Clearing by clicking
This example demonstrates how to combine user interaction with WebGL graphics operations by clearing the rendering context with a random color when the user clicks.
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.