[theora-dev] YUV question

illiminable ogg
Fri Jun 4 03:56:08 PDT 2004

----- Original Message -----
From: "Makc" <makc at ukrprombank.kiev.ua>
To: <theora-dev at xiph.org>
Sent: Friday, June 04, 2004 5:29 PM
Subject: [theora-dev] YUV question

> The statement I care to make here, is simply that there ain't neither
> such thing as "luminance", which details "the eye is more sensitive to",
> nor "chromaticity", which details the eye is less sensitive to. What we

Yes, it's true that luminance and chromaticity are somewhat abstract
notations. But it is true that the eye is more sensitive to luminance than
to chrominance. The eye has two major types of receptor cells, rods and
cones, rods are insensitive to colour, and are more sensitive to luminance
than the cones. This is a specialisation which leads to our relatively good
night vision. In low light the cones cannot oeprate properly, hence when we
are in low light we tend to see only the luminance as only our rods are
operating. Hence the feeling that when seeing in low light, things are kind
of in "black and white".

There are also around 10 times more rods than there are cones.

Of the cones there are three types, which detect the major chrominace
components. Interestingly only about 2% are sensitive to blue, but blue
perception is not as deficient as that would indicate, though it is the most
deficient of the three colours.

> said: "YUV color is used in... TV broadcasts... Only the Y component of a
> color TV signal is shown on black-and-white TVs". Besides TV standards,
> nothing holds you fron using something like Y = R/4 + G/2 + B/4, which
> would considerably speed up codec, or sticking with good old 12-bit RGB.

I know a bit more about visual perception than about video encoding (though
it's been a while), but it appears to me the selection of the coefficients
of the function relate to the sensitivity to the different colours. Green
tends to be overrepresented because the range of green sensitivity is about
30% wider than for either red or blue. Coupled with the fact that in the
electromagnetic spectrum green is between red and blue, hence has the
greatest overlap in all but the red and blue colour extremes.

I think you'd find if you just used those simple integer coefficients that
you would lose more valuable information than with the original
coefficients. The green is most imoprtant, that's why it's overrepresented
and the blue is least, that's why it's underrepresented.

As for 12 bit rgb, i'd imagine that it would not be of the same quality as
12bit yuv, for the reason stated in the articel you quoted. That that will
give equal importance to luminance and chrominance. I'm sure someone who
knows a bit more about the video side of it could give a better explanation.


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