Downloaded, unzipped, into Paint.NET/Effects folder, relaunched, Effects -> Photo Flood Fill
Thanks very much - that will come in very handy!
I suppose if I draw surfaces in different layers according to the angle the surface faces at, it'll be a lot easier to apply some kind of broadbrush colour effect to simulate lighting effects? Hmm. Well that is something to think about.
While you are here MadJik and since you are French, do you know of (and have you ever used) the free software 3D CAD package, "DraftSight" by Dassault Systemes, a French company I believe?
I have used DraftSight before for engineering design and it has got a 3D rendering capability though I've not used it much so I don't know precisely what its best performance is capable of in terms of lighting effects. This is another approach I am considering.
Perhaps using both packages, DraftSight and Paint.NET I could achieve good results, eventually?