text: // Zfighting.js (c) 2012 matsuda
  // Vertex shader program
  var VSHADER_SOURCE =
    'attribute vec4 a_Position;\n' +
    'attribute vec4 a_Color;\n' +
    'uniform mat4 u_ViewProjMatrix;\n' +
    'varying vec4 v_Color;\n' +
    'void main() {\n' +
    '  gl_Position = u_ViewProjMatrix * 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;
    }
  
    //Set clear color and enable the hidden surface removal function
    gl.clearColor(0, 0, 0, 1);
    gl.enable(gl.DEPTH_TEST);
  
    // Get the storage locations of u_ViewProjMatrix
    var u_ViewProjMatrix = gl.getUniformLocation(gl.program, 'u_ViewProjMatrix');
    if (!u_ViewProjMatrix) {
      console.log('Failed to get the storage locations of u_ViewProjMatrix');
      return;
    }
  
    var viewProjMatrix = new Matrix4();
    // Set the eye point, look-at point, and up vector.
    viewProjMatrix.setPerspective(30, canvas.width/canvas.height, 1, 100);
    viewProjMatrix.lookAt(3.06, 2.5, 10.0, 0, 0, -2, 0, 1, 0);
  
    // Pass the view projection matrix to u_ViewProjMatrix
    gl.uniformMatrix4fv(u_ViewProjMatrix, false, viewProjMatrix.elements);
  
    // Clear color and depth buffer
    gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
  
    // Enable the polygon offset function
    gl.enable(gl.POLYGON_OFFSET_FILL);
    // Draw the triangles
    gl.drawArrays(gl.TRIANGLES, 0, n/2);   // The green triangle
      gl.polygonOffset(1.0, 1.0);          // Set the polygon offset
    gl.drawArrays(gl.TRIANGLES, n/2, n/2); // The yellow triangle
  }
  
  function initVertexBuffers(gl) {
    var verticesColors = new Float32Array([
      // Vertex coordinates and color
       0.0,  2.5,  -5.0,  0.4,  1.0,  0.4, // The green triangle
      -2.5, -2.5,  -5.0,  0.4,  1.0,  0.4,
       2.5, -2.5,  -5.0,  1.0,  0.4,  0.4,
  
       0.0,  3.0,  -5.0,  1.0,  0.4,  0.4, // The yellow triagle
      -3.0, -3.0,  -5.0,  1.0,  1.0,  0.4,
       3.0, -3.0,  -5.0,  1.0,  1.0,  0.4,
    ]);
    var n = 6;
  
    // Create a buffer object
    var vertexColorbuffer = gl.createBuffer();
    if (!vertexColorbuffer) {
      console.log('Failed to create the buffer object');
      return -1;
    }
  
    // Write the vertex coordinates and color to the buffer object
    gl.bindBuffer(gl.ARRAY_BUFFER, vertexColorbuffer);
    gl.bufferData(gl.ARRAY_BUFFER, verticesColors, gl.STATIC_DRAW);
  
    var FSIZE = verticesColors.BYTES_PER_ELEMENT;
    // Assign the buffer object to a_Position and enable the assignment
    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);
    // Assign the buffer object to a_Color and enable the assignment
    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;
  }
  