Clearing by clicking

Clearing the rendering context with random colors

This example provides a simple illustration of how to combine WebGL and user interaction. Every time the user clicks the canvas or the button, the canvas is cleared with a new randomly chosen color.

Note how we embed the WebGL function calls inside the event handler function.

<p>A very simple WebGL program that still shows some color and
    user interaction.</p> 
<p>You can repeatedly click the empty canvas or the button below
    to change color.</p>
<canvas id="canvas-view">Your browser does not seem to support
    HTML5 canvas.</canvas>
<button id="color-switcher">Press here to switch color</button>

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

window.addEventListener("load", function setupWebGL (evt) {
  "use strict"

  // Cleaning after ourselves. The event handler removes
  // itself, because it only needs to run once.
  window.removeEventListener(evt.type, setupWebGL, false);

  // Adding the same click event handler to both canvas and
  // button.
  var canvas = document.querySelector("#canvas-view");
  var button = document.querySelector("#color-switcher");
  canvas.addEventListener("click", switchColor, false);
  button.addEventListener("click", switchColor, false);

  // A variable to hold the WebGLRenderingContext.
  var gl;

  // The click event handler.
  function switchColor () {
    // Referring to the externally defined gl variable.
    // If undefined, try to obtain the WebGLRenderingContext.
    // If failed, alert user of failure.
    // Otherwise, initialize the drawing buffer (the viewport).
    if (!gl) {
      gl = canvas.getContext("webgl")
        || canvas.getContext("experimental-webgl");
      if (!gl) {
        alert("Failed to get WebGL context.\n"
          + "Your browser or device may not support WebGL.");
        return;
      }
      gl.viewport(0, 0,
        gl.drawingBufferWidth, gl.drawingBufferHeight);
    }
    // Get a random color value using a helper function.
    var color = getRandomColor();
    // Set the clear color to the random color.
    gl.clearColor(color[0], color[1], color[2], 1.0);
    // Clear the context with the newly set color. This is
    // the function call that actually does the drawing.
    gl.clear(gl.COLOR_BUFFER_BIT);
  }

  // Random color helper function.
  function getRandomColor() {
    return [Math.random(), Math.random(), Math.random()];
  }

}, false);

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