HDR Color Volume and Tone Mapping

HDR expands a three-dimensional color volume

A gamut diagram shows chromaticity at one abstract brightness level. Real displays have limits that change with luminance. Color volume describes the set of colors a device can produce across brightness, from dark shadows to intense highlights.

A display may reach a wide gamut at moderate luminance but lose saturation in very bright colors. Another may produce high peak brightness only in small areas. HDR mastering has to respect both chromaticity and luminance.

PQ and HLG encode light differently

Perceptual Quantizer, commonly called PQ, maps signal values to absolute display luminance targets. Hybrid Log-Gamma, or HLG, is more scene-referred and designed for broadcast compatibility. Neither should be treated like ordinary sRGB gamma.

Applying SDR math to HDR values can create incorrect gradients, compositing, and brightness relationships. The signal must be interpreted with its transfer function and color primaries.

The mastering display defines a reference, not every viewer

HDR content is graded on a display with known peak luminance, black level, primaries, and behavior. Consumer displays differ widely. They may have lower peak brightness, different local dimming, smaller color volume, or aggressive picture modes.

Metadata can describe mastering conditions and content light levels, but the receiving display still decides how to adapt the signal.

Tone mapping compresses the source into the destination

Tone mapping remaps luminance when the target display cannot reproduce the mastered range. A simple clip loses highlight detail. A more controlled curve preserves detail while changing contrast. Local methods can protect bright regions but may alter the image structure.

Gamut mapping often occurs at the same time because highly saturated bright colors may lie outside the destination color volume. Reducing luminance alone may not solve the problem.

HDR interface and motion design

Bright interface elements can become uncomfortable or distract from content. Full-white surfaces, subtitles, logos, and overlays should be managed relative to the program. A highlight that is appropriate for a specular reflection may be excessive for persistent text.

Gradients and transparency need HDR-aware interpolation and compositing. Test transitions for banding, unexpected desaturation, and clipping.

A practical HDR workflow

Identify primaries, white point, transfer function, mastering luminance, and deliverable. Work on a calibrated reference display. Monitor scopes as well as the image. Review on representative consumer displays and create a deliberate SDR trim rather than relying on an unknown automatic conversion.

HexCheck can help reason about palette relationships and digital values, but final HDR judgment requires the actual signal path and display.

A disciplined verification checklist

Record the source primaries, white point, transfer function, mastering display, peak and black levels, metadata, output target, and viewing surround. Keep HDR and SDR decisions separate enough that each can be judged on its own hardware.

• Do not apply SDR math to encoded HDR values.
• Inspect both luminance and gamut mapping.
• Test persistent interface whites for comfort.
• Review multiple destination displays.
• Create an intentional SDR trim.

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