Apple Color 1.0 - Manual (Page 26)

26 Chapter 1 Color Correction Basics
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. These keying operations will have smoother and less noisy edges when you’re
working with 4:2:2 subsampled video. The chroma compression used by 4:1:1 and 4:2:0
subsampled video results in “blockiness” when you isolate the chroma, which affects
the mattes that are created by the HSB qualifiers.
However, it’s important to bear in mind that it is definitely possible to aggressively
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.
Bit Depth
Another factor that affects the quality of video images, and can have an affect on the
quality of your image adjustments, is the bit depth of the source media you’re working
with. With both RGB and Y´C
B
C
R
encoded media, the higher the bit depth, the more
image data is available, and the smoother both the image and your corrections will be.
The bit depth of your source media depends largely on how that media was originally
acquired.
Film Versus Video and Chroma Subsampling
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.
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 then to make the best of
what you have.
That said, 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.