We need shaders.

https://github.com/moderngl/moderngl/blob/master/examples/integration_pycairo.py

https://moderngl.readthedocs.io/en/latest/the_guide/rendering.html#rendering

https://moderngl.readthedocs.io/en/latest/the_guide/basic.html#shader-introduction

Thanks for working on this!

right now, one can't switch between CPU and GPU rendering without closing the app

I don't think it's necessary. Ideally, GPU rendering should work good for most users, and CPU rendering will be a fallback for some extreme cases. Also, switching rendering without restart will require switching between DrawingArea and GLArea on the fly, that's additional work.

https://github.com/moderngl/moderngl/blob/master/examples/integration_pycairo.py

As I understand this example, it renders cairo surface into GL texture of rectangle, maybe we can use the shader from here without changing.

Here are examples of using modernGL with GTK.GlArea and Gtk.DrawingArea. This problem is, I'm not sure if it worth the effort, maybe by using modernGL+Cairo we'll just re-do things that Gtk.DrawingArea automatically does for us and we'll not get any significant performance improvement. If you have time and motivation, feel free to try. I will continue looking for another way to avoid using Cairo at all.

I guess whether we get a performance boost depends on what is actually the bottleneck right now, cairo or CPU.

Anyway, I think for as long as we want to keep both CPU and GPU rendering available, we should render using moderngl+cairo, otherwise every new drawing mode will have to be implemented twice.

Once we drop CPU rendering, we should probably stop using cairo and turn to something more native in OpenGL/ModernGL.