Feedback gain near 1 causes runaway whiteout — leave headroom and decay each frame

In a feedback loop (feedback-trail), the previous frame is mixed back into the current frame. If the gain is exactly 1, the accumulated signal never decays and the image saturates to solid white or black (runaway). Even gain slightly above 1 produces exponential blowup within seconds. Safe practice: keep gain below 1 (typically 0.85–0.95 for trails), or pair feedback with a brightness-reducing transform so each frame dims slightly. The danger is heightened by other motions: rotation or scale compound the brightness across frames. Feedback is ‘the most alive-feeling technique and the most dangerous’ precisely because this threshold is sharp.

Examples

Hydra: src(o0).brightness(-0.02).out(o0) — the -0.02 is the decay. Without it, src(o0).out(o0) quickly whites out. GLSL ping-pong: multiply prev frame by 0.92 before adding current.

Assessment

Explain why feedback gain of exactly 1 causes whiteout. Describe two strategies to prevent runaway while still getting long-trail feedback. What happens if you combine full-gain feedback with a slow zoom?