逐片元光照，多着色方案切换

<div class="page-header"><h3>13、逐片元光照，多着色方案切换</h3></div>

<canvas id = "test13-canvas" width = "800" height = "600"></canvas>
<br/>

<input type="checkbox" id="lighting" checked /> 使用光照
<br/>
<input type="checkbox" id="per-fragment" checked /> 逐片元计算光照
<br/>
<input type="checkbox" id="textures" checked /> 使用纹理
<br/>
<h4>点光:</h4>
<table>
    <tr>
        <td><b>位置:</b></td>
        <td>X: <input type="text" id="lightPositionX" value="0.0" /></td>
        <td>Y: <input type="text" id="lightPositionY" value="0.0" /></td>
        <td>Z: <input type="text" id="lightPositionZ" value="-5.0" /></td>
    </tr>
    <tr>
        <td><b>颜色:</b></td>
        <td>R: <input type="text" id="pointR" value="0.8" /></td>
        <td>G: <input type="text" id="pointG" value="0.8" /></td>
        <td>B: <input type="text" id="pointB" value="0.8" /></td>
    </tr>
</table>
<h4>环境光:</h4>
<table style="border: 0; padding: 10px;">
    <tr>
        <td><b>颜色:</b></td>
        <td>R: <input type="text" id="ambientR" value="0.2" /></td>
        <td>G: <input type="text" id="ambientG" value="0.2" /></td>
        <td>B: <input type="text" id="ambientB" value="0.2" /></td>
    </tr>
</table>
<br/>
月球表面纹理图片来自 
<a href="http://maps.jpl.nasa.gov/">the Jet Propulsion Laboratory</a>.
<script id = "per-vertex-lighting-vs" type = "x-shader/x-vertex">
    attribute vec3 aVertexPosition;
    attribute vec3 aVertexNormal;
    attribute vec2 aTextureCoord;

    uniform mat4 uMVMatrix;
    uniform mat4 uPMatrix;
    uniform mat3 uNMatrix;

    uniform vec3 uAmbientColor;

    uniform vec3 uPointLightingLocation;
    uniform vec3 uPointLightingColor;

    uniform bool uUseLighting;

    varying vec2 vTextureCoord;
    varying vec3 vLightWeighting;

    void main(void)
    {
        vec4 mvPosition = uMVMatrix * vec4(aVertexPosition, 1.0);
        gl_Position = uPMatrix * mvPosition;
        vTextureCoord = aTextureCoord;

        if (!uUseLighting)
        {
            vLightWeighting = vec3(1.0, 1.0, 1.0);
        }
        else
        {
            vec3 lightDirection = 
                normalize(uPointLightingLocation - mvPosition.xyz);

            vec3 transformedNormal = uNMatrix * aVertexNormal;
            float directionalLightWeighting = 
                max(dot(transformedNormal, lightDirection), 0.0);
            vLightWeighting = 
                uAmbientColor + uPointLightingColor * directionalLightWeighting;
        }
    }
</script>

<script id = "per-vertex-lighting-fs" type = "x-shader/x-fragment">
    precision mediump float;

    varying vec2 vTextureCoord;
    varying vec3 vLightWeighting;

    uniform bool uUseTextures;

    uniform sampler2D uSampler;

    void main(void)
    {
        vec4 fragmentColor;
        if (uUseTextures)
        {
            fragmentColor = texture2D(uSampler, vec2(vTextureCoord.s, vTextureCoord.t));
        }
        else
        {
            fragmentColor = vec4(1.0, 1.0, 1.0, 1.0);
        }
        gl_FragColor = vec4(fragmentColor.rgb * vLightWeighting, fragmentColor.a);
    }
</script>




<script id = "per-fragment-lighting-vs" type = "x-shader/x-vertex">
    attribute vec3 aVertexPosition;
    attribute vec3 aVertexNormal;
    attribute vec2 aTextureCoord;

    uniform mat4 uMVMatrix;
    uniform mat4 uPMatrix;
    uniform mat3 uNMatrix;

    varying vec2 vTextureCoord;
    varying vec3 vTransformedNormal;
    varying vec4 vPosition;


    void main(void)
    {
        vPosition = uMVMatrix * vec4(aVertexPosition, 1.0);
        gl_Position = uPMatrix * vPosition;
        vTextureCoord = aTextureCoord;
        vTransformedNormal = uNMatrix * aVertexNormal;
    }
</script>
<script id = "per-fragment-lighting-fs" type = "x-shader/x-fragment">
    precision mediump float;

