Vortex flow with lunar field indicators

The lunes in the last video provided an extra dimension over my previous raster demos: arbitrary direction. It wasn't very meaningful for the photo rasterization, and I thought it could be used for something more general, namely vector fields. I have a number of demos involving those, so this could make a nice alternative display for the same functions.

With the varying area, direction and colour, the lune field has a lot of possibilities I haven't yet used. The hue still doubles as direction, but I'm now using saturation to indicate vorticity. I have done this with the particle colour itself in some of my 3D flow demos, but the separate field indicator makes this much clearer.

Now the technical curiosity, though it might be somewhat hidden due to the overlay: white rings around vortex centres. This means areas of zero vorticity. My vortices have a Gaussian rotation profile, so vorticity is positive around the centre and negative on the outskirts. This colouring only shows the magnitude, but the sign change through zero is clearly seen in some cases.