Highlight Trails on a rainy neon street (Hail Mary highlight trails). Starting footage: one 10 s vertical clip of a rainy night street with neon, lamps, headlights and wet reflections. What I asked the timeline agent: build a custom shader on my clip where only the brightest lights smear upward, expose six controls (threshold, softness, trail length, direction, taper, mix), and generate nothing. What the agent did: wrote the GLSL, put it on the clip, built a native 9:16 timeline with Screen-blend copies for glow, then A/B-tested its own shader on the frame, called it weak and rebuilt it from scratch. What I refined: stepped trails and a white wall flood. Denser sampling, higher threshold (0.72), shorter trail (0.33), taper 0.40. White clipping fell from about 28% to 3%. The walkthrough itself was assembled by the same agent in invideo Editor, including a second custom shader for the before/after split. invideo Editor is free to try.