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



Charles Poynton writes:
> 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. 
	[Randy K. Reck]  

	Charles,

	It concerns me that your otherwise informative white paper
(presented to Discreet) contains misleading statements and one very large
omission.

	It is misleading to state that three-quarters of the available
colors are lost upon conversion from R'G'B' to Y'CbCr. If the entire 601
R'G'B' color space cube is contained within the larger Y'CbCr cube, then a
conversion from Y'CbCr to R'G'B' with the same bit-depth will indeed result
in significant quantization error in all 3 dimensions. However, to use
phrase "lost colors" when merely referring to this missing precision WITHIN
the R'G'B' space confuses this problem with an even greater problem existing
in my little world: there are colors (in whole chunks) that are truly lost
going in the OTHER direction- Y'CbCr to R'G'B' !

	For example, when a 601 Y'CbCr color corrected telecine output goes
to the CGI domain (i.e. SGI Flame), only those Y'CbCr colors that fall
within the smaller R'G'B' color space are passed. You CANNOT assume that all
remaining Y'CbCr codewords are invalid colors (in everyone's ballpark).
There are colors overlooked here such as very saturated, bright reds and
even more saturated, bright blues. On an RGB waveform monitor, one of these
colors might show up as 100% red channel, 100% green channel and 160% (sic)
blue channel. Invalid, you say. Ah, but what a beautiful color for a sky, I
say. Not only can I view that color in an expensive telecine suite, I can
encode it "legally" to NTSC composite, broadcast it, and watch it on my old
20" consumer set at home. It seems that analog component levels well above
700mV hitting my cheapo TV or expensive Sony Evergreen have the same
pleasant effect: Impossibly blue skies.

	If one were to send such "invalid" Y'CbCr colors from a telecine to
a Flame, they would get violently chopped to the boundaries of that puny
R'G'B' cube. My impossibly blue sky would be white. Overly-saturated red
regions on faces might turn to solarized white splotches. In Quantel's world
(Y'CbCr), this would not happen. The popularity of FFFI systems have
actually led to restricted (read LOST) colors from telecine color correctors
when "legal RGB" limiting must be used. I believe that this is the message
that the folks at Discreet should have received in addition to your other
points.

	Randy Reck
	Humble Telecine Guy
	Encore Santa Monica


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