Auto-exposure
The renderer measures the scene's average brightness and adjusts exposure per frame, so that a dark cave and a sunlit street both land in a visible range.
An HDR-rendered image holds light values far beyond what a display can show. Auto-exposure mimics the eye's adaptation: it computes the scene's average luminance — often from a histogram or a downsampled mipmap of the frame — then scales the whole image so that average lands on a target level before tone mapping squeezes the rest down.
The transition is deliberately slow. Step from daylight into a tunnel and exposure eases down over a second or so, like a real eye adjusting to the dark. Do it instantly and it looks wrong; do it too sluggishly and the player is blinded. Sometimes called "eye adaptation". A classic trap: one bright lamp in view can drag the whole exposure down, so many engines weight the metering toward the centre of the screen.