Hello vertex attributes

Hello World program in GLSL

How to send input to a shader program by saving data in GPU memory.

<p>First encounter with attributes and sending data to GPU. Click
on the canvas to change the horizontal position of the square.</p>

<canvas>Your browser does not seem to support 
    HTML5 canvas.</canvas>

body {
  text-align : center;
}
canvas {
  width : 280px;
  height : 210px;
  margin : auto;
  padding : 0;
  border : none;
  background-color : black;
}
button {
  display : block;
  font-size : inherit;
  margin : auto;
  padding : 0.6em;
}

<script type="x-shader/x-vertex" id="vertex-shader">
#version 100
precision highp float;

attribute float position;

void main() {
  gl_Position = vec4(position, 0.0, 0.0, 1.0);
  gl_PointSize = 64.0;
}
</script>

<script type="x-shader/x-fragment" id="fragment-shader">
#version 100
precision mediump float;
void main() {
  gl_FragColor = vec4(0.18, 0.54, 0.34, 1.0);
}
</script>

;(function(){

"use strict"
window.addEventListener("load", setupWebGL, false);
var gl,
  program;
function setupWebGL (evt) {
  window.removeEventListener(evt.type, setupWebGL, false);
  if (!(gl = getRenderingContext()))
    return;

  var source = document.querySelector("#vertex-shader").innerHTML;
  var vertexShader = gl.createShader(gl.VERTEX_SHADER);
  gl.shaderSource(vertexShader,source);
  gl.compileShader(vertexShader);
  source = document.querySelector("#fragment-shader").innerHTML
  var fragmentShader = gl.createShader(gl.FRAGMENT_SHADER);
  gl.shaderSource(fragmentShader,source);
  gl.compileShader(fragmentShader);
  program = gl.createProgram();
  gl.attachShader(program, vertexShader);
  gl.attachShader(program, fragmentShader);
  gl.linkProgram(program);
  gl.detachShader(program, vertexShader);
  gl.detachShader(program, fragmentShader);
  gl.deleteShader(vertexShader);
  gl.deleteShader(fragmentShader);
  if (!gl.getProgramParameter(program, gl.LINK_STATUS)) {
    var linkErrLog = gl.getProgramInfoLog(program);
    cleanup();
    document.querySelector("p").innerHTML =
      "Shader program did not link successfully. "
      + "Error log: " + linkErrLog;
    return;
  }
   
  initializeAttributes();
  gl.useProgram(program);
  gl.drawArrays(gl.POINTS, 0, 1);
   
  document.querySelector("canvas").addEventListener("click",
    function (evt) {
      var clickXrelativToCanvas =
          evt.pageX - evt.target.offsetLeft;
      var clickXinWebGLCoords =
          2.0 * (clickXrelativToCanvas- gl.drawingBufferWidth/2)
          / gl.drawingBufferWidth;
      gl.bufferData(gl.ARRAY_BUFFER,
        new Float32Array([clickXinWebGLCoords]), gl.STATIC_DRAW);
      gl.drawArrays(gl.POINTS, 0, 1);
    }, false);
}

var buffer;
function initializeAttributes() {
  gl.enableVertexAttribArray(0);
  buffer = gl.createBuffer();
  gl.bindBuffer(gl.ARRAY_BUFFER, buffer);
  gl.bufferData(gl.ARRAY_BUFFER, new Float32Array([0.0]), gl.STATIC_DRAW);
  gl.vertexAttribPointer(0, 1, gl.FLOAT, false, 0, 0);
}

window.addEventListener("beforeunload", cleanup, true);
function cleanup() {
  gl.useProgram(null);
  if (buffer)
    gl.deleteBuffer(buffer);
  if (program)
    gl.deleteProgram(program);
}

function getRenderingContext() {
  var canvas = document.querySelector("canvas");
  canvas.width = canvas.clientWidth;
  canvas.height = canvas.clientHeight;
  var gl = canvas.getContext("webgl")
    || canvas.getContext("experimental-webgl");
  if (!gl) {
    var paragraph = document.querySelector("p");
    paragraph.innerHTML = "Failed to get WebGL context."
      + "Your browser or device may not support WebGL.";
    return null;
  }
  gl.viewport(0, 0,
    gl.drawingBufferWidth, gl.drawingBufferHeight);
  gl.clearColor(0.0, 0.0, 0.0, 1.0);
  gl.clear(gl.COLOR_BUFFER_BIT);
  return gl;
}

})();

The source code of this example is also available on GitHub.