Inside the Color Volume

Every color a human can see maps inside the horseshoe on the floor. x and y are projections of the CIE XYZ tristimulus values (x = X∕(X+Y+Z), y = Y∕(X+Y+Z)), so they encode chromaticity only, with luminance divided out. The curved boundary is monochromatic light, 380 to 700 nm; the straight base is the line of purples, colors with no single wavelength. The diagram is famously non-uniform: equal distances near green overstate perceptual difference compared to blue, which is why later metrics (u′v′, ΔE2000, ICtCp) exist.

A display mixes three primaries, so its gamut is the triangle they span on the floor. R (x, y)G (x, y)B (x, y) 601.630 .340.310 .595.155 .070 709.640 .330.300 .600.150 .060 P3.680 .320.265 .690.150 .060 2020.708 .292.170 .797.131 .046 AP1.713 .293.165 .830.128 .044 BT.2020's primaries sit on the spectral locus itself (monochromatic ~630, 532, and 467 nm sources), which is why only laser projection approaches full coverage. Share of CIE 1931 area: BT.709 ≈ 35.9%, DCI-P3 ≈ 53.6%, BT.2020 ≈ 75.8%.

The white curve across the floor is the Planckian locus: the chromaticity of a black-body radiator from 1667 K to 25,000 K; a display's correlated color temperature is the nearest point on it. The D-series illuminants model daylight phases: D50 (5003 K, print), D55 (5503 K), D65 (6504 K, broadcast and web), D93 (9305 K, legacy NTSC-J). DCI cinema white (x 0.314, y 0.351, ≈6300 K, visibly green of the curve) is the theatrical projection reference. Switching the white point above re-solves each volume's RGB↔XYZ matrix; watch the white column, the ridges, and every top surface move with it.

The vertical axis is luminance Y in cd/m² (nits) on a log₁₀ scale: four decades from 1 to 10,000. Human vision is roughly logarithmic, and the PQ EOTF (SMPTE ST 2084) that carries HDR was fit to the Barten contrast-sensitivity model, so equal steps up this axis approximate equal perceptual steps. Reference points: SDR diffuse white at 100; HDR mastering displays at 1,000 to 4,000; the PQ container's absolute ceiling at 10,000. How signal becomes luminance: the Transfer Function Explorer →

A gamut triangle says nothing about brightness; the volume is what a display can actually reproduce. The walls are vertical because any in-gamut chromaticity can be made at low luminance. The top sags because peak luminance is only reachable at white, where all three channels are maxed: each chromaticity tops out at peak × the luminance factor of its brightest RGB mix. For BT.2020, red carries 26.3% of luminance, green 67.8%, blue only 5.9%; those are the ridge lines running from the white peak down to each primary corner.

Rec. ITU-R BT.709 (1990) defines HD television and shares its primaries with sRGB. Reference viewing: 100 cd/m² peak, BT.1886 EOTF (γ ≈ 2.4), D65 white. It is the smallest volume in this model, and still the delivery target for the majority of broadcast and web content, which is why every wide-gamut grade eventually needs a trim pass or gamut map down to it.

P3 came from digital cinema; consumer HDR panels typically reach 95 to 100% of P3-D65, making it the practical mastering gamut, shown here at a 1,000 cd/m² mastering peak. BT.2020 is the container: BT.2100 pairs its primaries with the PQ and HLG transfer functions up to 10,000 cd/m². No commercial display fully covers 2020; real deliverables are graded P3-inside-2020, so the largest volume here is signal headroom, not any panel's reality. That gap between container and display is exactly what tone and gamut mapping negotiate.

Display-P3 and DCI-P3 share the exact same primaries, so on the CIE floor their gamut triangles sit right on top of each other. What differs is everything around the color. DCI-P3 is the digital-cinema theatrical standard: a green-shifted white near 6,300 K (x 0.314, y 0.351), a pure 2.6 gamma, and a 48 cd/m² projection reference, so its solid here is short and leans toward that warmer-green white. Display-P3 is Apple's consumer variant: the same primaries re-referenced to D65 with the sRGB tone curve, which is what phones, laptops and HDR panels actually ship, shown here at a 1,000 cd/m² mastering peak. Toggle DCI-P3 alongside P3 above: the footprint is identical, but the white point and height are not. That is why a cinema DCI-P3 master looks green and dim if a display treats it as Display-P3 without converting the white.

ACES AP1 is the working gamut of ACEScg and ACEScct, not a display standard. It wraps slightly outside BT.2020 near green so every display gamut fits inside with margin for grading math; natively it uses a ~D60 white and is scene-referred and unbounded, so the 10,000-nit top here is just the plot's container. Rec.601 (shown with SMPTE-C 525-line primaries) is the SD legacy space: slightly smaller and warmer-shifted than 709. Toggle both in VOLUMES above to compare.

Delivery formats pin these volumes down: HDR10 is PQ in a 2020 container with static MaxCLL/MaxFALL; Dolby Vision adds dynamic per-shot metadata; HLG serves live broadcast. Cameras capture wider-than-709 native gamuts, grades happen in ACES or camera space, and everything is gamut- and tone-mapped into the delivery volume. Whether a display actually reproduces its claimed volume is a measurement question: probe-verified ΔE, grayscale tracking, and gamut coverage, before and after. Book a calibration →