FlightGear and OpenGL Core Profile

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Objective

OpenGL ES This is a link to a Wikipedia article

Some FG users are very interested in seeing how FlightGear runs on low end hardware - because they may have a collection of it :)

Probably to do it initially, gles1 would have to be used - I think gles2 has no fixed function - and simgear/flightgear would have to be patched to use only OSG calls (if possible)

I know OSG can have gles{1,2} support without windowing compiled in

Background

1rightarrow.png See Howto:Optimizing FlightGear for mobile devices for the main article about this subject.

being able to run fgfs on such, comparatively low-powered, system is actually a good thing for fgfs as a whole - it can help us understand bottlenecks that are hardly visible on typical gaming rigs, but that may still show up over time (think leaking listeners/memory) - this sort of thing can also be considered the prerequisite for people wanting to target/build/run fgfs on other embedded hardware, such as thin clients with integrated GPUs or even mobile phones (think Android)

In other words, if the right people were to team up to specifically such hardware, this could also mean significant performance improvements for people on powerful gaming rigs.

Approach

For starters, we can try the FlightGear Headless option to build a fgfs version without any graphics at all.

The next step will be identifying and excluding problematic sources (those containing legacy/raw OpenGL code, e.g. using glBegin() or glEnable() respectively):

https://sourceforge.net/p/flightgear/flightgear/ci/next/tree/src/Cockpit/render_area_2d.cxx

void RenderArea2D::RenderQuad( const SGVec2f *p) {
    glBegin(GL_QUADS);
        glNormal3f(0.0f, 0.0f, 0.0f);
        glVertex2fv( p[0].data() );
        glVertex2fv( p[1].data() );
        glVertex2fv( p[2].data() );
        glVertex2fv( p[3].data() );
    glEnd();
}

Example: disabling PUI

Knowing that PUI contains legacy OpenGL code and knowing it's scheduled to be removed anyway because it isn't compatible with modern OpenGL, we will disable it completely without reviewing/porting individual PUI files. This means opening $FG_SRC/CMakeLists.txt to add a new option to disable PUI.

https://sourceforge.net/p/flightgear/flightgear/ci/next/tree/CMakeLists.txt

option(SYSTEM_CPPUNIT    "Set to ON to build Flightgear with the system's CppUnit library")
option(DISABLE_PUI    "Set to ON to build Flightgear without PUI support")

We will be adding #ifdef macros to the sources in question, and update CMakeLists.txt accordingly.

Ideally, in conjunction with a feature-specific build option to disable the corresponding feature (think PUI or the HUD).

Thus, after editing CMakeLists.txt, we need to open fg_init.cxx to prevent initalization of PUI: https://sourceforge.net/p/flightgear/flightgear/ci/next/tree/src/Main/fg_init.cxx#l1043

    ////////////////////////////////////////////////////////////////////
    // Create and register the XML GUI.
    ////////////////////////////////////////////////////////////////////
#if 0
    globals->add_subsystem("gui", new NewGUI, SGSubsystemMgr::INIT);
#endif


So, after editing fg_init.cxx to prevent the PUI GUI from getting initialized by FlightGear, we will need to find remaining hard-coded references to it, to fix those up and deal with PUI not being available.

1rightarrow.png See Developing using CMake for the main article about this subject.

We should add separate build options to explicitly disable these features individually, e.g.:

  • -DDISABLE_PUI
  • -DDISABLE_2DPANELS

Successfully disabling a feature means primarily:

  • adding a corresponding new build option to the top-level CMakeLists.txt (e.g. DISABLE_PUI)
  • opening fg_init.cxx and navigating to the lines where the feature/subsystem is initialized
  • wrapping the corresponding code in between #ifdef...#endif blocks
  • locating any remaining references to the subsystem in question and repeating the last step there

Affected Features / Sources

Feature Directory Notes Status
PUI $FG_SRC/GUI can be replaced via Phi Pending Pending
2D Panels $FG_SRC/Cockpit legacy 2D panels (could use Canvas) Pending Pending
Canvas Path (Shiva) $SG_SRC/canvas needs some thinking, probably native OpenVG on RPi [1] [2] or an OpenGL based implementation like nanovg [3] Pending Pending
Effects $FG_ROOT/Effects not important for the time being, will include Shader and probably involve a custom Compositor pipeline Pending Pending

Porting

OpenGL ES This is a link to a Wikipedia article

Vertex Buffer Objects

Canvas.Path (OpenVG)

Forum topics

See also

References

References