In the prior installments, I introduced the idea that different color spaces have different characteristics that grossly or subtly affect the look of an image on film or video. Every type of color film stock, whether for use as a camera negative or as a positive print, has its own associated color space, meaning the gamut of colors it can actually render. The same is true of high-definition video as opposed to standard-def video; each of the two uses a (very slightly) different color space.
|The CIE XYZ gamut|
In the diagram, according to its caption, "the innermost cube represents the colors that can be reproduced by a particular RGB display device." A television or video projector, whether high-def or standard-def, is just such an "RGB display device."
Furthermore, the AC caption continues, "the curved surface illustrated by colored contour lines represents the boundary of colors visible to human beings; that it lies outside the RGB cube shows that many [visible] colors cannot be reproduced by an RGB device." That much is fairly self-explanatory, if disappointing.
As f0r "the outermost figure, the white wireframe rhombohedron ... represents the colors that can be specified by the CIE XYZ primaries." The X, Y, and Z primaries, as defined by international standards of the CIE (Commission Internationale de l'Eclairage), define an all-encompassing color space. To some subset of XYZ any "lesser" color space, such as RGB, can be mapped. XYZ's compass is so large, it even includes "colors" beyond those the eye can actually see.
So the XYZ rhombohedron has many more colors (visible and otherwise) than the RGB cube has. (A "color" is some combination of "hue," such as orange or teal; "saturation," such as vivid orange or muted teal; and "value," such as light vivid orange or dark muted teal.) In particular, a range of extremely saturated colors that the eye is capable of seeing and responding to lie outside the RGB cube.
Color negative and print film uses a differently positioned cube, for a different color space. Film's color space is represented by a cube whose axes are C, M, and Y, not R, G, and B. C (for cyan), M (f0r magenta), and Y (for yellow) are the secondary colors or subtractive primaries. Each can be made by adding exactly two of the additive primaries: R (for red), G (for green), and B (for blue) in equal amounts. For example, adding equal amounts of R (red) and B (blue) produces M (magenta).
Color film uses a strategy of subtracting CMY secondary colors from white light, whereas TV and video use a strategy of adding RGB primaries to make white light. That's why film uses a CMY color space whose cube does not necessarily match RGB's in the diagram above.
(Actually, film uses a CMYK color space, where K stands for "blacK." By itself, CMY does not produce convincing blacks. So an independent "primary color," K, is used as a layer in color film stock to make blacks more solid and convincing. In what follows, I will simply ignore K.)
The CMY cube, were it to be drawn into the diagram, would (for one thing) have a whole different orientation than the RGB cube has, with its corners in different places than those of the RGB cube. For another thing, the primary-color chromaticities — the x and y coordinates of the specific cyan, magenta, and yellow hues that are used to define the CMY cube — make for a CMY cube has a different color gamut than the RGB cube.
If the CMY gamut were wider than RGB's, as it apparently is, that would mean that some of the colors available in CMY as used in film don't "make the cut" when film is transfered to video. Those colors would instead be represented by colors available in the RGB gamut which approximate the original colors.
Furthermore, for like reasons having to do with the two color-space gamuts not overlapping precisely at their edges, there may be colors available in RGB that can't be accurately reproduced in CMY.
In the view of Ed Milbourn at HDTV Magazine, as stated in "Ed's View - The 'Film Look'," today "the video color gamut is somewhat greater than that of film." (Here, Milbourn is not referring to the old, highly saturated look of classic Technicolor.) "This," he continues, "gives video the capability of reproducing a slightly wider range of colors than film, adding to the color 'snap' of HDTV video. All of these factors (and a few others) combine to give film color a more muted, realistic look than video."
So HDTV color is not, and cannot be, as cinematic as it would be if it used the CMY color space common to color film stocks instead of the narrower-gamut RGB color space upon which video is standardized!
More on HDTV color in Part 4 (which I may not get to for a while).