Re: [video] Re: question

[Richard Herveille <richard@asics.ws>]

> > > Hi Richard,
> > >
> > > Really thanks for your reply,
> > >
> > > For RGB converted into YUV,I see following equation,
> > >     Y = 0.2999R + 0.587G + 0.114B
> > >     U = 0.492(B-Y)
> > >     V = 0.877(R-Y)
> > > I am not sure you implement the design by this formula or anything else
> > > ?
> >
> > YUV is the term used for analog signals, YCrCb is the term for the
> > digital color space. They are not the same!!!
>
> I am really confused,in the book of "Video Demystified(2nd edition)"
> chapter 3 Color spaces, YUV color space,seems similar with the term of
> YCrCb,it says "YCbCr is a scaled and offset version of the YUV color
> space.Y is defined to have a nominal 8-bit range of 16-235;Cb and Cr are
> defined to have a nomainal range of 16-240." 

Ah you've got the bible. Makes life much easier.

You are right YCrCb is a scaled and offset version of YUV. But YUV is analog, 
YCrCb is digital. The book is confusing about this part, because it sometimes 
expresses the analog values in pure digital numbers. YUV is used by composite 
video, in that it is analog. YCrCb is defined in the ITU-R BT.601 specs for 
digital video, in that it is digital.

In the modern video world there's a very big grey area, therefore the book 
treats YUV and YCrCb in a similar fashion.

> Also the above equation is
> also from that chapter,only RGB has been gamma-corrected.Could you clarify
> for me ? Thanks,

There are a number of conversion formulas, mostly depending on what kind of 
system you are targeting. The formulas for NTSC and PAL (and all their 
variations) are slightly different. Choose the one that best fits you 
application.

Richard


>
> > > For Y/C generation,I think it need first create YCbCr format color
> > > space,
> >
> > YCrCb to YUV:
> > Y=0.591(Y-64)
> > U=0.504(Cb-512)
> > V=0.711(Cr-512)
> >
> > RGB to YUV
> > Y=0.151R + 0.297G + 0.058B
> > U=-0.074R - 0.147G + 0.221B
> > V=0.312R - 0.261G - 0.051B
> >
> > >and also there is some formula as the above one could create YCbCr
> > > signals,
> >
> > Just use the block available from the video_systems directory. It's in
> > the common subdirectory.
> >
> > >but how YCrCb convert into composite signal is still unclear for
> > > me,could you tell me how/where I can get the reference/document for
> > > this algorithm,
> >
> > The colors are qam modulated, so you need to generate this modulation
> > scheme. You need a sine/cosine translation to do this, I would suggest
> > using the cordic algorithm (also available from OpenCores) to do this.
> > Besides the colors you need to generate sync and blanking signals.
> >
> > I would suggest buying a good book (e.g. Video Demystified), or
> > outsourcing this project (I can help you with this).
>
> Really thanks for your help!
>

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