    varying vec2 vTextureCoord;
    varying vec3 vTransformedNormal;
    varying vec4 vPosition;

    uniform bool uUseLighting;
    uniform bool uUseTextures;

    uniform vec3 uAmbientColor;

    uniform vec3 uPointLightingLocation;
    uniform vec3 uPointLightingColor;

    uniform sampler2D uSampler;


    void main(void)
    {
        vec3 lightWeighting;
        if (!uUseLighting)
        {
            lightWeighting = vec3(1.0, 1.0, 1.0);
        }
        else
        {
            vec3 lightDirection = 
                normalize(uPointLightingLocation - vPosition.xyz);

            float directionalLightWeighting = 
                max(dot(normalize(vTransformedNormal), lightDirection), 0.0);
            lightWeighting = 
                uAmbientColor + uPointLightingColor * directionalLightWeighting;
        }

        vec4 fragmentColor;
        if (uUseTextures)
        {
            fragmentColor =
                texture2D(uSampler, vec2(vTextureCoord.s, vTextureCoord.t));
        }
        else
        {
            fragmentColor = vec4(1.0, 1.0, 1.0, 1.0);
        }
        gl_FragColor = vec4(fragmentColor.rgb * lightWeighting, fragmentColor.a);
    }
</script>
<script type="text/javascript">
    requestAnimFrame = window.requestAnimationFrame || 
        window.mozRequestAnimationFrame || 
        window.webkitRequestAnimationFrame || 
        window.msRequestAnimationFrame || 
        window.oRequestAnimationFrame || 
        function(callback) { setTimeout(callback, 1000 / 60); }; 
</script>
<script type="text/javascript">
$(document).ready(function ()
{
    webGLStart();
});

function webGLStart()
{
    var canvas = $("#test13-canvas");
    initGL(canvas[0]);
    initShaders();
    initBuffers();
    initTexture();

    gl.clearColor(0.0, 0.0, 0.0, 1.0);
    gl.enable(gl.DEPTH_TEST);

    waitTexture = setInterval("tick()", 100);
}

var textureFlag = [0, 0];
var startTick = false;
var waitTick;
function tick()
{
    if(textureFlag[0] == 1 && textureFlag[1] == 1)
    {
        startTick = true;
        textureFlag[0] = 2;
        textureFlag[1] = 2;
        clearInterval(waitTexture);
    }
    if(startTick)
    {
        requestAnimFrame(tick);
        drawScene();
        animate();
    }
}
var gl;
function initGL(canvas)
{
    try
    {
        gl = canvas.getContext("webgl") || canvas.getContext("experimental-webgl");
        gl.viewportWidth = canvas.width;
        gl.viewportHeight = canvas.height;
    }catch(e){}
    if(!gl)
    {
        alert("无法初始化“WebGL”。");
    }
}

var moonTexture;
var crateTexture;
function initTexture()
{
    moonTexture = gl.createTexture();
    moonTexture.image = new Image();
    moonTexture.image.onload = function()
    {
        handleLoadedTexture(moonTexture);
        textureFlag[0] = true;
    }
    moonTexture.image.src = "/Public/image/moon.gif";


    crateTexture = gl.createTexture();
    crateTexture.image = new Image();
    crateTexture.image.onload = function()
    {
        handleLoadedTexture(crateTexture);
        textureFlag[1] = true;
    }
    crateTexture.image.src = "/Public/image/crate.gif";
}
function handleLoadedTexture(texture)
{
    gl.pixelStorei(gl.UNPACK_FLIP_Y_WEBGL, 1);


    gl.bindTexture(gl.TEXTURE_2D, texture);
    gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, 
        gl.UNSIGNED_BYTE, texture.image);
    gl.texParameteri(gl.TEXTURE_2D, 
        gl.TEXTURE_MAG_FILTER, gl.LINEAR);
    gl.texParameteri(gl.TEXTURE_2D, 
        gl.TEXTURE_MIN_FILTER, gl.LINEAR_MIPMAP_NEAREST);

    gl.generateMipmap(gl.TEXTURE_2D);

    gl.bindTexture(gl.TEXTURE_2D, null);
}
function getShader(gl, id)
{
    var shaderScript = $("#" + id);
    if(!shaderScript.length)
    {
        return null;
    }
    var str = shaderScript.text();

