protected AbstractGwtGLExample() {
// create the button to launch the example
final PushButton button = new PushButton("Launch example!");
setWidget(0, 0, button);
WebGLContextAttributes contextAttributes = WebGLContextAttributes.create();
contextAttributes.setAlpha(false);
// create the WebGL Canvas
webGLCanvas = new WebGLCanvas(contextAttributes, "500px", "500px");
glContext = webGLCanvas.getGlContext();
glContext.viewport(0, 0, 500, 500);