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Converting Between R'G'B' and 4:2:2



TIG -

I wrote a white paper for Discreet Logic that explains the R'G'B' and
Y'CBCR 4:2:2 representations, and explains the technical aspects of
conversion between the two. I attach the abstract. The paper was
distributed at NAB, and is now available in its entirety at the ftp site of
Discreet Logic.

I think it will be of interest to those TIG members who exchange
imagery between the film, video, and CGI worlds.

C.

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Charles Poynton 
<mailto:poynton at poynton.com> [Mac Eudora, MIME, BinHex, uu, qpv]
<http://www.inforamp.net/~poynton/>
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MERGING COMPUTING WITH STUDIO VIDEO:  
CONVERTING BETWEEN R'G'B' AND 4:2:2

Charles Poynton
www.inforamp.net/~poynton

ABSTRACT

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 becominga 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.

[The entire paper is available in Acrobat PDF format, 130705 bytes.]

<ftp://www2.discreet.com/dl2/whitepaper/discreet_poynton_rgb_422.pdf>

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Thanks to Rich Torpey, Bob Lovejoy for support in 1998.
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1000 subscribers in 39 countries on Tue Jul 28 20:12:14 PDT 1998 
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