Color values in digital cinema are distributed in the form of tristimulus values of red, green, and blue (R, G, B). The container color space referenced by the RGB values is a normalized form of the CIE XYZ color space, defined in SMPTE ST 428-1 Image Characteristics and referred to as the X’Y’Z’ color space. Container color space refers to the reference color space in which the RGB values reside. In practical terms, this means that the RGB color values in digital cinema distributions can describe any color visible to the human eye. This has substantial value in that the distribution data format will readily accomodate emerging projectors and displays with wider color gamut.
Digital cinema displays, however, have practical limitations. Cinematographers and colorists need to know the minimum set of colors that will be correctly reproduced in the cinema. A minimum display gamut is needed for digital cinema projectors and displays, and this minimum gamut is DCI-P3, defined in SMPTE RP 431-2 Reference Projector. DCI-compliant projectors and displays must be capable of reproducing the DCI-P3 color gamut at a minimum. (This color space is shown in the next section.)
Digital cinema is unique in its specification of an all-encompassing color container in distribution, accompanied by a minimum color gamut for presentation. Likewise, digital cinema is also unique in its architecture for color presentation. In practice, the X’Y’Z’ color values in the Composition must be transformed to the color gamut of the projector or display. To do this properly, the color primaries of the projector or display must be measured by a technician and entered into a computational module within the display device. The module actively transforms incoming X’Y’Z’ color values to those required of the actual color gamut of the device. (See the illustration below.) The result is a perfect display of color as the colorist intended. This level of precision is only available in digital cinema.
Figure P-0. Color Transformation in Cinema