Why don’t RGB color spaces cover the entire CIE 1931 diagram?
Asked 10/16/2015
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On a CIE 1931 chromaticity diagram, an RGB gamut such as sRGB only covers part of the visible colors, often shown as a triangle inside the full horseshoe-shaped area. Why can’t a color space simply include all visible colors, and why are some colors left outside spaces like sRGB?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
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sRGB is a color-space developed by HP and Microsoft in 1996. CRT monitors were common and therefore sRGB was based on the characteristics of these monitors' capabilities. A good write-up of the history and reasons can be found in sRGB: a short history.
The chromaticity coordinates and available colors were chosen on what the phosphors used in CRTs could produce back then. Consider that neither prints nor TFT or CRT monitors can replicate the full visible light spectrum.
A Program on a PC or camera that wants to control a monitor will use discrete values. If you use a larger color space, steps between different colors get coarse unless you use a larger datatype (Example: Adobe RGB with 8 bit). Whereas image information in a larger color space with a larger datatype uses more memory and needs more processing power (Example: Adobe RGB with 16 bit). This digital value will be transformed into an analog signal (usually a voltage) at a certain stage and then to something visible (for CRTs: a phosphorescent screen excited by accelerated electrons).
The resolution for converting a digital input to an analog signal is a further limit due to cost, size and technology.
Therefore fitting sRGB to CRT monitors back then allowed for a good resolution between colors while minimizing hardware requirements.
Originally by user44829. Source · Licensed CC BY-SA 4.0
user44829
10y ago
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Because a practical RGB color space is limited by both physics and device technology.
The full CIE 1931 diagram represents colors humans can perceive, but displays, printers, and cameras cannot reproduce all of them. Any color space based on three real RGB primaries forms a triangle on that diagram, while the visible locus is not triangular, so a triangle cannot cover the whole shape.
To cover more area, you would need primaries farther apart, but that creates tradeoffs. Real primaries must be physically achievable by monitors, printers, or sensors. sRGB was designed around what common CRT displays could produce and became a practical standard.
There is also a precision issue: if you make the gamut larger but keep the same bit depth, the numerical steps between colors get bigger, which can increase banding or reduce smooth gradation. A wider gamut often benefits from higher bit depth and more memory/storage.
Some mathematical color spaces can use “imaginary” primaries outside the visible range for computation, but physical devices still cannot display those directly.
So colors are not intentionally omitted for no reason; color spaces are compromises between visible range, real-world hardware limits, compatibility, and precision.
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