
    quad.position.z = 0;
		scene.add( quad );

    // Whenever the user updates the input field, we need to parse and produce a
    // new shader to update what's being displayed.
    function update_expression() {
      var new_expression = $('#equation-input').val();
      var compiledExpression = pegParser.parse(new_expression);

    	var shaderMaterial = new THREE.ShaderMaterial({
        uniforms: uniforms,
    		vertexShader:   $('#vertexshader').text(),
    		fragmentShader: $('#fragmentshader').tex