Apple Color 1.5 - Manual (Page 26)

As you can probably guess, more color information is better when doing color correction.
For example, when you make large contrast adjustments to 4:1:1 or 4:2:0 subsampled
video, video noise in the image can become exaggerated; this happens most often with
underexposed footage. You’ll find that you can make the same or greater adjustments
to 4:2:2 subsampled video, and the resulting image will have much less grain and noise.
Greater contrast with less noise provides for a richer image overall. 4:4:4 allows the most
latitude, or flexibility, for making contrast adjustments with a minimum of artifacts and
noise.
Furthermore, it’s common to use chroma keying operations to isolate specific areas of
the picture for correction. This is done using the HSB qualifiers in the Secondaries room.
(For more information, see
Choosing a Region to Correct Using the HSL Qualifiers .) These
keying operations will have smoother and less noisy edges when you’re working with
4:2:2 or 4:4:4 subsampled video. The chroma compression used by 4:1:1 and 4:2:0
subsampled video results in macroblocks around the edges of the resulting matte when
you isolate the chroma, which can cause a “choppy” or “blocky” result in the correction
you’re trying to create.
Despite these limitations, it is very possible to color correct highly compressed video. By
paying attention to image noise as you stretch the contrast of poorly exposed footage,
you can focus your corrections on the areas of the picture where noise is minimized.
When doing secondary color correction to make targeted corrections to specific parts of
the image, you may find it a bit more time consuming to pull smooth secondary keys.
However, with care and patience, you can still achieve beautiful results.
Film Versus Video and Chroma Subsampling
With a bit of care you can color correct nearly any compressed video or image sequence
format with excellent results, and Color gives you the flexibility to use highly compressed
source formats including DV, HDV, and DVCPRO HD.
Standard and high definition video, on the other hand, is usually recorded with lower
chroma subsampling ratios (4:2:2 is typical even with higher-quality video formats, and
4:1:1 and 4:2:0 are common with prosumer formats) and higher compression ratios,
depending entirely upon the recording and video capture formats used. Since the
selected video format determines compression quality at the time of the shoot, there’s
nothing you can do about the lost image data, other than to make the best of what you
have.
In general, film footage is usually transferred with the maximum amount of image data
possible, especially when transferred as a completely uncompressed image sequence
(4:4:4) as part of a carefully managed digital intermediate workflow. This is one reason
for the higher quality of the average film workflow.
26 Chapter 1 Color Correction Basics