Stop the breathing

The film never sat identically in the projector gate, and old dyes respond unevenly to the lamp, so brightness flickers frame to frame. The cure sounds paradoxical: we don't force every frame to the same brightness (that would flatten a real sunset), we pull each frame toward a temporally smoothed target. Slow, intended lighting changes survive; the fast jitter gets averaged away.

The technique: measure once, apply once

It's a clean example of two-pass streaming. First pass: stream the shot and record just one number per frame, its perceptual brightness (green weighed far above blue, per BT.601):

lum = [float(frame.mean(axis=(0,1)) @ _BGR_LUMA) for frame in read_frames(src)]
target = rolling_mean(lum, window=31)          # the smooth curve
gain   = clip(target / lum, 0.5, 2.0)          # per-frame nudge, safety-clamped

Second pass: multiply every pixel of frame i by gain[i]. The same gain hits all three channels, so color is untouched, we only move brightness. The 0.5×-2.0× clamp stops a single freak dark or blown frame from exploding.

The upgrade: flicker is often local

A single gain for the whole frame misses the classic 8mm ailment, a projector lamp hot spot, or one edge of the film fading differently, so the left side breathes while the right sits still. The v2 pipeline measures a 4×4 grid of gains, smooths each independently, and blends them back into a smooth full-frame map.