Apple Color 1.5 User Manual
Page 25
The sampling ratio between the Y
′
, C
B
, and C
R
channels is notated as a three-value ratio.
There are four common chroma subsampling ratios:
• 4:4:4: 4:4:4 chroma subsampled media encodes completely uncompressed color, the
highest quality possible, as the color difference channels are sampled at the same rate
as the luma channel. 4:4:4 subsampled image data is typically obtained via telecine or
datacine to an image sequence or video format capable of containing it, and is generally
employed for digital intermediate and film workflows. RGB encoded images such as
DPX and Cineon image sequences and TIFF files are always 4:4:4.
The Apple ProRes 4444 codec lets you capture, transcode to, and master media at this
high quality. (The fourth 4 refers to the ability of Apple ProRes 4444 to preserve an
uncompressed alpha channel in addition to the three color channels; however, Color
doesn’t support alpha channels.)
Be aware that simply rendering at 4:4:4 doesn’t guarantee a high-quality result. If media
is not acquired at 4:4:4, then rendering at 4:4:4 will preserve the high quality of
corrections you make to the video, but it won’t add color information that wasn’t there
to begin with.
As of this writing, few digital acquisition formats are capable of recording 4:4:4 video,
but those that do include HDCAM SR, as well as certain digital cinema cameras, including
the RED, Thompson Viper FilmStream, and Genesis digital camera systems.
• 4:2:2: 4:2:2 is a chroma subsampling ratio typical for many high-quality standard and
high definition video acquisition and mastering formats, including Beta SP (an analog
format), Digital Betacam, Beta SX, IMX, DVCPRO 50, DVCPRO HD, HDCAM, and D-5 HD.
Although storing half the color information of 4:4:4, 4:2:2 is standard for video mastering
and broadcast. As their names imply, Apple Uncompressed 8-bit 4:2:2, Apple
Uncompressed 10-bit 4:2:2, Apple ProRes 422, and Apple ProRes 422 (HQ) all use 4:2:2
chroma subsampling.
• 4:1:1 and 4:2:0: 4:1:1 is typical for consumer and prosumer video formats including
DVCPRO 25 (NTSC and PAL), DV, and DVCam (NTSC).
4:2:0 is another consumer-oriented subsampling rate, used by DV (PAL), DVCAM (PAL),
and MPEG-2, as well as the high definition HDV and XDCAM HD formats.
Due to their low cost, producers of all types have flocked to these formats for acquisition,
despite the resulting limitations during post-production (discussed below). Regardless,
whatever the acquisition format, it is inadvisable to master using either 4:1:1 or 4:2:0
video formats.
It’s important to be aware of the advantages of higher chroma subsampling ratios in the
color correction process. Whenever you’re in a position to specify the transfer format with
which a project will be finished, make sure you ask for the highest-quality format your
system can handle. (For more information about high-quality finishing codecs, see
25
Chapter 1
Color Correction Basics