Quote Originally Posted by wysota View Post
Make sure you are setting up the buffers correctly. I would also advise to use Qt classes for VBO manipulation.
Thanks for your guidance. I solved the problem. The correct code for what I aimed for is:
Qt Code: 
  1. if(!initialized)
  2.     {
  3.         GLuint vertexId = shaderProgram->attributeLocation("vertex");
  4.         GLuint colorId  = shaderProgram->attributeLocation("color");
  5.  
  6.         glGenBuffers(2, VBO_ID);
  7.  
  8.         glGenVertexArrays(1, &VAO_ID);
  9.         glBindVertexArray(VAO_ID);
  10.  
  11.         glBindBuffer(GL_ARRAY_BUFFER, VBO_ID[0]);
  12.         glBufferData(GL_ARRAY_BUFFER, sizeof(QVector3D)*verticesSize, vertices.constData(), GL_STATIC_DRAW);
  13.         glVertexAttribPointer(vertexId, 3, GL_FLOAT, GL_FALSE, 0, 0);// param - 1 ->0
  14.         glEnableVertexAttribArray(vertexId);
  15.  
  16.         glBindBuffer(GL_ARRAY_BUFFER, VBO_ID[1]);
  17.         glBufferData(GL_ARRAY_BUFFER, sizeof(float)*verticesSize, colors, GL_STATIC_DRAW);
  18.         glVertexAttribPointer(colorId, 1, GL_FLOAT, GL_FALSE, 0, 0);
  19.         glEnableVertexAttribArray(colorId);
  20.  
  21.         glBindBuffer(GL_ARRAY_BUFFER, 0);
  22.  
  23.         vertices.clear();
  24.  
  25.         free(colors);
  26.  
  27.         initialized = true;
  28.     }
  29.  
  30.     glBindVertexArray(VAO_ID);
  31.  
  32.     int vertexLocation = shaderProgram->attributeLocation("vertex");
  33.     int colorLocation = shaderProgram->attributeLocation("color");
  34.  
  35.     shaderProgram->enableAttributeArray(vertexLocation);
  36.     shaderProgram->enableAttributeArray(colorLocation);
  37.  
  38.     glEnableClientState(GL_VERTEX_ARRAY);
  39.  
  40.     glEnable    (GL_BLEND);
  41.     glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
  42.     glDrawArrays(GL_TRIANGLES, 0, 6*numVerts);
  43.  
  44.     glFlush();
  45.  
  46.     glBindVertexArray(0);
  47.  
  48.     glDisableVertexAttribArray(0);
  49.     glDisableVertexAttribArray(1);
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