TV Tone Mapping Explained: What HDR Must Fit

What does tone mapping do on a TV?

Tone mapping is the television's job of fitting an HDR picture into the brightness and color it can actually produce. HDR video can describe highlights far brighter than most screens reach, so the set has to decide what to reproduce exactly, what to compress, and what to give up. A well-behaved mapping leaves faces and rooms close to how they were graded and rolls the brightest highlights off gently instead of cutting them flat. How a TV makes those choices shapes a sunlit cloud more than any badge on the box.

Why can HDR content be brighter than the TV?

HDR10, the most common HDR encoding according to the HDR10+ system whitepaper, uses PQ (the perceptual quantizer), a transfer curve that assigns each code value an absolute brightness rather than a share of whatever the screen can manage. Screen brightness is measured in candelas per square metre (cd/m², often called nits). The ITU's Report ITU-R BT.2390, describes the PQ curve as able to operate from 10 000 cd/m² down to less than 0.001 cd/m².

The same report describes the ideal reference monitor as one rendering black down to or below 0.005 cd/m² and highlights up to 10 000 cd/m², viewed in a dim room with a neutral grey surround at 5 cd/m². Then it names the everyday reality: content is often watched in brighter rooms and "on monitors that cannot display the deepest blacks or brightest highlights that the PQ signal can convey," so the display's response must be changed in a process it calls display mapping. Tone mapping is the familiar name for that adjustment.

The ITU's production guidance, Report ITU-R BT.2408, puts it just as plainly: monitors "may not reach the full extent of the BT.2100 gamut or the 10 000 cd/m2 limit of the PQ signal." Films and shows are judged on a mastering display, which the HDR10+ whitepaper describes as sitting in a darkened room and typically having greater capabilities than consumer sets. The gap between that room and your living room is the space tone mapping has to cross. The basics are in what HDR means on a TV.

What does the TV know about the content it receives?

It depends on the format. In HDR10, the HDR10+ whitepaper explains, static metadata defined in SMPTE ST 2086 carries the minimum and maximum mastering luminance and the chromaticities of the mastering display, while two further values, Maximum Content Light Level (MaxCLL) and Maximum Frame Average Light Level (MaxFALL), are defined in the CTA-861 and Blu-ray standards. All of it "applies a single set of data for an entire title." The HDR10+ explainer on static metadata describes that data as including the brightest and average light levels in the program.

Dynamic formats send guidance that changes during the program. The HDR10+ whitepaper says its metadata signals, per scene or per frame, "the binned statistics of all pixel values," and that a TV which does not support it ignores it and shows the plain HDR10 version. Dolby's content-creation page says Dolby Vision analysis generates "three core metadata values for each shot that represent the minimum, average, and maximum luminance levels," that colorists can steer the result with artistic trims, and that the metadata travels to Dolby Vision TVs, which map each shot to the device's capabilities. Dolby's consumer overview says certified TVs use it to guide mapping "based on the specific capability of the display."

These are the format owners describing their own systems. Metadata is guidance; the television's processing still draws the curve, which is why two sets fed the same film can disagree. When nothing useful is known, BT.2408's worked method allows the mastering range to be assumed as 0 to 10 000 cd/m², the entire PQ range.

What are the options when a highlight is too bright?

BT.2390 walks through the choices using a glossy object whose reflection exceeds the screen's maximum:

• Scale everything down. The whole range is renormalised so the highlight fits. The report notes this is seldom selected, and for good reason: the entire picture dims to make room for one sparkle.
• Hard clip. Everything below the limit stays accurate and the highlight is simply truncated. The report says detail inside the highlight is replaced with constant values, producing flat regions that look artificial. A cloud becomes a white shape with no texture.
• Soft clip, or knee. Brightness is compressed progressively as it approaches the limit, so the highlight's shape and internal detail are somewhat preserved. The report calls this typical best practice.

The third option is the one worth wanting from a television. The interesting differences lie in where the compression starts and how steeply it bends.

Where does the roll-off begin?

BT.2408's Annex 5 gives a reference method built on exactly this idea. "The central region of the tone mapping curve is defined as a 1:1 mapping," it says, with a knee roll-off calculated using a hermite spline (a smooth curve joined to the straight section) to reduce the luminance range to what the display can show. A separate toe gently lifts the blacks; elsewhere the report describes the function as providing "a toe and knee to gracefully roll off the highlights and shadows."

The knee starts at KS = 1.5 × maxLum − 0.5, where maxLum is the display's peak expressed as a fraction of the mastering range in PQ signal terms. If the display matches the master, maxLum is 1 and the knee starts at 1, meaning nothing is compressed. At a maxLum of 0.8 the knee starts at 0.7; at 0.6 it starts at 0.4. These are normalised PQ signal values, not nits, but the direction is the lesson: the further a screen falls short of the master, the lower in the picture the compression has to begin, and the more of the image it touches.

