Colour space. Gamut is not gamma.

A colour space gives coordinates a defined colour meaning. RGB is only one model: XYZ and Lab are not specified by three display primaries in the same way. An RGB space needs primaries and a reference white; nonlinearly encoded coordinates also need a transfer function. Describing an entire file additionally requires its range, bit depth, RGB/YCbCr representation and metadata.

Red, green and blue primaries have specific CIE xy coordinates. Connect them on a chromaticity diagram and you obtain the triangle of possible positive RGB mixtures. Rec.709/sRGB, Display P3 and Rec.2020 differ primarily in the positions of these points, so the same RGB triplet can represent different chromaticities.

Gamut is the set of colours a system can represent or reproduce under specified constraints. An RGB triangle in CIE xy describes chromaticity limits for nonnegative mixtures of the primaries, but omits luminance. Its area is not a percentage of all perceptible colours.

A fuller description needs value-range and luminance constraints. A floating-point RGB basis can retain negative components and values above 1; this is not the same as a monitor’s physical gamut. Wide working primaries do not expand the display by themselves.

sRGB, Rec.709, Display P3 and Rec.2020 use D65 white. Display P3 is not interchangeable with every “P3”: theatrical DCI-P3 has a different reference white and transfer characteristic. ACES white is often called D60, but D60-like is more precise: its coordinates are separately specified and do not equal exact CIE D60. Matching white-point chromaticities also do not imply identical display spectra.

An encoding function determines the numbers used to store linear RGB components. It does not move the primaries. Standard sRGB and Display P3 share a piecewise function but have different primaries, so a shared function does not mean a shared space.

The lab deliberately separates target primaries and encoding. P3 primaries plus a 2.4 power is a teaching combination, not standard Display P3. During conversion, changing encoding changes output codes while preserving XYZ before preview clipping. Reassigning the same codes changes interpretation.

Rec.709 can be encoded as YCbCr 4:2:0 10-bit video range, RGB full range or a floating-point buffer. These can describe the same colour system while storing numbers very differently. Codec, bit depth, chroma subsampling and range belong to encoding and transport; they do not replace a colour-space description.

A colour-space name defines how coordinates should be interpreted. If a Rec.709 signal is merely labelled Rec.2020 without conversion, the system does not create new real colours; it reads the same numbers in a different coordinate system and the image becomes wrong. A proper conversion requires a transform, not just metadata relabelling.

Camera space conveniently describes a specific sensor and encoding. A working space should provide headroom for grading and predictable math. An output/display space constrains the result for a particular display or delivery standard. A good colour-managed pipeline does not force one space to perform all three roles equally.

A conversion between RGB bases with known primaries decodes the input function, passes through colourimetric coordinates and encodes the output. Different white points may require chromatic adaptation. This experiment uses shared D65, so no additional adaptation is applied.

Assignment is a different operation: it gives the numbers another description without that conversion. Appearance may change even while the codes stay unchanged. A correct conversion can preserve XYZ without clipping; a physically limited output requires a separate gamut-mapping choice, not necessarily simple RGB clipping to 0 and 1.

RGB is a colour model. A gamut is the available range of colours. A colour space defines a particular coordinate system and reference conditions. An ICC profile describes a device or standard space together with transforms used for colour management. Casual speech often mixes these terms, but pipeline diagnosis requires keeping them separate.

Input Colour Space answers 'which RGB geometry are these numbers in?', Input Gamma asks 'how are they encoded relative to light?', Output Colour Space asks 'which RGB geometry should they become?', and Output Gamma asks 'how should the result be encoded?'. Once those four answers are known, most of CST stops being mysterious.