    var shader;
    if(shaderScript[0].type == "x-shader/x-fragment")
    {
        shader = gl.createShader(gl.FRAGMENT_SHADER);
    }
    else if(shaderScript[0].type == "x-shader/x-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 createProgram(vertexShaderID, fragmentShaderID)
{
    var vertexShader = getShader(gl, vertexShaderID);
    var fragmentShader = getShader(gl, fragmentShaderID);

    var program = gl.createProgram();
    gl.attachShader(program, vertexShader);
    gl.attachShader(program, fragmentShader);
    gl.linkProgram(program);
    if(!gl.getProgramParameter(program, gl.LINK_STATUS))
    {
        alert("无法初始化“Shader”。");
    }

    program.vertexPositionAttribute = 
        gl.getAttribLocation(program, "aVertexPosition");
    gl.enableVertexAttribArray(program.vertexPositionAttribute);

    program.vertexNormalAttribute = 
        gl.getAttribLocation(program, "aVertexNormal");
    gl.enableVertexAttribArray(program.vertexNormalAttribute);

    program.textureCoordAttribute = 
        gl.getAttribLocation(program, "aTextureCoord");
    gl.enableVertexAttribArray(program.textureCoordAttribute);

    program.pMatrixUniform = 
        gl.getUniformLocation(program, "uPMatrix");
    program.mvMatrixUniform = 
        gl.getUniformLocation(program, "uMVMatrix");
    program.nMatrixUniform  = 
        gl.getUniformLocation(program, "uNMatrix");
    program.samplerUniform = 
        gl.getUniformLocation(program, "uSampler");
    program.useTexturesUniform = 
        gl.getUniformLocation(program, "uUseTextures");
    program.useLightingUniform = 
        gl.getUniformLocation(program, "uUseLighting");
    program.ambientColorUniform = 
        gl.getUniformLocation(program, "uAmbientColor");
    program.pointLightingLocationUniform  = 
        gl.getUniformLocation(program, "uPointLightingLocation");
    program.pointLightingColorUniform  = 
        gl.getUniformLocation(program, "uPointLightingColor");
    return program;
}
var currentProgram;
var perVertexProgram;
var perFragmentProgram;
function initShaders()
{
    perVertexProgram = 
        createProgram("per-vertex-lighting-vs", "per-vertex-lighting-fs");
    perFragmentProgram = 
        createProgram("per-fragment-lighting-vs", "per-fragment-lighting-fs");
}

var mvMatrix = mat4.create();

var mvMatrixStack = [];

var pMatrix = mat4.create();

function mvPushMatrix()
{
    var copy = mat4.clone(mvMatrix);
    mvMatrixStack.push(copy);
}
function mvPopMatrix()
{
    if(mvMatrixStack.length == 0)
    {
        throw "不合法的矩阵出栈操作!";
    }
    mvMatrix = mvMatrixStack.pop();
}

function setMatrixUniforms()
{
    gl.uniformMatrix4fv(currentProgram.pMatrixUniform, false, pMatrix);
    gl.uniformMatrix4fv(currentProgram.mvMatrixUniform, false, mvMatrix);

    var normalMatrix = mat3.create();

    mat3.fromMat4(normalMatrix, mvMatrix);
    mat3.invert(normalMatrix, normalMatrix);
    mat3.transpose(normalMatrix, normalMatrix);

    gl.uniformMatrix3fv(currentProgram.nMatrixUniform, false, normalMatrix);
}

var cubeVertexPositionBUffer;
var cubeVertexTextureCoordBuffer;
var cubeVertexIndexBuffer;
var cubeVertexNormalBuffer;

var moonVertexPositionBuffer;
var moonVertexNormalBuffer;
var moonVertexTextureCoordBuffer;
var moonVertexIndexBuffer;

function initBuffers()
{
    //箱子
    cubeVertexPositionBuffer = gl.createBuffer();
    gl.bindBuffer(gl.ARRAY_BUFFER, cubeVertexPositionBuffer);
    vertices = [
                // 正面
                -1.0, -1.0,  1.0,
                 1.0, -1.0,  1.0,
                 1.0,  1.0,  1.0,
                -1.0,  1.0,  1.0,

                // 背面
                -1.0, -1.0, -1.0,
                -1.0,  1.0, -1.0,
                 1.0,  1.0, -1.0,
                 1.0, -1.0, -1.0,