For production monitors, by contrast, the same report recommends a hard clip to the display's capabilities plus a way to identify out-of-range pixels, because a colorist needs to see the truth. A living-room television has a different brief: a pleasant, detailed picture from a signal it cannot fully reproduce.

Why do midtones and faces matter most?

Because that is where most of the picture lives. BT.2408 sets HDR Reference White, the level of a 100% reflectance white card, at 203 cd/m², and lists an 18% grey card at 26 cd/m². In an analysis of 107 HDR images graded at 1 000 cd/m², it found that 99% of the images had between 5.6% and 14% of their pixels above 203 cd/m². The rest of each frame sat at or below reference white.

That makes the 1:1 region precious. A mapping that holds skin, walls and foliage where the grade put them, and spends its compression on the bright minority, keeps the image feeling like the original. A mapping built around the single brightest moment has to protect that moment everywhere. The HDR10+ whitepaper argues that dynamic tone mapping exists "to limit the dimming and desaturation of the display that happens from static tone mapping," which is the format owner's case. The trade itself is real: protect every highlight with one fixed curve and some ordinary scenes look dimmer than they need to.

Room light pushes the other way. BT.2390 notes that to suit a brighter environment the display may lift the black level, and on a dim display it may change system gamma or impose a knee. A brighter HDR mode for daytime viewing trades some accuracy for visibility, which is sometimes the right call.

What happens to color during tone mapping?

Color is where clumsy mapping shows itself. Describing how standard dynamic range handles highlights, BT.2390 notes that when bright colored highlights pass through a shoulder, the color shifts toward white. It also gives a worked example of why curves are better applied to luminance than to each color channel separately: squaring the channel values (0.25, 0.75, 0.25) gives (0.0625, 0.5625, 0.0625), and the green-to-red ratio jumps from 3:1 to 9:1, so the pixel becomes a visibly different green. The HDR10+ whitepaper makes the same point about its own approach, describing a tone curve that holds color ratios constant so only luminance changes.

Brightness is not the only limit. BT.2390 notes that where a display cannot cover the delivered color range, gamut mapping brings those colors inside what the screen can show. Our color gamut guide explains why the signal's color range and the screen's are different things.

Does HLG need tone mapping too?

HLG (hybrid log-gamma), an HDR format designed with broadcasters in mind, works differently. BT.2390 says it "requires no production metadata," and the signal is relative rather than absolute, so the display scales it to its own peak. What the display adjusts is system gamma, using the report's formula γ = 1.2 + 0.42 log10(LW/1 000), where LW is the display's nominal peak luminance. The report says the BBC's tests showed a consistent look from 400 to 2 000 cd/m² by varying gamma this way. Working the formula through: a 1 000 cd/m² display uses 1.2, a 500 cd/m² display about 1.07, and a 2 000 cd/m² display about 1.33.

Which TV settings change tone mapping?

The labels are model-specific, so the manual is the authority on what each one does. In broad terms, look for:

• The HDR picture mode your TV switches into when it detects HDR. Confirm which mode is active before adjusting anything.
• A dynamic or scene-analysing tone mapping option, offered on some sets, which studies the picture itself rather than relying only on static metadata. It can lift dim-looking HDR10 titles or push scenes beyond the grade; judge it on content you know.
• Brightness-related HDR controls, which on LCD sets interact with the backlight. Local dimming decides how bright a small highlight can actually get, and therefore how much mapping is needed.

Some games and consoles include an HDR calibration screen that asks where detail disappears. Run it in the TV mode you will actually play in, since the answer depends on the TV's own processing. For a reversible order of operations, start with the first settings to change.

How can you judge tone mapping at home?

A bright sky with clouds shows whether texture survives or turns into flat white patches. A sunlit face shows whether midtones have been dragged down to protect highlights. Saturated lights, such as neon signs or a red tail lamp, show whether bright color washes toward white. A dark room lit by one lamp shows how shadows are handled.

Change one setting at a time, in your usual lighting, using the same scene. The goal is not the brightest picture in the room. It is the one where the sky still has weather in it and the people still look like people.

Sources

• Report ITU-R BT.2390-11: High dynamic range television for production and international programme exchange
• Report ITU-R BT.2408-8: Suggested guidance for operational practices in high dynamic range television production
• HDR10+ Technologies: HDR10+ System Whitepaper
• HDR10+: One Instruction for a Whole Movie: What Static Metadata Gets Wrong
• Dolby Professional: Dolby Vision for content creators
• Dolby Professional: Discover the Dolby difference