Apple Color 1.5 - Manual (Page 27)

Bit Depth Explained
Another factor that affects the quality of video images, and can have an effect on the
quality of your image adjustments, is the bit depth of the source media you’re working
with. With both RGB and Y
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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
differences between images at different bit depths is most readily apparent in gradients
such as skies, where lower bit depths show banding, and higher bit depths do not.
The bit depth of your source media depends largely on how that media was originally
acquired. Most of the media you’ll receive falls into one of the following bit depths, all of
which Color supports:
• 8-bit: Most standard and high definition consumer and professional digital video formats
capture 8-bit image data, including DV and DVCPRO-25, DVCPRO 50, HDV, DVCPRO
HD, HDCAM, and so on.
• 10-bit: Many video capture interfaces allow the uncompressed capture of analog and
digital video at 10-bit resolution.
• 10-bit log: By storing data logarithmically, rather than linearly, a wider contrast ratio
(such as that of film) can be represented by a 10-bit data space. 10-bit log files are often
recorded from datacine scans using the Cineon and DPX image sequence formats.
• 12-bit: Some cameras, such as the RED ONE, capture digital images at 12-bit, providing
for even smoother transitions in gradients.
• 16-bit: It has been said that it takes 16 bits of linear data to match the contrast ratio
that can be stored in a 10-bit log file. Since linear data is easier for computers to process,
this is another data space that’s available in some image formats.
• Floating Point: The highest level of image-processing quality available. Refers to the
use of floating-point math to store and calculate fractional data. This means that values
higher than 1 can be used to store data that would otherwise be rounded down using
the integer-based 8-bit, 10-bit, 12-bit, and 16-bit depths. Floating Point is a
processor-intensive bit depth to work with.
Higher bit depths accommodate more image data by using a greater range of numbers
to represent the tonal range that’s available. This is apparent when looking at the numeric
ranges used by the two bit depths most commonly associated with video.
• 8-bit images use a full range of 0–255 to store each color channel. (Y
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video uses
a narrower range of 16–235 to accommodate super-black and super-white.) 255 isn’t
a lot of values, and the result can be subtly visible “stairstepping” in areas of the picture
with narrow gradients (such as skies).
27 Chapter 1 Color Correction Basics