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- [23] Usenet echoes (21:200/1) -------------------------------- COMP.GRAPHICS -
 Msg  : 5 of 34                                                                 
 From : R18926@waccvm.corp.mot.com          2:5030/144.99   22 Mar 94  12:19:00 
 To   : All                                                                     
 Subj : Re: Does YCbCr = YUV?                                                   
--------------------------------------------------------------------------------

      Lee_Walker_at_712@ccmail.gsfc.nasa.gov (Lee Walker)
in message: <2mkrpj$1pc@paperboy.gsfc.nasa.gov>

wrote:-

LW> Is the YUV color model the same thing as the YCbCr color
LW> model?  I can't seem to find any solid references on
LW> anything  other than YUV. (I don't have access to CCIR 601).
LW>
LW> How do I go from YCbCr to RGB?

The answer I'm afraid is a definite YES and NO!!

The YUV colour space description is for ANALOGUE encoded video
using the PAL system.

        Y =   0.299 * R + 0.587 * G + 0.114 * B
        U = - 0.147 * R - 0.289 * G + 0.437 * B
        V =   0.615 * R - 0.515 * G - 0.100 * B

Notice that   U = 0.493 * (B-Y)  &  V = 0.877 * (R-Y)

These equations are actually based on a hypothetical transmission
system that uses the NTSC primaries and Illuminant C. A real PAL
system uses a reference white of Illuminant D and PAL primaries
but the equations stuck!!

YCbCr is the DIGITAL representation of B-Y & R-Y using the following
equations:-

              Y =   262 * R + 514 * G + 100 * B + 64
             Cb = - 151 * R - 297 * G + 448 * B + 512
             Cr =   448 * R - 375 * G -  73 * B + 512

where R, G & B are taken as ranging from 0 - 1023 {10 bit}
and Y ranges from 64 to 943 inc and Cb & Cr range from  64 to 960 inc.

To go back from YCbCr to RGB do the following:-

              R = 1023 * Y + 1434 * Cr
              G = 1023 * Y - 352 * Cb - 731 * Cr
              B = 1023 * Y + 1807 * Cb

You might like to look at:-

                Colour Science in TV and Display Systems
                by W N Sproson - ISBN 0-85274-413-7

and you will find a very useful copy of CCIR 601 in the Philips
Semiconductors Desktop Video Data Handbook {1992} pp 2-29 to 2-38 inc.

Hope that this helps,
John

John Baraclough       email: r18926@waccvm.sps.mot.com
Motorola Ltd
East Kilbride
GLASGOW  G75 0TG
Scotland

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