# arc An electrical discharge crossing a gap, drawn on a ribbon standing between the two things it jumps between. `uv.y` runs from one terminal to the other and `uv.x` across the ribbon; the shader draws a channel that wanders across it, pinned at both ends because that is where it is attached. There is a white-hot core inside a wider sheath, a second channel that only appears while the strike is at its hardest, and a re-strike flicker — a gap does not conduct once and stop, it conducts in bursts until the voltage across it collapses. One material serves a whole rack of arcs through lane 0 of the per-instance data: `.x` is life, 1 at the strike falling to 0; `.y` is a seed so no two channels wander together; `.z` is a per-arc brightness multiplier. An arc whose life has reached zero returns nothing at all. `core_w` and `glow_w` are the two widths as fractions of the ribbon, `jag` how far the channel wanders, `branch` how strong the second channel is. Draw it with an `additive`, `cull: none`, `depth_write: false` material.