1.1.3 WebGL vs. Three.js

为了比较说明Three.js能大大简化WebGL的开发，我们使用最简单的例子进行比较：渲染黑色背景下的白色正方形和三角形。效果如图：

Three.js需要30行左右的代码：

例1.1.1

var renderer = new THREE.WebGLRenderer({
    canvas: document.getElementById('mainCanvas')
});
renderer.setClearColor(0x000000); // black

var scene = new THREE.Scene();

var camera = new THREE.PerspectiveCamera(45, 4 / 3, 1, 1000);
camera.position.set(0, 0, 5);
camera.lookAt(new THREE.Vector3(0, 0, 0));
scene.add(camera);

var material = new THREE.MeshBasicMaterial({
        color: 0xffffff // white
});
// plane
var planeGeo = new THREE.PlaneGeometry(1.5, 1.5);
var plane = new THREE.Mesh(planeGeo, material);
plane.position.x = 1;
scene.add(plane);

// triangle
var triGeo = new THREE.Geometry();
triGeo.vertices = [new THREE.Vector3(0, -0.8, 0),
        new THREE.Vector3(-2, -0.8, 0), new THREE.Vector3(-1, 0.8, 0)];
triGeo.faces.push(new THREE.Face3(0, 2, 1));
var triangle = new THREE.Mesh(triGeo, material);
scene.add(triangle);

renderer.render(scene, camera);

如果接触过图形学知识，这里的代码应该很容易理解，如果不懂也没关系，接下来几章会进行详细说明。所以在此就不花费篇章解释这几行代码了。

以下摘录实现相同功能的WebGL代码，来自博客http://learningwebgl.com/blog/?p=28。

var gl;
function initGL(canvas) {
    try {
        gl = canvas.getContext("experimental-webgl");
        gl.viewportWidth = canvas.width;
        gl.viewportHeight = canvas.height;
    } catch (e) {
    }
    if (!gl) {
        alert("Could not initialise WebGL, sorry :-(");
    }
}

function getShader(gl, id) {
    var shaderScript = document.getElementById(id);
    if (!shaderScript) {
        return null;
    }

    var str = "";
    var k = shaderScript.firstChild;
    while (k) {
        if (k.nodeType == 3) {
            str += k.textContent;
        }
        k = k.nextSibling;
    }

    var shader;
    if (shaderScript.type == "x-shader/x-fragment") {
        shader = gl.createShader(gl.FRAGMENT_SHADER);
    } else if (shaderScript.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;
}

var shaderProgram;

function initShaders() {
    var fragmentShader = getShader(gl, "shader-fs");
    var vertexShader = getShader(gl, "shader-vs");

    shaderProgram = gl.createProgram();
    gl.attachShader(shaderProgram, vertexShader);
    gl.attachShader(shaderProgram, fragmentShader);
    gl.linkProgram(shaderProgram);

    if (!gl.getProgramParameter(shaderProgram, gl.LINK_STATUS)) {
        alert("Could not initialise shaders");
    }

    gl.useProgram(shaderProgram);

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

    shaderProgram.pMatrixUniform = gl.getUniformLocation(shaderProgram, "uPMatrix");
    shaderProgram.mvMatrixUniform = gl.getUniformLocation(shaderProgram, "uMVMatrix");
}

var mvMatrix = mat4.create();
var pMatrix = mat4.create();

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

var triangleVertexPositionBuffer;
var squareVertexPositionBuffer;

function initBuffers() {
    triangleVertexPositionBuffer = gl.createBuffer();
    gl.bindBuffer(gl.ARRAY_BUFFER, triangleVertexPositionBuffer);
    var vertices = [
         0.0,  1.0,  0.0,
        -1.0, -1.0,  0.0,
         1.0, -1.0,  0.0
    ];
    gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(vertices), gl.STATIC_DRAW);
    triangleVertexPositionBuffer.itemSize = 3;
    triangleVertexPositionBuffer.numItems = 3;

    squareVertexPositionBuffer = gl.createBuffer();
    gl.bindBuffer(gl.ARRAY_BUFFER, squareVertexPositionBuffer);
    vertices = [
         1.0,  1.0,  0.0,
        -1.0,  1.0,  0.0,
         1.0, -1.0,  0.0,
        -1.0, -1.0,  0.0
    ];
    gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(vertices), gl.STATIC_DRAW);
    squareVertexPositionBuffer.itemSize = 3;
    squareVertexPositionBuffer.numItems = 4;
}

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

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

    mat4.identity(mvMatrix);

    mat4.translate(mvMatrix, [-1.5, 0.0, -7.0]);
    gl.bindBuffer(gl.ARRAY_BUFFER, triangleVertexPositionBuffer);
    gl.vertexAttribPointer(shaderProgram.vertexPositionAttribute, triangleVertexPositionBuffer.itemSize, gl.FLOAT, false, 0, 0);
    setMatrixUniforms();
    gl.drawArrays(gl.TRIANGLES, 0, triangleVertexPositionBuffer.numItems);

    mat4.translate(mvMatrix, [3.0, 0.0, 0.0]);
    gl.bindBuffer(gl.ARRAY_BUFFER, squareVertexPositionBuffer);
    gl.vertexAttribPointer(shaderProgram.vertexPositionAttribute, squareVertexPositionBuffer.itemSize, gl.FLOAT, false, 0, 0);
    setMatrixUniforms();
    gl.drawArrays(gl.TRIANGLE_STRIP, 0, squareVertexPositionBuffer.numItems);
}

function webGLStart() {
    var canvas = document.getElementById("lesson01-canvas");
    initGL(canvas);
    initShaders();
    initBuffers();

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

    drawScene();
}

从上面的代码我们不难发现，使用原生WebGL接口实现同样功能需要5倍多的代码量，而且很多代码对于没有图形学基础的程序员是很难看懂的。由这个例子我们可以看出，使用Three.js开发要比WebGL更快更高效。尤其对图形学知识不熟悉的程序员而言，使用Three.js能够降低学习成本，提高三维图形程序开发的效率。