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Global Shader Editing: Make Bright Objects Become White

How do I get objects of a higher exposure to quickly become whiter as they get brighter? Here’s the effect I’m trying to get.
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Lower their saturation.

It’s mathematically quite simple: A pure bright blue (0,255,255) will never turn white, regardless of how bright you multiply it. A slightly less saturated, but visually almost identical tint (10,255,255) will need to be multiplied by 25.5 (=255/10) to reach white in overbright areas. While a significantly less saturated but still quite bright blue (127,255, 255) will only need to be multiplied by ~2 (=255/127).

Note that UE4 uses a tonemapper by default which will make the math a bit more complex as its settings will have a big influence on the way blown out colors are handled. But the overall solution is the same: lower your saturation.