                // 顶部
                -1.0,  1.0, -1.0,
                -1.0,  1.0,  1.0,
                 1.0,  1.0,  1.0,
                 1.0,  1.0, -1.0,

                // 底部
                -1.0, -1.0, -1.0,
                 1.0, -1.0, -1.0,
                 1.0, -1.0,  1.0,
                -1.0, -1.0,  1.0,

                // 右侧面
                 1.0, -1.0, -1.0,
                 1.0,  1.0, -1.0,
                 1.0,  1.0,  1.0,
                 1.0, -1.0,  1.0,

                // 左侧面
                -1.0, -1.0, -1.0,
                -1.0, -1.0,  1.0,
                -1.0,  1.0,  1.0,
                -1.0,  1.0, -1.0,
                ];
    gl.bufferData(gl.ARRAY_BUFFER, 
        new Float32Array(vertices), gl.STATIC_DRAW);
    cubeVertexPositionBuffer.itemSize = 3;
    cubeVertexPositionBuffer.numItems = 24;

    cubeVertexNormalBuffer = gl.createBuffer();
    gl.bindBuffer(gl.ARRAY_BUFFER, cubeVertexNormalBuffer);
    var vertexNormals = [
                      // 正面
                        0.0,  0.0,  1.0,
                        0.0,  0.0,  1.0,
                        0.0,  0.0,  1.0,
                        0.0,  0.0,  1.0,

                      // 背面
                        0.0,  0.0, -1.0,
                        0.0,  0.0, -1.0,
                        0.0,  0.0, -1.0,
                        0.0,  0.0, -1.0,

                      // 顶部
                        0.0,  1.0,  0.0,
                        0.0,  1.0,  0.0,
                        0.0,  1.0,  0.0,
                        0.0,  1.0,  0.0,

                      // 底部
                        0.0, -1.0,  0.0,
                        0.0, -1.0,  0.0,
                        0.0, -1.0,  0.0,
                        0.0, -1.0,  0.0,

                      // 右侧面
                        1.0,  0.0,  0.0,
                        1.0,  0.0,  0.0,
                        1.0,  0.0,  0.0,
                        1.0,  0.0,  0.0,

                      // 左侧面
                      -1.0,  0.0,  0.0,
                      -1.0,  0.0,  0.0,
                      -1.0,  0.0,  0.0,
                      -1.0,  0.0,  0.0,
                    ];
    gl.bufferData(gl.ARRAY_BUFFER, 
        new Float32Array(vertexNormals), gl.STATIC_DRAW);
    cubeVertexNormalBuffer.itemSize = 3;
    cubeVertexNormalBuffer.numItems = 24;

    cubeVertexTextureCoordBuffer = gl.createBuffer();

    gl.bindBuffer(gl.ARRAY_BUFFER, cubeVertexTextureCoordBuffer);
    textureCoords = [
                     // 正面
                     0.0, 0.0,
                     1.0, 0.0,
                     1.0, 1.0,
                     0.0, 1.0,

                     // 背面
                     1.0, 0.0,
                     1.0, 1.0,
                     0.0, 1.0,
                     0.0, 0.0,

                     // 顶部
                     0.0, 1.0,
                     0.0, 0.0,
                     1.0, 0.0,
                     1.0, 1.0,

                     // 底部
                     1.0, 1.0,
                     0.0, 1.0,
                     0.0, 0.0,
                     1.0, 0.0,

                     // 右侧面
                     1.0, 0.0,
                     1.0, 1.0,
                     0.0, 1.0,
                     0.0, 0.0,

                     // 左侧面
                     0.0, 0.0,
                     1.0, 0.0,
                     1.0, 1.0,
                     0.0, 1.0,
                    ];
    gl.bufferData(gl.ARRAY_BUFFER, 
        new Float32Array(textureCoords), gl.STATIC_DRAW);
    cubeVertexTextureCoordBuffer.itemSize = 2;
    cubeVertexTextureCoordBuffer.numItems = 24;

    cubeVertexIndexBuffer = gl.createBuffer();
    gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, cubeVertexIndexBuffer);
    var cubeVertexIndices = 
        [
          0, 1, 2,        0, 2, 3,    // 正面
          4, 5, 6,        4, 6, 7,    // 背面
          8, 9, 10,        8, 10, 11,  // 顶部
          12, 13, 14,    12, 14, 15, // 底部
          16, 17, 18,    16, 18, 19, // 右侧面
          20, 21, 22,    20, 22, 23  // 左侧面
        ];
    gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, 
        new Uint16Array(cubeVertexIndices), gl.STATIC_DRAW);
    cubeVertexIndexBuffer.itemSize = 1;
    cubeVertexIndexBuffer.numItems = 36;

