Sorry about the double-post, wrong key sequence.
I kind of get the feeling this thread is in need of some
clarification regarding RGB, YUV, 4-1-1, 4-2-0, 4-2-2 and 4-4-4. I'll
threat you all as idiots and try to explain it from scratch.
A digital image is built up by pixels.
If the picture is in gray-scale you only need one number to define
the tone of each pixel. The defining number is somewhere in a
predefined range, the greater the range the more possible tones. If
the range is only 0 and 1 you can define only black and white. If the
range consists of more than about 200 possible variations you can
define a seemingly unbroken grayscale between black and white (8 bits
equals 256 variations). If you need to make vast manipulations of the
tones of your pixels you might need an even greater range of possible
tones in your original pixels (that's why there is such things as 16
bit and 32 bit grayscale images).
In a RGB-format image the color is defined by 3 grayscale channels
(that is three separate grayscale images really), one for red, one
for green and one for blue. It is also possible to ad other channels
to an image, such as alpha-channels for transparency or z-channels
for depth or any other channel you find useful. Every pixel in your
image could be defined by 1, 3, 4, 5 or even more channels, each
consisting of 1, 2, 4, 8, 10, 16, or 32 bits.
YUV also contains 3 channels, but the luminance (brightness)
information (Y) of the image is separated from the color information
(u and v). The human brain is much less sensitive for changes in
color than for changes in luminance. Therefore if the color
information is separated from the luminance you can save space by
removing some of the information in the color-channels while
retaining approximately the same visual qualities. 4-1-1 means you
have only one pixel in every color channel per pixel in the luminance-
channel (4-2-0 is a variant where the color-pixels alternate between
u and v every frame (or field?), the compression is the same). 4-2-2
means 1 color-channel pixel per 2 luminance pixels. In this respect
RGB is actually always 4-4-4.
4-2-2 or 4-1-1 reduces space in a great way, but if you need to make
manipulations that moves information from color to luminance the
method might be a rather bad idéa. Worse even, if you need to use
color-information for creating an alpha-channel (blue-screen/green-
screen f.ex.) you might experience serious loss of quality.
regards, Pär