Color in 1080p24 and Electronic Cinema: Converting between R′G′B′ and 4:2:2
Charles Poynton
An important challenge in the standardization of electronic cinema is the choice of a color representation. Among the possibilities are R′G′B′ and Y′CBCR. In this paper, I explore the relationship between the two. — Film, video, and computer-generated imagery (CGI) all start with red, green, and blue (RGB) tristimulus components proportional to intensity—“linear light.” A nonlinear transfer function is applied to RGB to give gamma corrected R′G′B′. This is the native color representation of video cameras, computer monitors, video monitors, and television. — The human visual system has poor color acuity. If R′G′B′ is transformed into luma and chroma, then color detail can be discarded without the viewer noticing. This enables a substantial saving in data capacity—in “bandwidth,” or in storage space. Because studio video equipment has historically operated near the limit of realtime capture, recording, processing, and transmission capabilities, the subsampled Y′CBCR 4:2:2 format has been the workhorse of studio video for more than a decade. — The disadvantage of 4:2:2 is its lossy compression. Upon “matrixing” from 8-bit R′G′B′ to 8-bit Y′CBCR, three-quarters of the available colors are lost. Upon 4:2:2 subsampling, half the color detail is discarded. But production staff are facing increasing demands for quality, and increasing demands to integrate video production with film and CGI. The lossy compression of 4:2:2 is becoming a major disadvantage.