Vertex shader to create and animate geometry in QQuickItem

Question

I'm reading through the comments on this Qt bug report:

https://bugreports.qt-project.org/browse/QTBUG-32741

That bug report suggests using a vertex shader with the QSGGeometry and animating in C++. I know the QML side supports shaders, but how do you access a shader from a QSGGeometry material that is handled via a C++ subclass of QQuickItem (as I interpret the bug report to suggest)? The vertex shaders accessed within QML are generally for deforming existing geometry, not really for creating new geometry or animating geometry, from what I can tell.

Or is the report suggesting to bypass QML completely for this task?

It would be pretty cool to pass data to a vertex shader for raw drawing and have the GL viewport be the Quick Item, but I don't think the QML shaders are designed for this.

Answer 1

In your subclass of QQuickItem override updatePaintNode() method should create (and update when needed) instance of QSGGeometryNode and set it up with a QSGGeometry configured for specific geometry type. That will allow you to directly control Vertex Buffer Object (just one, but with arbitrary layout of vertex attributes) and use your custom shaders.

See "Custom Geometry" example in qt documentation. Full project is in official repository.

Even more interesting example is "Texture in SGNode". It uses QQuickWindow::beforeRendering() signal to be able to run completely arbitrary OpenGL code. In this example custom rendering goes to Frame Buffer Object. Later this FBO is used as texture in a QSGSimpleTextureNode subclass.