Color Spaces and Color Transfer Functions
An RGB value is meaningless without a color space and transfer function. The color space defines the gamut. The transfer function defines how tonal values are encoded.
Always state both. P3 primaries, for example, may use gamma 2.6 for theatrical projection, the sRGB curve for Display P3, or PQ for HDR mastering. Naming only "P3" leaves the encoding ambiguous.
For a more detailed explanation of color science and the processes involved, see Cinematic Color by Jeremy Selan.
Apply these concepts through a documented color-management system. See ACES and Color Management and OpenColorIO.
Color Spaces
Color spaces define the gamut a camera can encode or a display can reproduce.
Display Color Spaces
| Color space | Description |
|---|---|
| sRGB | The standard color space used by most computer and mobile phone displays, with an implied approximate gamma of 2.2. |
| Rec.709 | The standard HDTV color space and the reference target for home video and broadcast mastering. The same primaries and white point as sRGB. It specifies a camera-side OETF, not a display gamma: the reference display EOTF is BT.1886, approximately 2.4. |
| Rec.2020 | The wide-gamut UHDTV standard and, through BT.2100, the colorimetric container for essentially all current HDR delivery, including HDR10, Dolby Vision, HLG broadcast, UHD Blu-ray, and HDR streaming. No shipping display reproduces the full gamut because its primaries are monochromatic. Content is delivered in the Rec.2020 container but almost always mastered to a P3-D65 volume inside it. |
| DCI-P3 | The standard gamut and white point used in theatrical digital cinema projection: standardized as P3DCI (see below). Substantially wider than Rec.709 (about 36% more area in CIE 1931 xy, mostly in the greens and reds), with an implicit gamma of 2.6 and the DCI white point (0.3140, 0.3510). |
| Display P3 | P3 primaries and a D65 white point with the sRGB transfer function: a registered ICC/CSS encoding and the default display color space on Apple hardware since 2015. Not to be confused with DCI-P3 (gamma 2.6, DCI white) or P3-D65 PQ (HDR mastering). They share primaries and nothing else. |
Three P3 color spaces are standardized by SMPTE ST 2113, sharing one primary set and differing
only in white point. The standard also fixes the naming convention: "P3" followed by the white-point
name, with no separators, so P3D65 is the conformant spelling and D65P3 is not:
| Color space | White point | Description |
|---|---|---|
| P3D65 | 0.3127, 0.3290 | P3 primaries on a D65 white. Common for HDR home video mastering. |
| P3D60 | 0.32168, 0.33767 | P3 primaries on the ACES white point (ST 2065-1). |
| P3DCI | 0.3140, 0.3510 | P3 primaries on the DCI reference projector white: the "DCI-P3" of the table above. |
Camera Color Spaces
Camera gamuts are defined as "virtual primaries" as they are not necessarily indicative of color ranges a camera sensor can register. The primaries often exceed the human vision spectrum as plotted on a CIE 1931 chromaticity diagram. Comparing the virtual primaries of different cameras does not indicate whether a camera actually has broader sensitivity than another, but rather serves as a comparison of the color encoding models used by each camera.
| Color space | Description |
|---|---|
| ARRI Wide Gamut 3 (AWG3) | The acquisition gamut of AMIRA and ALEXA models through ALEXA Mini LF, paired with LogC3. |
| ARRI Wide Gamut 4 (AWG4) | The wider acquisition gamut introduced with the ALEXA 35 under REVEAL color science, paired with LogC4. Also used by ALEXA 35 Xtreme and ALEXA 265. |
| REDWideGamutRGB | A camera color space designed to contain all colors a RED camera can capture without clipping. It has replaced the earlier DRAGONcolor / DRAGONcolor2 encodings. |
| Sony S-Gamut3.Cine | The practical de facto working space for Sony professional cinema cameras: a slightly smaller, more grading-friendly variant of the fuller S-Gamut3. |
Transfer Functions
Transfer functions define the relationship between RGB tonal values and their equivalents either in the "scene" if scene-referred, or on a display if display-referred. Transfer functions are often called color encodings, encoding spaces, and gamma spaces.
Camera Transfer Functions
| Transfer function | Description |
|---|---|
| Cineon | The standard log encoding of film scans. |
| ARRI LogC3 | The exposure-index-dependent log encoding used by ARRIRAW and ProRes from AMIRA and ALEXA models through Mini LF. |
| ARRI LogC4 | The EI-independent successor introduced with the ALEXA 35. Different mid-gray placement (0.2784 vs LogC3's ~0.391), and ARRI specifies 12-bit as the minimum quantization: never 10-bit. Always state the version number. |
| REDlogFilm | RED's Cineon-style log curve from the legacy (pre-IPP2) pipeline. RED describes it as "RED's version of the Cineon transfer function" rather than an exact match. In IPP2 it survives only as a selectable output gamma, not as the acquisition encoding. |
| RED Log3G10 | An alternate log curve optimized for HDR grading applications. (Also see Log3G12.) |
| Sony S-Log3 | The latest logarithmic encoding utilized by Sony professional cinema cameras. |
Display Transfer Functions
| Transfer function | Description |
|---|---|
| Gamma 2.2 | Material mastered for sRGB computer displays. Viewed in a normal, bright surround (office or home interior). |
| Gamma 2.4 | A target for material mastered for Rec.709 displays. Viewed in a dim surround (dim room with low ambient light). |
| Gamma 2.6 | Digital cinema projection gamma. Viewed in a dark room (theater, little or no ambient light). |
Linear Transfer Functions
| Transfer function | Description |
|---|---|
| Scene-Linear | Gamma 1.0 with middle gray encoded at 0.18. Floating point values exceed 1.0. |
| Normalized Linear | Gamma 1.0 with arbitrary mapping of signal bounded between 0.0 and 1.0. |
DCI-X'Y'Z'
DCI-X'Y'Z' is a device-independent container rather than a conventional RGB color space. Digital Cinema Initiatives1 selected it so a DCP can map directly to compliant projectors without a manufacturer-specific color conversion.
DCI-X'Y'Z' uses a 2.6 power encoding2 with headroom for multiple white points.
Use DCI-X'Y'Z' only as a display-referred digital-cinema delivery format, not as a working space. DCDMs and DCPs use this encoding.
Why the normalization constant is 52.37
SMPTE ST 428-1 normalizes X'Y'Z' by 52.37 rather than the 48 cd/m² reference luminance. This preserves 48 cd/m² while making room for D55, D61, and D65 white points. At the DCI projector white, Y' reaches code value 3960, leaving 135 values before the 12-bit maximum.3
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Hence X'Y'Z' (pronounced X-prime, Y-prime, Z-prime), prime implying a gamma, rather than XYZ. ↩
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SMPTE ST 428-1, Annex B.1, which works the calculation and reports the same three code values together with the 135-code-value headroom figure. The D55 / D61 / D65 rationale is the standard's own, from footnote 4 to the transfer-function equation. The 48 cd/m² reference luminance it preserves is specified in SMPTE ST 431-1, Table 5.2. ↩