimationFrame(draw);
}
draw();
</script>
<style>
/* This is designed to work with a white background */
canvas {
background-color: white;
}
</style>
</body>
<!--
© Adam Murray 2022
https://adammurray.blog/
Creative Commons License
Attribution-NonCommercial-ShareAlike 4.0 International
https://creativecommons.org/licenses/by-nc-sa/4.0/
-->