Vortice Umbra: A companion piece to Vortice Lux.

Concentric ellipses rotate over time, producing phasing patterns. The ellipses are rendered with drop-shadows, with the direction varying over time. This produces emergent patterns light and dark interplay. Vortice Umbra features a dark background.

Features:

Spacing: A wider spacing produces bolder patterns and emphasis on the light and dark. All integer sizes from sixteen to forty-eight are equally likely.

Eccentricity: This affects the shape of the ellipses. An eccentricity close to one produces a subtle effect, whilst lower eccentricities of 0.9 emphasise the changes in the spiral.

Rotation: There are three speeds; a relatively-rapid speed, a steady, almost-default speed, and a slower, more contemplative version. The default speed is most likely.

Highlights: The drop-shadow effect is more likely to be light, producing a brighter edge to each set of ellipses. A slightly darker version is also available, with a probability of around a quarter.