var VSHADER_SOURCE =

  'attribute vec4 a_Position;\n' +

  'attribute vec4 a_Color;\n' +

  'uniform mat4 u_MvpMatrix;\n' +

  'varying vec4 v_Color;\n' +

  'void main() {\n' +

  '  gl_Position = u_MvpMatrix * a_Position;\n' +

  '  v_Color = a_Color;\n' +

  '}\n';

// Fragment shader program

var FSHADER_SOURCE =

  '#ifdef GL_ES\n' +

  'precision mediump float;\n' +

  '#endif\n' +

  'varying vec4 v_Color;\n' +

  'void main() {\n' +

  '  gl_FragColor = v_Color;\n' +

  '}\n';

function main() {

  // Retrieve <canvas> element

  var canvas = document.getElementById('webgl');

  // Get the rendering context for WebGL

  var gl = getWebGLContext(canvas);

  if (!gl) {

    console.log('Failed to get the rendering context for WebGL');

    return;

  }

  // Initialize shaders

  if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {

    console.log('Failed to intialize shaders.');

    return;

  }

  // Set the vertex coordinates and color (the blue triangle is in the front)

  var n = initVertexBuffers(gl);

  if (n < ) {

    console.log('Failed to set the vertex information');

    return;

  }

  // Specify the color for clearing <canvas>

  gl.clearColor(, , , );
　　②

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

  //开启深度检测（基于Z轴，前面的自动遮住后面的，这样不用在意图形先后绘制决定层级的问题）

  gl.enable(gl.DEPTH_TEST);

  //开启多边形偏移解决不同物体Z轴值一样的问题（基于所在面与视区的角度设置z轴偏移量）

  gl.enable(gl.POLYGON_OFFSET_FILL);

  // Get the storage location of u_MvpMatrix

  var u_MvpMatrix = gl.getUniformLocation(gl.program, 'u_MvpMatrix');

  if (!u_MvpMatrix) {

    console.log('Failed to get the storage location of u_MvpMatrix');

    return;

  }

  var modelMatrix = new Matrix4(); // Model matrix

  var viewMatrix = new Matrix4();  // View matrix

  var projMatrix = new Matrix4();  // Projection matrix

  var mvpMatrix = new Matrix4();   // Model view projection matrix

　　①
//模型矩阵，用于计算模型的运动（平移、旋转、缩放）

  modelMatrix.setTranslate(0.75, , );

  //视图矩阵（人看物体的位置）

  viewMatrix.setLookAt(, , , , , -, , , );

  //透视投影矩阵（相对应的还有正视投影，但是3D下一般用透视投影，看起来更符合人类视角）

  projMatrix.setPerspective(, canvas.width/canvas.height, , );

  //上述三种矩阵相乘

  mvpMatrix.set(projMatrix).multiply(viewMatrix).multiply(modelMatrix);

  gl.uniformMatrix4fv(u_MvpMatrix, false, mvpMatrix.elements);

  gl.clear(gl.COLOR_BUFFER_BIT);   // Clear <canvas>

  gl.drawArrays(gl.TRIANGLES, , n);   // Draw the triangles

　　③
//绘制之前设置多边形偏移参数值

  gl.polygonOffset(1.0, 1.0);

  //几何体位置移动再画一次（放到右边）

  modelMatrix.setTranslate(-0.75, , );

  mvpMatrix.set(projMatrix).multiply(viewMatrix).multiply(modelMatrix);

  gl.uniformMatrix4fv(u_MvpMatrix, false, mvpMatrix.elements);

  gl.drawArrays(gl.TRIANGLES, , n);   // Draw the triangles

}

function initVertexBuffers(gl) {
　　④
var verticesColors = new Float32Array([

    // Vertex coordinates and color

     0.0,  1.0,  -4.0,  0.4,  1.0,  0.4, // The back green one

    -0.5, -1.0,  -4.0,  0.4,  1.0,  0.4,

     0.5, -1.0,  -4.0,  1.0,  0.4,  0.4, 

     0.0,  1.0,  -2.0,  1.0,  1.0,  0.4, // The middle yellow one

    -0.5, -1.0,  -2.0,  1.0,  1.0,  0.4,

     0.5, -1.0,  -2.0,  1.0,  0.4,  0.4, 

     0.0,  1.0,   0.0,  0.4,  0.4,  1.0,  // The front blue one

    -0.5, -1.0,   0.0,  0.4,  0.4,  1.0,

     0.5, -1.0,   0.0,  1.0,  0.4,  0.4,

  ]);

  var n = ;

  // Create a buffer object

  var vertexColorBuffer = gl.createBuffer();

  if (!vertexColorBuffer) {

    console.log('Failed to create the buffer object');

    return -;

  }

  // Write the vertex information and enable it

  gl.bindBuffer(gl.ARRAY_BUFFER, vertexColorBuffer);

  gl.bufferData(gl.ARRAY_BUFFER, verticesColors, gl.STATIC_DRAW);

  var FSIZE = verticesColors.BYTES_PER_ELEMENT;

  var a_Position = gl.getAttribLocation(gl.program, 'a_Position');

  if(a_Position < ) {

    console.log('Failed to get the storage location of a_Position');

    return -;

  }

  gl.vertexAttribPointer(a_Position, , gl.FLOAT, false, FSIZE * , );

  gl.enableVertexAttribArray(a_Position);

  var a_Color = gl.getAttribLocation(gl.program, 'a_Color');

  if(a_Color < ) {

    console.log('Failed to get the storage location of a_Color');

    return -;

  }

  gl.vertexAttribPointer(a_Color, , gl.FLOAT, false, FSIZE * , FSIZE * );

  gl.enableVertexAttribArray(a_Color);

  return n;

}