DCI-P3: the cinema colour space

DCI-P3 defines a set of red, green and blue primaries and a white point. A compliant cinema projector must be able to reproduce that gamut at the reference brightness. The picture inside a DCP is not actually stored as P3, though: it is encoded in X'Y'Z', a device-independent space that can describe any colour the eye can see. The projector's job is to map that onto its own P3-or-better primaries.

That is why the same DCP looks the same on two properly calibrated projectors from different manufacturers — and why a badly calibrated one is the single most common reason a picture looks "off" in one room and not in another.

Two practical things. First, alternative content — a concert, a live broadcast, an advert made for television — is usually graded in Rec. 709 and has to be converted properly on the way into a DCP; if it is not, colours will be oversaturated or dull on the big screen. Second, calibration drifts, particularly as a lamp ages or a laser module wears. A yearly check against the reference is cheap insurance for the picture you are charging for.