libQGLViewer how to draw withouth clearing buffer

Question

I'm using libQGLViewer in my project to draw the trajectory of a robot. To this end, I started from the code of the simpleViewer provided in the examples.

To do my stuff, I put these lines of code in the draw function:

void Viewer::draw()
{

  glPushMatrix();
  for (auto pose : poses)
  {
    std::vector<float> m;
    poseToVector(pose, m);

    glPushMatrix();
    multMatrix(m);
    drawReferenceSystem();
    glPopMatrix();
  }
  glPopMatrix();
}

where the poseToVector , multMatrix and drawReferenceSystem functions are OpenGL wrappers to implement the desired functionalities.

Now, my problem is that the poses vector is filled at each iteration of my code with a new pose. Thus, what I'm doing now is that at iteration 1 I'm drawing pose 1, at iteration 2 I'm drawing pose 1 and 2, at iteration 3 I'm drawing pose 1,2 and 3, and so on.

For this reason, I was wondering if there was a more efficient way to perform this operation. However, if I just get the back of the vector I will see at each time instant only the last pose. I guess that this is because every time the draw function is called the frame buffer is cleared.

Therefore, my question is: is it possible to avoid clearing the buffer or I'm just saying bullshit?

Answer 1

preDraw clears the screen and resets matrices. You could subclass QGLViewer and override it to drop the clearing behavior.

The default implementation is:

void QGLViewer::preDraw() {
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); // this line should be removed here or in a sublass.

  // GL_PROJECTION matrix
  camera()->loadProjectionMatrix();
  // GL_MODELVIEW matrix
  camera()->loadModelViewMatrix();

  Q_EMIT drawNeeded();
}