    //月亮
    var latitudeBands = 30;
    var longitudeBands = 30;
    var radius = 1;
    var vertexPositionData = [];
    var normalData = [];
    var textureCoordData = [];
    for(var latNumber = 0; latNumber <= latitudeBands; latNumber ++)
    {
        var theta = latNumber * Math.PI / latitudeBands;
        var sinTheta = Math.sin(theta);
        var cosTheta = Math.cos(theta);
        for(var longNumber = 0; longNumber <= longitudeBands; longNumber ++)
        {
            var phi = longNumber * 2 * Math.PI / longitudeBands;
            var sinPhi = Math.sin(phi);
            var cosPhi = Math.cos(phi);
            var x = cosPhi * sinTheta;
            var y = cosTheta;
            var z = sinPhi * sinTheta;
            var u = 1 - (longNumber / longitudeBands);
            var v = 1 - (latNumber / latitudeBands);

            normalData.push(x);
            normalData.push(y);
            normalData.push(z);
            textureCoordData.push(u);
            textureCoordData.push(v);
            vertexPositionData.push(radius * x);
            vertexPositionData.push(radius * y);
            vertexPositionData.push(radius * z);
        }
    }
    var indexData = [];
    for(var latNumber = 0; latNumber < latitudeBands; latNumber ++)
    {
        for(var longNumber = 0; longNumber < longitudeBands; longNumber ++)
        {
            var first = (latNumber * (longitudeBands + 1)) + longNumber;
            var second = first + longitudeBands + 1;
            indexData.push(first);
            indexData.push(second);
            indexData.push(first + 1);
            indexData.push(second);
            indexData.push(second + 1);
            indexData.push(first + 1);
        }
    }
    moonVertexNormalBuffer = gl.createBuffer();
    gl.bindBuffer(gl.ARRAY_BUFFER, moonVertexNormalBuffer);
    gl.bufferData(gl.ARRAY_BUFFER, 
        new Float32Array(normalData), gl.STATIC_DRAW);
    moonVertexNormalBuffer.itemSize = 3;
    moonVertexNormalBuffer.numItems = normalData.length / 3;

    moonVertexTextureCoordBuffer = gl.createBuffer();
    gl.bindBuffer(gl.ARRAY_BUFFER, moonVertexTextureCoordBuffer);
    gl.bufferData(gl.ARRAY_BUFFER, 
        new Float32Array(textureCoordData), gl.STATIC_DRAW);
    moonVertexTextureCoordBuffer.itemSize = 2;
    moonVertexTextureCoordBuffer.numItems = textureCoordData.length / 2;

    moonVertexPositionBuffer = gl.createBuffer();
    gl.bindBuffer(gl.ARRAY_BUFFER, moonVertexPositionBuffer);
    gl.bufferData(gl.ARRAY_BUFFER, 
        new Float32Array(vertexPositionData), gl.STATIC_DRAW);
    moonVertexPositionBuffer.itemSize = 3;
    moonVertexPositionBuffer.numItems = vertexPositionData.length / 3;

    moonVertexIndexBuffer = gl.createBuffer();
    gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, moonVertexIndexBuffer);
    gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, 
        new Uint16Array(indexData), gl.STATIC_DRAW);
    moonVertexIndexBuffer.itemSize = 1;
    moonVertexIndexBuffer.numItems = indexData.length;
}

var moonAngle = 180;
var cubeAngle = 0;

function drawScene()
{
    gl.viewport(0, 0, gl.viewportWidth, gl.viewportHeight);
    gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);

    mat4.perspective(pMatrix, 45, 
        gl.viewportWidth / gl.viewportHeight, 0.1, 100.0);

    var perFragmentLighting = $("#per-fragment").is(":checked");
    if(perFragmentLighting)
    {
        currentProgram = perFragmentProgram;
    }
    else
    {
        currentProgram = perVertexProgram;
    }
    gl.useProgram(currentProgram);

    var lighting = $("#lighting").is(":checked");
    gl.uniform1i(currentProgram.useLightingUniform, lighting);
    if(lighting)
    {
        gl.uniform3f(
            currentProgram.ambientColorUniform,
            parseFloat($("#ambientR").val()),
            parseFloat($("#ambientG").val()),
            parseFloat($("#ambientB").val())
            );
        gl.uniform3f(
            currentProgram.pointLightingLocationUniform,
            parseFloat($("#lightPositionX").val()),
            parseFloat($("#lightPositionY").val()),
            parseFloat($("#lightPositionZ").val())
        );

        gl.uniform3f(
            currentProgram.pointLightingColorUniform,
            parseFloat($("#pointR").val()),
            parseFloat($("#pointG").val()),
            parseFloat($("#pointB").val())
        );

