function Context(canvas)

 {

     this.canvas = canvas;

     try 

     {

 	this.gl = canvas.getContext("experimental-webgl");

 	this.viewport = {'width': canvas.width,

 			 'height': canvas.height};

     } 

     catch(e) 

     {

 	alert(e);

     }

     if (!this.gl) alert("Failed: WebGL init.");

     this.mvMatrix = mat4.create();

     this.pMatrix = mat4.create();

     this.shader = new Shader(this);  

 }

 Context.prototype.updateMatrixUniforms = function()

 {

     var gl = this.gl;

     var program = this.shader;

     var pMatrix = this.pMatrix;

     var mvMatrix = this.mvMatrix;

     gl.uniformMatrix4fv(program.pMatrixUniform, false, pMatrix);

     gl.uniformMatrix4fv(program.mvMatrixUniform, false, mvMatrix);

     var normalMatrix = mat3.create();

     mat4.toInverseMat3(mvMatrix, normalMatrix);

     mat3.transpose(normalMatrix);

     gl.uniformMatrix3fv(program.nMatrixUniform, false, normalMatrix);

 }

 Context.prototype.expand = function()

 { 

     var width = window.innerWidth;

     var height = window.innerHeight;

     this.canvas.width = width;

     this.canvas.height = height;

     this.viewport.width = width;

     this.viewport.height = height;

 }

 function Shader(context)

 {

     var gl = context.gl;

     var fragment = loadShader(gl, "fragment-shader");

     var vertex = loadShader(gl, "vertex-shader");

     this.program = gl.createProgram();

     gl.attachShader(this.program, vertex);

     gl.attachShader(this.program, fragment);

     gl.linkProgram(this.program);

     if (!gl.getProgramParameter(this.program, gl.LINK_STATUS))

     {

 	alert("Failed: Shader init.");

     }

     gl.useProgram(this.program);

     this.vertexPosition = gl.getAttribLocation(this.program, "aVertexPosition");

     gl.enableVertexAttribArray(this.vertexPosition);

     this.vertexNormal = gl.getAttribLocation(this.program, "aVertexNormal");

     gl.enableVertexAttribArray(this.vertexNormal);

     this.vertexColour = gl.getAttribLocation(this.program, "aVertexColour");

     gl.enableVertexAttribArray(this.vertexColour);    

     this.pMatrixUniform = gl.getUniformLocation(this.program, "uPMatrix");

     this.mvMatrixUniform = gl.getUniformLocation(this.program, "uMVMatrix");    

     this.nMatrixUniform = gl.getUniformLocation(this.program, "uNMatrix");

     //this.samplerUniform = gl.getUniformLocation(this.program, "uSampler");

     this.useLightingUniform = gl.getUniformLocation(this.program, "uUseLighting");

     this.ambientColourUniform = gl.getUniformLocation(this.program, "uAmbientColour");

     this.lightingDirectionUniform = gl.getUniformLocation(this.program, "uLightingDirection");

     this.directionalColourUniform = gl.getUniformLocation(this.program, "uDirectionalColour");

     function loadShader(gl, id)

     {

 	var script = document.getElementById(id);

 	if (!script) return null;

 	var str = "";

 	var child = script.firstChild;

 	while (child)

 	{

 	    if (child.nodeType == 3) str += child.textContent;

 	    child = child.nextSibling;

 	}

 	var shader;

 	var common = "x-shader/x-";

 	if (script.type == common + "fragment") shader = gl.createShader(gl.FRAGMENT_SHADER);

 	else if (script.type == common + "vertex") shader = gl.createShader(gl.VERTEX_SHADER);

 	else return null;

 	gl.shaderSource(shader, str);

 	gl.compileShader(shader);

 	if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS))

         {

 	    alert(gl.getShaderInfoLog(shader));

 	    return null;

 	}

 	return shader;

     }

 }

 function Renderer(camera, context)

 {

     this.camera = camera;

     this.context = context;

     this.gl = context.gl;

     this.matrixStack = [];

 }

 Renderer.prototype.update = function(scene)

 { 

     var gl = this.context.gl;

     var viewport = this.context.viewport;

     var shader = this.context.shader;

     var mvMatrix = this.context.mvMatrix;

     var pMatrix = this.context.pMatrix;

     var camera = this.camera;

     gl.viewport(0, 0, viewport.width, viewport.height);

     gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);

     mat4.perspective(45, viewport.width / viewport.height, 0.1, 100.0, pMatrix);

     mat4.identity(mvMatrix);

     camera.update();

     mat4.multiply(mvMatrix, camera.matrix); 

     //this.pushMatrix(mvMatrix);

     gl.bindBuffer(gl.ARRAY_BUFFER, scene.positionBuffer);

     gl.vertexAttribPointer(shader.vertexPosition, scene.positionBuffer.itemSize, gl.FLOAT, false, 0, 0);

     gl.bindBuffer(gl.ARRAY_BUFFER, scene.normalBuffer);

     gl.vertexAttribPointer(shader.vertexNormal, scene.normalBuffer.itemSize, gl.FLOAT, false, 0, 0);

     gl.bindBuffer(gl.ARRAY_BUFFER, scene.colourBuffer);

     gl.vertexAttribPointer(shader.vertexColour, scene.colourBuffer.itemSize, gl.FLOAT, false, 0, 0);

     var lighting = true;

     gl.uniform1i(shader.useLightingUniform, lighting);

     if (lighting)

     {   

 	var uni = 0.02;

 	var ambient = {"r": uni, "g": uni, "b": uni};

    	gl.uniform3f(shader.ambientColourUniform, ambient.r, ambient.g, ambient.b);

 	var lightingDir = vec3.create();

 	vec3.add(lightingDir, [0.0, -1.0, 0.0]);

 	vec3.normalize(lightingDir);

 	vec3.scale(lightingDir, -1);     

 	gl.uniform3fv(shader.lightingDirectionUniform, lightingDir);

 	uni = 0.5;

 	var directional =  {"r": uni, "g": uni, "b": uni};

 	gl.uniform3f(shader.directionalColourUniform, directional.r, directional.g, directional.b);

     }

     gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, scene.indexBuffer);

     this.context.updateMatrixUniforms();

     gl.drawElements(gl.TRIANGLES, scene.indexBuffer.numItems, gl.UNSIGNED_SHORT, 0);

     //gl.drawArrays(gl.TRIANGLE_STRIP, 0, scene.positionBuffer.numItems);

     //mvMatrix = this.popMatrix();

     if (this.next) this.next.draw(scene);

 }

 Renderer.prototype.pushMatrix = function(matrix)

 {

     var copy = mat4.create();

     mat4.set(matrix, copy);

     this.matrixStack.push(copy);

 }

 Renderer.prototype.popMatrix = function()    

 {

     if (this.matrixStack.length > 0) return this.matrixStack.pop();

 }