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RE: Converting Between R'G'B' and 4:2:2 (The Impossibly Blue Sky)





> -----Original Message-----
> From:	Charles Poynton [SMTP:poynton at poynton.com]
> Sent:	Saturday, August 01, 1998 1:50 PM
> To:	multiple recipients of
> Subject:	Re: [TIG] Converting Between R'G'B' and 4:2:2 (The
> Impossibly Blue Sky)
> 
	Charles Poynton writes: 

> Digital video ( la Rec. 601)
> accommodates headroom and footroom equivalent to about 7% below and 9%
> above full scale.
	 ... 
>  In video and film production, part
> of what we do involves art. But we wish to distribute our art to the
> viewers in a manner that allows us to predict what colors the viewers will
> see.  
> 
	Charles,

	If I had only described the problem of converting 109% blue (or red
or green) from Y'CbCr into the unforgiving, no-headroom world of Discreet's
R'G'B', my point would stand: some colors are lost (clipped). You listed
black level concerns in the "Practical Suggestions" of your R'G'B'-4:2:2
white paper but did not mention the headroom danger. That is concern #1
regarding that topic, on my block.

	Although 160% blue sounds obscene, I contend that the resulting
color IS predictable on the consumer sets that I have unscientifically
tested. (some practice, no theory) If you are still asking "but WHY
bother?", then I can only refer you to commercial people who demand that the
envelope be pushed.

	If you dismiss those impossible colors as heresy, then you may not
be interested in the discussion of phosphor limitations. For example, why is
it that the ATSC did not address the fact that NTSC red is actually
reddish-orange in Kelley's color-designation regions on the CIE chromaticity
diagram. (and to my eyes) Since film dyes can occupy a larger area on the
diagram, wouldn't this be a perfect time (infancy of DTV) to make up some of
the difference, or is spatial and temporal resolution everything? Since we
are stuck with tristimulus values to convey perceptual colors, let us not
forget those truly lost colors: tristimulus values outside of the NTSC
phosphor triangle.
>  
	Randy Reck
	Encore Santa Monica

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