This repo exists as an example of the Scene Graph interfering (flicking) with the Qt3D forward renderer.

Build

The project is setup to build with CMake, i.e.

# Linux

cmake -DQt_VERSION_RELEASE=5 -DQt_VERSION_MAJOR=8 -DQt_VERSION_MINOR=0 -DQt_DIR=/opt/Qt5.8.0/5.8/gcc_64 ${SOURCE}

# Windows

cmake -DQt_VERSION_RELEASE=5 -DQt_VERSION_MAJOR=8 -DQt_VERSION_MINOR=0 -DQt_DIR=c:/opt/Qt5.8.0/5.8/msvc2013_64 %SOURCE%

Not all the Qt options above are required, the CMakeList does have defaults for them, but the explicit example above is good if you have a non-default installation.

Once CMake generates build files, the project should be able to build. The program will be output to ${BUILD_DIR}/app.

The CMake files also included code to create an installer with the nullsoft scriptable install system (NSIS). Because the flicking issue only occurs on a Windows Surface, which in our case does not have a compiler, we run the installer and transfer it to the Surface.

The installer can be run in MSVC using the Install and Package targets. Not relevant, but the Linux installer currently does not configure the rpath on some of the build targets, so to run this example from an installed version in Linux, the LD_LIBRARY_PATH needs to be set.

Code

The Qt3D renderer is implemented as:

1. Off the root, a Scene3d is used, which instantiates ShipScene
2. ShipScene creates a ViewportFrameGraphs and adds it as a component
3. ViewportFrameGraphs is modelled off the multi viewport example
4. The renderer calls ClearBuffer with a fully opaque colour

The scene graph, modelled after the Scene Graph - OpenGL Under QML example.

1. In main.qml, a Ground object is created. This is just a wrapper to GroundModel.
2. GroundModel, a QQuickItem attaches to &QQuickWindow::afterRendering which syncs its state to GroundRenderer and calls GroundRenderer::paint
3. In this example, the renderer does not clear before drawing.

Discussion

Before setting up this example, I was connecting to &QQuickWindow::beforeRendering, calling clear in the scene graph, and ClearBuffers with a transparent colour in ViewportFrameGraphs. I felt that this example though was simpler and displayed the same flickering issue.

Another weird eccentricity is that when the SceneGraph is simply initialized (not never drawn to), the QML displays the 3D model upsidedown. i.e. the Top camera shows the ship model from underneath, the back from the front, and the user camera from the bottom right instead of the top left. I haven't yet spent time debugging this though.