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Re: Flame/Inferno Coloring

A correspondent to the Telecine list writes,

> in almost every Flame/Inferno suite there is ...
>   [conversion] ... from 4:4:4 RGB to 4:2:2 YUV

Or more properly, 4:2:2 Y'CbCr. 

Y'CbCr is scaled so that _each_ of Cb and Cr  have identical excursion. In Rec. 601, the reference excursion is +-112 centered on code 128, that is, codes 16 through 240 inclusive. (In PbPr scaling, the reference excursion is +-0.5, represented abstractly, or +-350 mV in SMPTE component analog video.)

Proper Y'UV is scaled so that the _combination_ of luma plus chroma makes a composite NTSC or PAL signal whose excursion is limited to +133 1/3 percent of the blanking-to-white range. 

In Y'CbCr, the scale factors are determined individually in each of Cb and Cr. In Y'UV, the scale factors are determined according to a combination of U and V. 

Computer guys - and even some broadcast guys - often use YUV to refer to any generic set of color difference components, no matter what the scale factors. However, for accurate color reproduction you need to get the scale factors right, and this goal is aided by naming them correctly. (Computer guys have trouble getting accurate color; this is one of the many reasons.)

On a closely related topic, in video we do not use luminance as defined and standardized by the CIE (and given the symbol Y). Instead, we use a nonlinear quantity Y', which I call luma. Most broadcast guys use the term "luminance," and the symbol Y, to refer to the video quantity. This is sloppy use of words and symbols.

I'll have more to say on these topics in a few weeks time.


Charles Poynton 
<mailto:poynton at poynton.com> [Mac Eudora, MIME, BinHex, uu, qpv]

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