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 < 0) {
    console.log(‘Failed to set the vertex information‘);
    return;
  }

  // Specify the color for clearing <canvas>
  gl.clearColor(0, 0, 0, 1);
　　②
  gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
  //開啟隱藏面消除（基於Z軸，前面的自動遮住後面的，這樣不用在意圖形先後繪制決定層級的問題）
  gl.enable(gl.DEPTH_TEST);
  //開啟多邊形偏移解決不同物體Z軸值一樣的問題（基於所在面與視區的角度設置z軸偏移量）
  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
  //模型矩陣，用於計算模型的運動（平移、旋轉、縮放）
  modelMatrix.setTranslate(0.75, 0, 0);
  //視圖矩陣（人開物體的位置）
  viewMatrix.setLookAt(0, 0, 5, 0, 0, -100, 0, 1, 0);
  //透視投影矩陣（相對應的還有正視投影，但是3D下一般用透視投影，看起來更符合人類視角）
  projMatrix.setPerspective(30, canvas.width/canvas.height, 1, 100);
  //上述三種矩陣相乘
  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, 0, n);   // Draw the triangles

　　③
  //繪制之前設置多邊形偏移參數值
  gl.polygonOffset(1.0, 1.0);

  //幾何體位置移動再畫一次（放到右邊）
  modelMatrix.setTranslate(-0.75, 0, 0);
  mvpMatrix.set(projMatrix).multiply(viewMatrix).multiply(modelMatrix);
  gl.uniformMatrix4fv(u_MvpMatrix, false, mvpMatrix.elements);
  gl.drawArrays(gl.TRIANGLES, 0, 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 = 9;

  // Create a buffer object
  var vertexColorBuffer = gl.createBuffer();  
  if (!vertexColorBuffer) {
    console.log(‘Failed to create the buffer object‘);
    return -1;
  }

  // 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 < 0) {
    console.log(‘Failed to get the storage location of a_Position‘);
    return -1;
  }
  gl.vertexAttribPointer(a_Position, 3, gl.FLOAT, false, FSIZE * 6, 0);
  gl.enableVertexAttribArray(a_Position);

  var a_Color = gl.getAttribLocation(gl.program, ‘a_Color‘);
  if(a_Color < 0) {
    console.log(‘Failed to get the storage location of a_Color‘);
    return -1;
  }
  gl.vertexAttribPointer(a_Color, 3, gl.FLOAT, false, FSIZE * 6, FSIZE * 3);
  gl.enableVertexAttribArray(a_Color);

  return n;
}

WebGL編程指南案例解析之3D視圖視區問題