4:2:0 - more than one type of Cb Cr sampling direction ?

[Movie-Maker]

Hi,
I stumbled upon something I read a few days ago but for the life of me I can't find it now. It said something to the effect that even within the 4:2:0 format, there are differences in the way Cb and Cr pixels are sampled. The general and most common idea found is that all pixels on all lines are sampled for Y, but Cb and Cr are sampled on alternate lines only. So all pixels on the first line would be only Y+Cb and the next line would be only Y+Cr and so on.

Looks like there is yet another way of sampling Cb Cr in 4:2:0. The entire line is either all Y or all Cb or all Cr. So all pixels on the first line would be all Y, the next line would be all Cb and the third line would be all Cr.

First of all, is this correct ? Are there such different 'directions' in which the Cb Cr sampling goes ?
If so then is there any standard for this ? Atleast theoritically it sounds like it should have some visual implications between the different types of sampling directions. I never seen such an option in any encoder to specify what type of direction to use for 4:2:0 encoding.

Lastly, is there any type of direction that alternates Cb Cr horizontally ? i.e. on a line all pixels would be sampled for Y and every alternate pixel, in the horizontal direction, would be sampled alternatively for Cb and Cr, alongwith Y.

[edDV]

Lastly, is there any type of direction that alternates Cb Cr horizontally ? i.e. on a line all pixels would be sampled for Y and every alternate pixel, in the horizontal direction, would be sampled alternatively for Cb and Cr, alongwith Y.

No, 4:2:2 ties both Cb and Cr to alternate Y samples. 4:1:1 ties both Cb and Cr to every forth Y sample.
This means Y, Cb, Cr samples are co-sited in 2d space.

4:2:0 associates Cb and Cr to pairs of vertical Y samples.

See here:http://www.pcmag.com/encyclopedia_term/0,2542,t=chroma+subsampling&i=57460,00.asp

[Movie-Maker]

so there is only one way of associating Cb Cr samples to Y samples in 4:2:0 ?
(never mind my comment on the last sampling scheme I was talking about)

[edDV]

so there is only one way of associating Cb Cr samples to Y samples in 4:2:0 ?
(never mind my comment on the last sampling scheme I was talking about)

As the link above shows, Mpeg1 4:2:0 sited Cr and Cb over four Y pixels. MPeg2 and other Rec601 based formats site Cb and Cr over alternate vertical pairs of Y samples.