Cinematography-Theory-And-Practice

Figure 9.15. Component outputson the camera: Y, Pr, Pb.ColorspaceCameras can render the colors of a scene differently. One of theprimary factors is the colorspace being used. Colorspace is a videostandard that defines how the colors will be handled — differentcolorspaces render the colors of a scene slightly differently. Somecameras are capable of using any one of a number of different colorspaceconfigurations; these can be selected in the camera menus.There is no agreed upon standard, but Rec 709 (a colorspace definedby SMPTE) is frequently used in HD.Measuring Color Space on the VectorscopeThere are many different colorspace systems and notations used toderive and define color video, as we discussed earlier in this chapter.Figures 9.22 and 9.23 show the same test chart in two differentcolorspaces. You can see how a different colorspace changes colorrendition of the image.Color Difference Signals: B-Y and R-YProcessing all information as R, G, B is inefficient because eachseparate channel contains both color information and grayscale(luminance) information, which is redundant. As we recall from thechapter on color, black-and-white (grayscale) actually conveys thegreat majority of information about an image. Engineers realizedthat there was no need to repeat the black-and-white information forevery channel. For this reason, most video systems distill the chromainformation into color difference signals. Luminance is notated as Y,since B already stands for blue.There are many systems in use, but basically, color difference isderived by taking the blue component and subtracting the luminance(grayscale) information: B-Y (blue minus luminance) andfor the red channel, luminance is subtracted from red: R-Y. This iscalled component video. This is abbreviated in various ways; on thecamera in Figure 9.15, it is Y, PB, PR.Encoded ColorOne characteristic of human vision is we can’t see fine detail nearly aswell for changes in color as we can for changes in luminance. In otherwords, the picture won’t suffer very much if we reduce the bandwidthof the color components, provided we can maintain essentially fullbandwidth of the luminance signal (bandwidth is the rate of informationflow). Even a full bandwidth luminance signal doesn’t havevery much energy in the upper end of its spectrum; the higher-frequencysignals are quite a bit lower amplitude almost all the time.HD cinematography161