Excellent explanation.

I'd like to add that no light source can lie outside the horseshoe of the CIE xyz diagram: pure wavelengths are points on the curved line, everything that mixes them moves towards the inside of the space. So you're stuck with triangles that fit within it.

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layer8 2 hours ago | parent | next [-]

I was wondering about that, since some color spaces have their primaries outside the horseshoe. Thanks for clarifying.

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extesy an hour ago | parent | prev [-]

What does it mean then for ProPhoto RGB triangle to be outside of the horseshoe on that diagram?

	▲	AprilArcus an hour ago \| parent [-]
		It uses nonphysical "imaginary" primaries that have meaning within the coordinate system but not within the human perceptual system.