[This article was first published in the March, 2005, issue of
Larry's
Final Cut Pro Newsletter.]
One of our on-going discussions is the issue of color sampling; which
we describe using terms like 4:1:1, 4:2:2, 4:4:4 and, with DVD and
HDV, 4:2:0.
And, I thought I pretty well understood what these meant. I should
have known better, because someone who DOES know better wrote a nice
article illustrating the difference.
Allow me to present Graeme Nattress (http://www.nattress.com) with a short and highly lucid description of what all the numbers mean.
(For those of you who worry about such things, "colour" is
the Canadian spelling...)
Color Terminology
Component digital video is often referred to as YUV, but this is not
an accurate description of component digital video, but instead refers
to a set of intermediate qualities used in the formation of analogue
composite video. The correct terminology for the components of digital
video is Y' Cb Cr.
The Y' represents luma, the ' is important, reminding us that it is
non-linear (gamma corrected). The Cb and Cr are chroma components that
are often wrongly referred to as U and V. The Cb is calculated as B'
- Y', and Cr is R' - Y'. If you are interested in more details, check
out Charles Poynton's excellent articles at http://www.poynton.com.
Chroma Sampling
The CCDs that capture our image inside our video camera, capture colour
by the use of Red, Green, and Blue filters. The phosphors in our CRT
monitor, and filters in our LCD monitor and DLP projector also use
Red, Green and Blue filters.
Why is it then, that digital video is stored and manipulated as separate
luma and chroma components? Separate luma and chroma are used so that
the resolution of the chroma can be reduced with respect to the resolution
of the luma so that large savings can be made in the amount of data
that needs to be transmitted -- it is a form of compression. This reduction
in the resolution of the chroma components works perceptually as our
human vision systems are not as able to see fine details in colour
as we are in lightness.
There are a number of methods of chroma sampling, and they each have
terminology that refers to the chroma resolution (the second and third
numbers) as compared to the luma resolution (first number).
4:4:4
Full resolution luma is represented by the number 4, and as the chroma
components Cb and Cr are also 4, there is no reduction in resolution.
4:4:4 sampling is mostly used for RGB images, although it can be used
for Y'CbCr, although no camera records 4:4:4 Y'CbCr.
4:2:2
Full resolution luma, and half (2/4 = 0.5) resolution horizontally
on the chroma components. This is the traditional broadcast standard
for chroma sampling and is used by DigiBeta, DVCpro50 etc.
4:1:1
Full resolution luma and quarter (1/4 = 0.25) resolution chroma components.
This is the system used by NTSC DV and PAL DVCPro.
4:2:0
Full resolution luma, and half resolution in the horizontal direction
and vertical direction for the chroma components. 4:2:0 is a very complex
chroma sampling with many variants depending on whether the video is
progressive or interlaced, or if it is being used by PAL DV or MPEG2.
4:2:0 compresses the resolution of the colour to 1/4, just like 4:1:1
compresses the resolution of the colour, but whereas the compression
in 4:1:1 is horizontal only, the compression in 4:2:0 is horizontal
and vertical.
This picture, above, illustrates PAL DV 4:2:0 chroma sampling.
This article is from "Larry's Final Cut Pro HD Newsletter," a
very cool, FREE, monthly, Final Cut newsletter -- subscribe at Larry's
web site: www.larryjordan.biz.
Larry Jordan is a post-production consultant and an Apple-Certified
Trainer in Digital Media with over 25 years experience as producer,
director and editor with network, local and corporate credits. Based
in Los Angeles, he's a member of both the Directors Guild of America
and the Producers Guild of America.
Any references to trademarks or products are used for editorial purposes
only. Text copyright 2005 by Larry Jordan. All rights reserved.
Larry Jordan is a post-production consultant and an Apple-Certified Trainer in Digital Media with over 25 years experience as producer, director and editor with network, local and corporate credits. Based in Los Angeles, he's a member of both the Directors Guild of America and the Producers Guild of America.
The information in this article is believed to be accurate at the time of publication. However, the author assumes no liability in case things go wrong. Please use your best judgment in applying these suggestions.
The opinions expressed in this article are solely those of the author. This newsletter has not been reviewed or sanctioned by Apple or any other third party. All trademarks are the property of their respective owners and are mentioned here for editorial purposes only.
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