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  #include "geometryengine.h"
  
  #include <QVector2D>
  #include <QVector3D>
  
  struct VertexData
  {
      QVector3D position;
      QVector2D texCoord;
  };
  
  //! [0]
  GeometryEngine::GeometryEngine()
      : indexBuf(QOpenGLBuffer::IndexBuffer)
  {
      initializeOpenGLFunctions();
  
      // Generate 2 VBOs
      arrayBuf.create();
      indexBuf.create();
  
      // Initializes cube geometry and transfers it to VBOs
      initCubeGeometry();
  }
  
  GeometryEngine::~GeometryEngine()
  {
      arrayBuf.destroy();
      indexBuf.destroy();
  }
  //! [0]
  
  void GeometryEngine::initCubeGeometry()
  {
      // For cube we would need only 8 vertices but we have to
      // duplicate vertex for each face because texture coordinate
      // is different.
      VertexData vertices[] = {
          // Vertex data for face 0
          {QVector3D(-1.0f, -1.0f,  1.0f), QVector2D(0.0f, 0.0f)},  // v0
          {QVector3D( 1.0f, -1.0f,  1.0f), QVector2D(0.33f, 0.0f)}, // v1
          {QVector3D(-1.0f,  1.0f,  1.0f), QVector2D(0.0f, 0.5f)},  // v2
          {QVector3D( 1.0f,  1.0f,  1.0f), QVector2D(0.33f, 0.5f)}, // v3
  
          // Vertex data for face 1
          {QVector3D( 1.0f, -1.0f,  1.0f), QVector2D( 0.0f, 0.5f)}, // v4
          {QVector3D( 1.0f, -1.0f, -1.0f), QVector2D(0.33f, 0.5f)}, // v5
          {QVector3D( 1.0f,  1.0f,  1.0f), QVector2D(0.0f, 1.0f)},  // v6
          {QVector3D( 1.0f,  1.0f, -1.0f), QVector2D(0.33f, 1.0f)}, // v7
  
          // Vertex data for face 2
          {QVector3D( 1.0f, -1.0f, -1.0f), QVector2D(0.66f, 0.5f)}, // v8
          {QVector3D(-1.0f, -1.0f, -1.0f), QVector2D(1.0f, 0.5f)},  // v9
          {QVector3D( 1.0f,  1.0f, -1.0f), QVector2D(0.66f, 1.0f)}, // v10
          {QVector3D(-1.0f,  1.0f, -1.0f), QVector2D(1.0f, 1.0f)},  // v11
  
          // Vertex data for face 3
          {QVector3D(-1.0f, -1.0f, -1.0f), QVector2D(0.66f, 0.0f)}, // v12
          {QVector3D(-1.0f, -1.0f,  1.0f), QVector2D(1.0f, 0.0f)},  // v13
          {QVector3D(-1.0f,  1.0f, -1.0f), QVector2D(0.66f, 0.5f)}, // v14
          {QVector3D(-1.0f,  1.0f,  1.0f), QVector2D(1.0f, 0.5f)},  // v15
  
          // Vertex data for face 4
          {QVector3D(-1.0f, -1.0f, -1.0f), QVector2D(0.33f, 0.0f)}, // v16
          {QVector3D( 1.0f, -1.0f, -1.0f), QVector2D(0.66f, 0.0f)}, // v17
          {QVector3D(-1.0f, -1.0f,  1.0f), QVector2D(0.33f, 0.5f)}, // v18
          {QVector3D( 1.0f, -1.0f,  1.0f), QVector2D(0.66f, 0.5f)}, // v19
  
          // Vertex data for face 5
          {QVector3D(-1.0f,  1.0f,  1.0f), QVector2D(0.33f, 0.5f)}, // v20
          {QVector3D( 1.0f,  1.0f,  1.0f), QVector2D(0.66f, 0.5f)}, // v21
          {QVector3D(-1.0f,  1.0f, -1.0f), QVector2D(0.33f, 1.0f)}, // v22
          {QVector3D( 1.0f,  1.0f, -1.0f), QVector2D(0.66f, 1.0f)}  // v23
      };
  
      // Indices for drawing cube faces using triangle strips.
      // Triangle strips can be connected by duplicating indices
      // between the strips. If connecting strips have opposite
      // vertex order then last index of the first strip and first
      // index of the second strip needs to be duplicated. If
      // connecting strips have same vertex order then only last
      // index of the first strip needs to be duplicated.
      GLushort indices[] = {
           0,  1,  2,  3,  3,     // Face 0 - triangle strip ( v0,  v1,  v2,  v3)
           4,  4,  5,  6,  7,  7, // Face 1 - triangle strip ( v4,  v5,  v6,  v7)
           8,  8,  9, 10, 11, 11, // Face 2 - triangle strip ( v8,  v9, v10, v11)
          12, 12, 13, 14, 15, 15, // Face 3 - triangle strip (v12, v13, v14, v15)
          16, 16, 17, 18, 19, 19, // Face 4 - triangle strip (v16, v17, v18, v19)
          20, 20, 21, 22, 23      // Face 5 - triangle strip (v20, v21, v22, v23)
      };
  
  //! [1]
      // Transfer vertex data to VBO 0
      arrayBuf.bind();
      arrayBuf.allocate(vertices, 24 * sizeof(VertexData));
  
      // Transfer index data to VBO 1
      indexBuf.bind();
      indexBuf.allocate(indices, 34 * sizeof(GLushort));
  //! [1]
  }
  
  //! [2]
  void GeometryEngine::drawCubeGeometry(QOpenGLShaderProgram *program)
  {
      // Tell OpenGL which VBOs to use
      arrayBuf.bind();
      indexBuf.bind();
  
      // Offset for position
      quintptr offset = 0;
  
      // Tell OpenGL programmable pipeline how to locate vertex position data
      int vertexLocation = program->attributeLocation("a_position");
      program->enableAttributeArray(vertexLocation);
      program->setAttributeBuffer(vertexLocation, GL_FLOAT, offset, 3, sizeof(VertexData));
  
      // Offset for texture coordinate
      offset += sizeof(QVector3D);
  
      // Tell OpenGL programmable pipeline how to locate vertex texture coordinate data
      int texcoordLocation = program->attributeLocation("a_texcoord");
      program->enableAttributeArray(texcoordLocation);
      program->setAttributeBuffer(texcoordLocation, GL_FLOAT, offset, 2, sizeof(VertexData));
  
      // Draw cube geometry using indices from VBO 1
      glDrawElements(GL_TRIANGLE_STRIP, 34, GL_UNSIGNED_SHORT, 0);
  }
  //! [2]
  