Image Processing Reference
In-Depth Information
Table 5-7 Luma-Related Terminology
Original red, green, and blue source signals
Component analog video values. Matrixed luma Y' plus the two
color-difference signals, which have been analog filtered to about half
the luma bandwidth
Component digital video values. Matrixed luma plus scaled
color-difference values
The blue and red difference components are combined and further
down-sampled before being mixed with the luma signal to form the
NTSC or PAL broadcast signal. This is decoded back into component
values before your compression system operates on it.
A legacy NTSC coding scheme that is obsolete but is referred to in older
the red and blue channels. Provided the green channel is left alone, the effects are very
hard to distinguish from the original, even if the blue channel is severely blurred. The
most minimal blurring of the green channel is immediately visible. Reducing the detail in
the red and blue channels also reduces the overall detail in the brightness channel and that
is not desirable.
Changing the color space in the Photoshop application to LAB demonstrates the sep-
aration of luma and chroma. The luma channel is a monochrome image. The A and B chan-
nels are the chroma. They will withstand considerable blurring without affecting the
apparent quality of the picture. There is no doubt that it is affected, because some infor-
mation has been discarded, however, the human eye is designed to cope with this sort of
degradation so the perceived effects are minimal.
You can use this knowledge to get better results when cleaning up the images before
compression. If you want to sharpen the image, only apply the sharpening to the luma
channel and not to the RGB image. That way you will avoid getting blotchy effects on skin
What Does 4:2:2 Notation Mean?
If the luma and the two color-difference channels (chroma) are coded into a digital form,
acceptable images are produced by sampling less information from the chroma. This is
done by sampling a smaller number of bits-per-pixel or fewer pixels across the sampled
area's chroma channels. The distribution of this sampling is not immediately clear from
the naming scheme.
The first digit, “4,” is an historical convention that means the sample rate is approx-
imately 4 times the frequency of the color sub-carrier signal. These samples are taken of
the luma signal and are considered to be one unit of sampling in the horizontal axis.
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