The tonemapper is doing the work
The glow note says to push the saturated hue past 1.0 and let the core whiten on its own. That instruction is only true if something maps your HDR values down with a curve. Clamp instead and the effect does not merely weaken, it inverts into an artifact.
Left, the core is a flat slab of cyan with a hard border, because every pixel above 1.0 became exactly the same color. Right, the same numbers roll off smoothly and the center goes white while the halo keeps its hue. Nothing about the glow changed. Only the last line did.
Why a clamp destroys the hue
Take a linear cyan of (0.10, 0.85, 1.00) and scale it by a gain
g. Under a hard clamp the channels hit the ceiling one at a time, and
between those thresholds the ratio between channels is being changed:
g = 1.00: blue saturates. Above this the hue starts shifting.g = 1.18: green saturates. Now only red still carries any information.g = 10.0: red finally saturates and the pixel is white.
So a clamp does eventually reach white, but it gets there by flattening everything in between into bands of a color you did not author. Those are the marks on the top strip below.
The top strip goes cyan, then a hard turquoise step, then a hard pale step, then white, with visible boundaries. The bottom moves continuously from saturated to white, which is what a real bright light does and what the eye expects.
The practical consequence
If a project renders to an LDR target, or has no tonemapper in the chain, the neon recipe silently becomes a no-op: everything above 1.0 collapses to the same value and the three-layer stack is doing arithmetic nobody will ever see. It is worth checking which one you actually have before tuning gains for an hour.
Rules of thumb
- The whitening is the curve's doing. No curve, no whitening.
- A hard clamp shifts hue between the first and last channel saturation. It is not a mild version of a tonemap.
- Author in linear and tonemap once, at the very end, after everything has been added.
- Keep the halo near 1 and let only the core exceed it, so the curve has something left to roll off.
- Dither after the tonemap, before the 8-bit write. The rolled-off region is where banding shows worst.
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