Merging Computing
with Studio Video:Converting Between R'G'B' and 4:2:2
Merging Computing
with Studio Video:Copyright © 1998-02-12 Charles Poynton
In this paper, I explain the R'G'B' and Y'CBCR 4:2:2 representations, and explain the technical aspects of conversion between the two. I conclude by suggesting steps that can be taken during production and post-production to avoid difficulty with the conversion.
Film, video, and computer-generated imagery (CGI) all start with red, green, and blue (RGB) intensity components. In video and computer graphics, a nonlinear transfer function is applied to RGB intensities 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 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 it's lossy compression. Upon conversion 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.
Owing to the enormous computing and storage capacity of general-purpose workstations, it is now practical to do production directly in R'G'B' (or as it's known in studio video terminology, 4:4:4). To integrate traditional studio video equipment into the new digital studio, conversion between R'G'B' and 4:2:2 is necessary.
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