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# geometryengine.cpp Example File

opengl/cube/geometryengine.cpp

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     #include "geometryengine.h"

     #include <QVector2D>
     #include <QVector3D>

     struct VertexData
     {
         QVector3D position;
         QVector2D texCoord;
     };

     GeometryEngine::GeometryEngine() : vboIds(new GLuint[2])
     {
     }

     GeometryEngine::~GeometryEngine()
     {
         glDeleteBuffers(2, vboIds);
         delete[] vboIds;
     }

     void GeometryEngine::init()
     {
         initializeGLFunctions();

         // Generate 2 VBOs
         glGenBuffers(2, vboIds);

         // Initializes cube geometry and transfers it to VBOs
         initCubeGeometry();
     }

     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(-1.0, -1.0,  1.0), QVector2D(0.0, 0.0)},  // v0
             {QVector3D( 1.0, -1.0,  1.0), QVector2D(0.33, 0.0)}, // v1
             {QVector3D(-1.0,  1.0,  1.0), QVector2D(0.0, 0.5)},  // v2
             {QVector3D( 1.0,  1.0,  1.0), QVector2D(0.33, 0.5)}, // v3

             // Vertex data for face 1
             {QVector3D( 1.0, -1.0,  1.0), QVector2D( 0.0, 0.5)}, // v4
             {QVector3D( 1.0, -1.0, -1.0), QVector2D(0.33, 0.5)}, // v5
             {QVector3D( 1.0,  1.0,  1.0), QVector2D(0.0, 1.0)},  // v6
             {QVector3D( 1.0,  1.0, -1.0), QVector2D(0.33, 1.0)}, // v7

             // Vertex data for face 2
             {QVector3D( 1.0, -1.0, -1.0), QVector2D(0.66, 0.5)}, // v8
             {QVector3D(-1.0, -1.0, -1.0), QVector2D(1.0, 0.5)},  // v9
             {QVector3D( 1.0,  1.0, -1.0), QVector2D(0.66, 1.0)}, // v10
             {QVector3D(-1.0,  1.0, -1.0), QVector2D(1.0, 1.0)},  // v11

             // Vertex data for face 3
             {QVector3D(-1.0, -1.0, -1.0), QVector2D(0.66, 0.0)}, // v12
             {QVector3D(-1.0, -1.0,  1.0), QVector2D(1.0, 0.0)},  // v13
             {QVector3D(-1.0,  1.0, -1.0), QVector2D(0.66, 0.5)}, // v14
             {QVector3D(-1.0,  1.0,  1.0), QVector2D(1.0, 0.5)},  // v15

             // Vertex data for face 4
             {QVector3D(-1.0, -1.0, -1.0), QVector2D(0.33, 0.0)}, // v16
             {QVector3D( 1.0, -1.0, -1.0), QVector2D(0.66, 0.0)}, // v17
             {QVector3D(-1.0, -1.0,  1.0), QVector2D(0.33, 0.5)}, // v18
             {QVector3D( 1.0, -1.0,  1.0), QVector2D(0.66, 0.5)}, // v19

             // Vertex data for face 5
             {QVector3D(-1.0,  1.0,  1.0), QVector2D(0.33, 0.5)}, // v20
             {QVector3D( 1.0,  1.0,  1.0), QVector2D(0.66, 0.5)}, // v21
             {QVector3D(-1.0,  1.0, -1.0), QVector2D(0.33, 1.0)}, // v22
             {QVector3D( 1.0,  1.0, -1.0), QVector2D(0.66, 1.0)}  // 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)
         };

         // Transfer vertex data to VBO 0
         glBindBuffer(GL_ARRAY_BUFFER, vboIds[0]);
         glBufferData(GL_ARRAY_BUFFER, 24 * sizeof(VertexData), vertices, GL_STATIC_DRAW);

         // Transfer index data to VBO 1
         glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, vboIds[1]);
         glBufferData(GL_ELEMENT_ARRAY_BUFFER, 34 * sizeof(GLushort), indices, GL_STATIC_DRAW);
     }

     void GeometryEngine::drawCubeGeometry(QGLShaderProgram *program)
     {
         // Tell OpenGL which VBOs to use
         glBindBuffer(GL_ARRAY_BUFFER, vboIds[0]);
         glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, vboIds[1]);

         // Offset for position
         int offset = 0;

         // Tell OpenGL programmable pipeline how to locate vertex position data
         int vertexLocation = program->attributeLocation("a_position");
         program->enableAttributeArray(vertexLocation);
         glVertexAttribPointer(vertexLocation, 3, GL_FLOAT, GL_FALSE, sizeof(VertexData), (const void *)offset);

         // 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);
         glVertexAttribPointer(texcoordLocation, 2, GL_FLOAT, GL_FALSE, sizeof(VertexData), (const void *)offset);

         // Draw cube geometry using indices from VBO 1
         glDrawElements(GL_TRIANGLE_STRIP, 34, GL_UNSIGNED_SHORT, 0);
     }