<!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>Basic Mouse/Touch Interactions</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>
<script id="fragmentShader" type="x-shader/x-fragment">
#version 300 es
precision highp float;
uniform vec2 canvasSize;
uniform vec2 pointerPosition;
out vec4 fragColor;
void main() {
vec2 coord = gl_FragCoord.xy - canvasSize/2.;
float minDim = min(canvasSize.x, canvasSize.y);
float outsideOfCircle = step(minDim/10., length(coord - pointerPosition));
// green outside circle, blue inside:
fragColor = vec4(0, outsideOfCircle, 1.-outsideOfCircle, 1);
}
</script>
<script 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");
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]];
const vertexBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
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 pointerPositionUniform = gl.getUniformLocation(program, 'pointerPosition');
const canvasSizeUniform = gl.getUniformLocation(program, 'canvasSize');
function draw(pointerEvent) {
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);
if (pointerEvent) {
const canvasRect = canvas.getBoundingClientRect();
// offset by canvas.width/2 and canvas.height/2 like we do with gl_FragCoord in the shader:
const x = pointerEvent.clientX - canvasRect.left - canvasRect.width / 2;
const y = -(pointerEvent.clientY - canvasRect.top - canvasRect.height / 2);
gl.uniform2f(pointerPositionUniform, x, y);
} else {
gl.uniform2f(pointerPositionUniform, 0, 0);
}
gl.drawArrays(gl.TRIANGLE_STRIP, 0, vertices.length);
}
draw();
// Only start dragging from the canvas...
canvas.addEventListener('pointerdown', (event) => {
this.pointerdown = true;
draw(event);
});
// but allow dragging around the entire window:
window.addEventListener('pointermove', (event) => {
if (this.pointerdown) draw(event);
});
window.addEventListener('pointerup', () => this.pointerdown = false);
// And don't scroll when sliding around the canvas on mobile:
canvas.addEventListener('touchmove', (event) => event.preventDefault());
window.addEventListener('resize', () => draw());
</script>
</body>
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