The real world spans a brightness range a screen cannot show — from deep shadow to the sun, a ratio of millions to one, squeezed into 256 steps. Tone mapping bends that huge range onto the display so the midtones look right, but it pays at the top: the brightest, most-different values all get pushed together into the same white. Slide the exposure and watch distinct highlights collapse into one.
A tone-map operator compresses an unbounded HDR range [0,∞) into [0,1) — here Reinhard’s x/(1+x) — monotonically, so ordering is preserved, then quantizes to 8 bits. Monotonic-but-saturating means midtones keep their spacing while highlights crowd the ceiling: two very different bright values (e.g. exposure 800 and 4000) both map above 254.5 and round to the SAME 255. A fail-loud self-check throws unless the map is monotonic AND two distinct highlights collide after 8-bit quantization. The scene had more light than the picture can hold; the order survives, the highlight detail does not. ◆ real imaging, node-verified.
Reinhard global tone-mapping shown (local operators preserve more highlight detail at the cost of halos); the monotonic-compression + highlight-collision behaviour is the exact, general consequence of mapping an unbounded range into a bounded, quantized one.