    }

    var textures = $("#textures").is(":checked");
    gl.uniform1i(currentProgram.useTexturesUniform, textures);

    mat4.identity(mvMatrix);
    mat4.translate(mvMatrix, mvMatrix, [0, 0, -5]);
    mat4.rotate(mvMatrix, mvMatrix, degToRad(30), [1, 0, 0]);

    mvPushMatrix();

    mat4.rotate(mvMatrix, mvMatrix, degToRad(moonAngle), [0, 1, 0]);
    mat4.translate(mvMatrix, mvMatrix, [2, 0, 0]);

    gl.activeTexture(gl.TEXTURE0);
    gl.bindTexture(gl.TEXTURE_2D, moonTexture);
    gl.uniform1i(currentProgram.samplerUniform, 0);

    gl.bindBuffer(gl.ARRAY_BUFFER, moonVertexTextureCoordBuffer);
    gl.vertexAttribPointer(currentProgram.textureCoordAttribute, 
        moonVertexTextureCoordBuffer.itemSize, gl.FLOAT, false, 0, 0);

    gl.bindBuffer(gl.ARRAY_BUFFER, moonVertexPositionBuffer);
    gl.vertexAttribPointer(currentProgram.vertexPositionAttribute, 
        moonVertexPositionBuffer.itemSize, gl.FLOAT, false, 0, 0);

    gl.bindBuffer(gl.ARRAY_BUFFER, moonVertexNormalBuffer);
    gl.vertexAttribPointer(currentProgram.vertexNormalAttribute, 
        moonVertexNormalBuffer.itemSize, gl.FLOAT, false, 0, 0);

    gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, moonVertexIndexBuffer);
    setMatrixUniforms();
    gl.drawElements(gl.TRIANGLES, 
        moonVertexIndexBuffer.numItems, gl.UNSIGNED_SHORT, 0);

    mvPopMatrix();

    mvPushMatrix();
    mat4.rotate(mvMatrix, mvMatrix, degToRad(cubeAngle), [0, 1, 0]);
    mat4.translate(mvMatrix, mvMatrix, [1.25, 0, 0]);

    gl.bindBuffer(gl.ARRAY_BUFFER, cubeVertexPositionBuffer);
    gl.vertexAttribPointer(currentProgram.vertexPositionAttribute, 
        cubeVertexPositionBuffer.itemSize, gl.FLOAT, false, 0, 0);

    gl.bindBuffer(gl.ARRAY_BUFFER, cubeVertexNormalBuffer);
    gl.vertexAttribPointer(currentProgram.vertexNormalAttribute, 
        cubeVertexNormalBuffer.itemSize, gl.FLOAT, false, 0, 0);

    gl.bindBuffer(gl.ARRAY_BUFFER, cubeVertexTextureCoordBuffer);
    gl.vertexAttribPointer(currentProgram.textureCoordAttribute, 
        cubeVertexTextureCoordBuffer.itemSize, gl.FLOAT, false, 0, 0);

    gl.activeTexture(gl.TEXTURE0);
    gl.bindTexture(gl.TEXTURE_2D, crateTexture);
    gl.uniform1i(currentProgram.samplerUniform, 0);

    gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, cubeVertexIndexBuffer);
    setMatrixUniforms();
    gl.drawElements(gl.TRIANGLES, 
        cubeVertexIndexBuffer.numItems, gl.UNSIGNED_SHORT, 0);
    mvPopMatrix();
}

function degToRad(degrees)
{
    return degrees * Math.PI / 180;
}
var lastTime = 0;
function animate()
{
    var timeNow = new Date().getTime();
    if(lastTime != 0)
    {
        var elapsed = timeNow - lastTime;

         moonAngle += 0.05 * elapsed;
         cubeAngle += 0.05 * elapsed;
    }
    lastTime = timeNow;
}
</script>