aurora

# aurora An auroral curtain, drawn as light rather than as a lit surface. The mesh it shades is a vertical XY sheet whose normal runs along +Z, with `uv.x` across it and `uv.y` up it — 0 at the base, 1 at the top. `fragment()` builds the ray structure out of fBm across the width, gates it with a slow envelope so sheets arrive and leave, and shapes it vertically with a hard lower edge fading to nothing at the top. Colour runs green at the base through cyan in the middle to violet at the top, and the violet only reaches full weight as `storm` does. `vertex()` folds the sheet out of its own plane, reading its clock from `v.time` so the previous-frame evaluation reports where the fold stood then and the curtain publishes a real motion vector. Drive `storm` from `carrington.timeline.storm(t)` and `intensity` from how much of the sky the shot wants. `ripple` is the fold amplitude in metres, `ray_scale` how many ray periods fit across the sheet, and `drift` how fast they scroll. Draw it with an `additive`, `cull: none`, `depth_write: false` material.