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




-----Original Message-----
From: Randy K. Reck <rreck at encorevideo.com>
To: Charles Poynton <poynton at poynton.com>; multiple recipients of
<telecine at alegria.com>
Date: den 31 juli 1998 02:11
Subject: RE: [TIG] Converting Between R'G'B' and 4:2:2 (The Impossibly Blue
Sky)

> 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' !

Randy, well put.

But Charles has a point,  but beeing so succint he might get into a fight...

This is a common way of thinking - it is VERY important to understand in
which form the source originated in. Colour difference has a circular color
volume and RGB is cubical. If a transformation goes from YCbCr to RGB then
the output will be a RGB cube INSIDE the YCbCr cylinder. If a transform is
made from RGB to YCbCr then the output will be a cylinder INSIDE the RGB
cube.

But there is a twist. The colour diffrence format can allow any saturated
color at any luma level, RGB can not. This is in my mind real important as
our eyes sense in RGB but brain reads (psychovisual perception) it color
difference like.

I hope it will not start the old Spirit thread again but its luma/chroma
array is not there by coincidence.

/Mike Reichel




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