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.