Apple Shake 4 - Manual (Page 438)

438 Chapter 15 Image Processing Basics
The logarithmic image does not appear to have a pure black or white. Its graph shows
that the highlights are flattened (compressed), and that the blacks have more
attention. This is why Cineon frames typically seem to have a very low contrast, mostly
in the highlights. Because the Y axis represents the output data, it contains less data
than the X axis. You can therefore store this in a smaller file than you might otherwise
use, to which the Cineon 10-bit format is nicely adapted as good balance between
color range and speed.
Log compression is not inherent or unique to the Cineon file format. You can store
logarithmic color data in any file type, and you can store linear color data into a .cin file.
The easiest way to think about a logarithmic file is as a “digital negative,” a concept
encouraged by Kodak literature. Unfortunately, Kodak decided to not actually invert the
image. While the idea of a negative piece of film is easy to understand, this concept
may cause some confusion. To see it properly, you must invert the film. In the same
manner, you cannot work with a “digital negative” in your composite. You must first
convert the image, in this case from the filmic log color to the digital linear color. This is
called a log to lin color correction, and is performed with the Color–LogLin node.
It is important to remember that log to lin (or lin to log) conversion is simply a color
correction. This workflow is explained below. However, it is first necessary to discuss
how this color correction works.
A piece of film negative has up to approximately 13 stops of latitude in exposure. A
stop is defined as a doubling of the brightness. The “digital negative,” the Cineon file,
contains approximately 11 stops. A positive print cannot display as much range as a
negative. The same rule applies to a computer monitor—it can only display about 7
stops of latitude. To be properly handled by the computer, information must be
discarded. In a simplified example, an image contains a rounded-off range of 0 to 11
stops for black to white. Since the monitor can only display about 7 stops (for example,
steps 1 through 8), the rest of the image (below 1 and above 8) is thrown away. These 7
steps, only a portion of the original negative, are then stretched back to 0 (black) and
11 (white). Because what used to be dark gray to medium-bright gray is now black to
white, the image appears to have a higher contrast. However, information has been
thrown away in the process. This loss is permanent when working in 8 or 16 bits, but
can be retrieved when working in 32-bit float. For more information, see “
Logarithmic
Color and Float Bit Depth ” on page 444.
The extraction process is one way to control exposure. If you select the higher portions
of the image (for example, 4 to 11 rather than 1 to 8), the image appears darker
because you are remapping the brighter parts of the image down to black. This is the
same as selecting to expose your camera for the brightest portion of your scene. The
opposite occurs when selecting the lower portions of the image brightness.