Tone mapping is the process of converting the luminance and color range of an image into the narrower range that a target display can reproduce, while preserving as much of the creative intent, detail and contrast as possible. In HDR video, it is what allows content mastered at, for example, 1,000 or 4,000 nits to play on a television that peaks at 600 nits, or to be converted into SDR for older screens.
Tone mapping is applied by the display, the playback device or a conversion tool. Its behavior depends on the HDR format, the available HDR metadata and the capabilities of the screen, which is why the same HDR10 film can look noticeably different on two televisions.
Why HDR video needs tone mapping
HDR video needs tone mapping because the mastering range exceeds most displays. The PQ transfer function used by HDR10, HDR10+ and Dolby Vision encodes absolute luminance up to 10,000 nits, and content is commonly graded on reference monitors of 1,000 nits or more. Consumer screens vary widely in peak luminance and color volume, so the signal must be adapted to each one.
| Stage or device | Typical peak luminance | Tone mapping needed |
|---|---|---|
| PQ signal container | Up to 10,000 nits | Defines the encoding range |
| Mastering monitor | 1,000 to 4,000 nits | Reference for the grade |
| High-end consumer TV | About 1,000 to 2,000 nits | Mild, mainly for 4,000-nit masters |
| Mid-range TV or monitor | About 400 to 800 nits | Substantial |
| SDR display | About 100 to 300 nits | Full HDR-to-SDR conversion |
Without tone mapping, a display would simply clip everything above its peak, turning bright skies, specular highlights and lamps into flat white patches. Tone mapping replaces that hard limit with a gradual roll-off that keeps some detail in the brightest parts of the image.
How a tone mapping curve works
A tone mapping operator is essentially a transfer curve that leaves most of the image unchanged and compresses only the upper range. Shadows and midtones, including skin tones around 100 to 200 nits, are usually reproduced one-to-one, while values above a knee point are progressively compressed into the remaining headroom of the display.
| Curve region | Behavior | Visual result |
|---|---|---|
| Shadows and midtones | Tracked one-to-one | Faces and scene detail as graded |
| Knee point | Start of compression | Chosen below display peak |
| Roll-off region | Increasing compression | Highlights keep gradation |
| Display peak | Asymptotic limit | Brightest content reaches screen maximum |
| Black level | Optional lift or compression | Detail kept on displays with raised blacks |
Color must be handled as carefully as luminance. Compressing each RGB channel independently can shift hues and desaturate highlights, so better operators work in perceptual color spaces such as ICtCp and apply gamut mapping to bring wide color gamut values into the display’s color volume.
Static and dynamic metadata
Tone mapping quality depends heavily on what the display knows about the content. HDR formats differ mainly in the metadata they provide to guide the curve, from none at all to scene-by-scene instructions created during mastering.
| Format | Metadata | Tone mapping approach |
|---|---|---|
| HDR10 | Static: mastering display, MaxCLL, MaxFALL | One curve for the whole program |
| HDR10+ | Dynamic, per scene or frame | Curve adapted to each scene |
| Dolby Vision | Dynamic, with trim passes from the grade | Curve guided by colorist-approved trims |
| HLG | None required | Scene-referred signal, display adapts system gamma |
With static metadata, a display must choose a single curve for the brightest scene in the program, which can make dark scenes look dimmer than necessary. Dynamic metadata lets each scene use the full capability of the screen, and Dolby Vision trim passes let the colorist approve how the image maps to lower peak luminance levels.
Tone mapping compared with related processes
Tone mapping is often confused with neighboring operations in the HDR workflow. The differences lie in the direction of conversion and whether the target is a specific display or a standard format.
| Operation | Direction | Typical place |
|---|---|---|
| Display tone mapping | HDR master to a specific HDR display | Inside the TV or monitor |
| HDR-to-SDR conversion | HDR to SDR Rec. 709 | Broadcast, streaming, deliverables |
| Inverse tone mapping | SDR to HDR | Upconversion of archive or live SDR sources |
| Gamut mapping | Wide color gamut to smaller gamut | Paired with tone mapping |
| Photographic HDR tone mapping | Bracketed exposures to a single image | Still photography, not video delivery |
HDR-to-SDR conversion is a specialized form of tone mapping with a fixed target of about 100 nits and Rec. 709 color. Broadcasters often use standardized conversion methods so that SDR viewers receive a consistent picture during simultaneous HDR and SDR transmission.
Common problems and practical advice
Most tone mapping problems are visible as lost highlight detail, dull images or shifted colors. Many can be prevented during mastering by checking how the grade behaves on lower-peak displays.
- Missing or wrong MaxCLL values can make displays apply an unsuitable curve; verify metadata at delivery.
- Overly aggressive roll-off flattens highlights and makes HDR look like dim SDR.
- Dynamic tone mapping on the TV may brighten dark scenes against the director’s intent; filmmaker modes reduce this.
- Hue shifts in bright saturated colors, such as neon signs, indicate per-channel compression without gamut mapping.
- Grading only on a 4,000-nit monitor without checking 1,000-nit or SDR versions hides problems most viewers will see.
A non-technical view
HDR movies are made with extremely bright highlights, such as sunlight glinting on water. Many televisions cannot get that bright. Tone mapping is the television’s way of fitting the picture into what it can actually show.
Imagine fitting a tall person into a short doorway by asking them to bend only at the neck rather than shrinking their whole body. Tone mapping leaves normal brightness alone and gently squeezes only the brightest parts, so the picture keeps its detail instead of turning into flat white patches.
A concrete example
A streaming series is graded in Dolby Vision on a 4,000-nit reference monitor. The final scene shows a character standing against a sunset, with the sun near 3,000 nits and the face at about 150 nits. The colorist also creates trim passes for 1,000 nits and 100-nit SDR, adjusting how the sky rolls off at each level.
A viewer watches on a mid-range television that peaks at about 600 nits. The television reads the dynamic metadata for the scene, keeps the face at its graded brightness, and compresses the sunset smoothly into the top of its range. The sky keeps its color gradation instead of clipping, and the scene looks close to the colorist’s intent despite the television reaching only a fraction of the mastering brightness.