glBindFramebuffer(GL_FRAMEBUFFER, fbo);
  glFramebufferTexture2D (GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, current, 0);

  glDrawBuffer(GL_COLOR_ATTACHMENT0);
  glReadBuffer(GL_COLOR_ATTACHMENT0);

  if (glCheckFramebufferStatus(GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE) {
    printf ("invalid framebuffer status");
    exit(1);
  }

  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

  glDisable(GL_DEPTH_TEST);
  glEnable(GL_BLEND);
  glBlendFunc(GL_ONE, GL_ONE);

  glPushMatrix();

  glutSolidCube(1);
  glPopMatrix();