Add Metal support to MinimalScene and Qt Application (#323)

* [Metal] Add Metal support to MinimalScene and Qt Application

Allow MinimalScene to use either OpenGL or Metal render systems

Add classes to abstract out render hardware interfaces:
- IgnCameraTextureRhi
- RenderThreadRhi
- TextureNodeRhi

Add implementations for OpenGL and Metal in:
- MinimalSceneRhiOpenGL
- MinimalSceneRhiMetal
- Modify the existing MinimalScene class to forward rendering calls to the (virtual) render hardware interface functions.
- Modify plugin CMakeLists.txt to conditionally compile Metal support if the platform is macOS
- Update Application.cc to support either OpenGL or Metal

Refactor MinimalScene to use GraphicAPI
- Use the GraphicsAPI enum to set the choice of graphics interface (OpenGL or Metal).
- Change the MinimalScene plugin element to <graphics_api>...</graphics_api>

Add documentation and confirm to code style
- Document the OpenGL and Metal render interface classes
- Revert default engine to ogre

Set graphics API to Metal for macOS, otherwise use OpenGL
- Update Application.cc to only use Metal for macOS
- Fix issues with code check

Signed-off-by: Rhys Mainwaring <rhys.mainwaring@me.com>
Co-authored-by: Ian Chen <ichen@osrfoundation.org>
Co-authored-by: Alejandro Hernández Cordero <ahcorde@gmail.com>

src/Application.cc

@@ -89,6 +89,29 @@ Application::Application(int &_argc, char **_argv, const WindowType _type)
 {
   igndbg << "Initializing application." << std::endl;
 
+#if __APPLE__
+  // Use the Metal graphics API on macOS.
+  igndbg << "Qt using Metal graphics interface" << std::endl;
+  QQuickWindow::setSceneGraphBackend(QSGRendererInterface::MetalRhi);
+
+  // TODO(srmainwaring): implement facility for overriding the default
+  //    graphics API in macOS, in which case there are restrictions on
+  //    the version of OpenGL used.
+  //
+  // macOS only supports OpenGL <= 4.1.
+  // This must be called before the main window is shown.
+  // QSurfaceFormat format(QSurfaceFormat::DeprecatedFunctions);
+  // format.setDepthBufferSize(24);
+  // format.setStencilBufferSize(8);
+  // format.setVersion(4, 1);
+  // format.setProfile(QSurfaceFormat::CoreProfile);
+  // format.setRenderableType(QSurfaceFormat::OpenGL);
+  // QSurfaceFormat::setDefaultFormat(format);
+#else
+  // Otherwise use OpenGL
+  igndbg << "Qt using OpenGL graphics interface" << std::endl;
+#endif
+
   // Configure console
   common::Console::SetPrefix("[GUI] ");
 

src/plugins/minimal_scene/CMakeLists.txt

@@ -1,9 +1,40 @@
+set(SOURCES
+  MinimalScene.cc
+  MinimalSceneRhi.cc
+  MinimalSceneRhiOpenGL.cc
+)
+
+set(PROJECT_LINK_LIBS "")
+
+# Objective-C sources for macOS
+if (APPLE)
+  set(SOURCES
+    ${SOURCES}
+    MinimalSceneRhiMetal.mm
+  )
+
+  set(PROJECT_LINK_LIBS
+    "-framework AppKit"
+    "-framework Metal"
+  )
+endif()
+
 ign_gui_add_plugin(MinimalScene
   SOURCES
-    MinimalScene.cc
+    ${SOURCES}
   QT_HEADERS
     MinimalScene.hh
   PUBLIC_LINK_LIBS
    ignition-rendering${IGN_RENDERING_VER}::ignition-rendering${IGN_RENDERING_VER}
+   ${PROJECT_LINK_LIBS}
 )
 
+# Enable ARC on selected source files
+if (APPLE)
+  set_source_files_properties(
+      MinimalSceneRhiMetal.mm
+      PROPERTIES
+      COMPILE_FLAGS
+          "-fobjc-arc"
+  )
+endif()