A camera sensor, or a real scene in front of you, can span a brightness range no consumer display can fully reproduce. Something has to give — either detail gets thrown away, or the whole range gets squeezed to fit. Tone mapping is the general name for that squeeze: a function that takes an input brightness value and remaps it to an output value that fits the target range, ideally without destroying what made the highlights and shadows look different from each other in the first place.
The simplest version: a straight line
The crudest possible tone mapping is linear scaling — multiply every pixel by the same factor. That works fine for the shadows and midtones, but it does nothing to solve the actual problem: if your brightest pixel is already at the display's ceiling, scaling everything down just makes the whole image dimmer without buying you any extra headroom for highlights specifically. Real tone-mapping curves are non-linear for exactly this reason — they leave the parts of the range that already fit alone, and only compress the parts that don't.
The knee: where compression actually starts
Most practical tone-mapping curves are identity below a threshold (a pixel that's already comfortably within range passes through completely unchanged) and compressed above it. That threshold is usually called the knee. Where you put the knee, and how sharply the curve bends there, changes the whole character of the result:
- Hard knee — identity, then an abrupt cap. Simple, but it can create a visible band where detail suddenly stops.
- Soft knee — identity, then a gradual curve (often built from a
tanhor similar function) that asymptotically approaches the ceiling instead of hitting it. Highlights near the knee stay separated from each other instead of getting flattened together.
tanh-based soft-clip compresses everything above it toward white — the same idea real HDR-grading tools use to preview an HDR grade on an SDR monitor.Two directions, same math
Tone mapping almost always comes up in the "down" direction — HDR or scene-referred brightness compressed down to fit an SDR display. But the same underlying idea runs in reverse too: taking an SDR image and deliberately stretching its highlights back out toward HDR headroom. That's what Encode's Highlight HDR and Full HDR modes actually do — they're inverse tone-mapping curves, remapping a limited input range up toward a target peak brightness instead of down. Superwhite mode is a more conservative version of the same idea, restricted to a fixed, very high threshold so it only touches pixels that were already close to blown out.
Why this matters beyond one slider
Every HDR mode described on this site — Superwhite, Highlight HDR, Full HDR, and the gain map that ships in an exported file — is a tone-mapping curve under the hood, parameterized by a threshold, an intensity, and a target peak brightness. Understanding the curve shape is what makes the difference between "why doesn't this control do anything" and "this control only affects pixels above the knee" concrete instead of mysterious.