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<td width="100%">this reffers to common terms for y'cbcr format<br>
which separates the 'luminance' information in an<br>
image from the chroma information, allowing the resolution of<br>
chroma information to be reduced, exploiting the lack of color<br>
acuity inherent in the human visual system.<br>
<br>
there are a couple of issues with the terms in this description<br>
as explained to me by charles poynton. firstly the common use of<br>
the term luminance for y' is a misrepresentation and a distinction<br>
should be drawn between this term and true cie luminance, for<br>
example by using the term "luma", secondly the u v terms for<br>
historical reasons are associated with adjusted levels as<br>
broadcast from television transmitters and digital systems<br>
actually use y'cbcr.<br>
<br>
so yuv should more accurately mean y'cbcr.<br>
<br>
the 4:1:1 denotes the degree of compression of chroma information<br>
and makes less sense with newer formats, suffice to say that 4:1:1<br>
stores full resolution y'(luma) but 1/4 resolution of cb and cr<br>
information sampled along the horizontal axis or line.<br>
<br>
y'cbcr 4:2:2 is actually a more commonly used professional<br>
format where chroma components are only halved along the<br>
horizontal axis or line.<br>
<br>
y'cbcr 4:2:0 is used for 1/4 resolution chroma but with half<br>
resolution horizontally and half resolution vertically.<br>
<br>
4:4:4 denotes equal resolution of luminance and chroma.<br>
<br>
to generate the standard cie luminance term, from an rgb triplet<br>
y = .2125r + .7154g + .0721b, this is *different* from the y'<br>
described above.<br>
<br>
luma as stored in these video formats is actually<br>
y' = .299r'+.587g'+.114b' where the r' g' and b' terms<br>
represent scalar values which are non linear and represent<br>
gamma corrected perceptually uniform values when represented<br>
by an integer (gamma == .45).<br>
<br>
chroma terms can then be produced from these valuse:<br>
cb = b'-y'<br>
cr = r'-y'<br>
<br>
to reconstruct an rgb image from a 'yuv4:1:1' you must<br>
first resample the single cbcr sample to 4 cbcr samples<br>
to match the luminance, once this is done you should<br>
regenerate rgb triplets by reversing the above arithmetic:<br>
<br>
b' = cb +y'<br>
r' = cr +y'<br>
g' = (y'-.299r'-.114b')/.587<br>
<br>
r' b' g' represent an rgb triplet with a built in gamma<br>
correction of .45, this should look perfect when displayed<br>
on a machine with no gamma correction and a crt display<br>
(which typically have gamma transfer functions of around<br>
2.5-3.0)<br>
<br>
note that because chroma information may be negative, the<br>
cb cr values stored in any file format mey have to be adjusted<br>
accordingly. for example you would subtract 127 from an 8 bit<br>
chroma value stored in a file.<br>
<br>
don't use the luminance equation above, it is included to show<br>
the difference between luminance and luma or y' which is<br>
actually stored in the file.<br>
<br>
for more in depth information read charles a. poyntons<br>
excellent book "a technical introduction to digital video"<br>
pub, john wiley & sons, inc. isbn 0-471-12253-x<br>
<br>
cheers,angus.<br>
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