An event sent by an mrpt::opengl::COpenGLViewport just after clearing the viewport and setting the GL_PROJECTION matrix, and before calling the scene OpenGL drawing primitives.
While handling this event you can call OpenGL glBegin(),glEnd(),gl* functions or those in mrpt::opengl::gl_utils to draw stuff in the back of the normal objects contained in the COpenGLScene.
After processing this event, COpenGLViewport will change the OpenGL matrix mode into "GL_MODELVIEW" and load an identity matrix to continue rendering the scene objects as usual. Any change done to the GL_PROJECTION will have effects, so do a glPushMatrix()/glPopMatrix() if that is not your intention.
IMPORTANTE NOTICE: Event handlers in your observer class will most likely be invoked from an internal GUI thread of MRPT, so all your code in the handler must be thread safe.
Definition at line 481 of file COpenGLViewport.h.