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Chapter 2 Creating and Managing Projects 185
About Bit Depth
When working with images, an increase in bits per pixel for those images means more
color information becomes available. A one-bit-per-pixel bitmap image is comprised of
purely black or white image pixels (no shades of gray).
A grayscale image comprises image pixels with 256 levels of gray. Each pixel requires 8
bits to represent the 256 shades of gray. Therefore, the bit depth of a grayscale image is
8 (2
8
= 256).
An RGB image can comprise image pixels with 256 shades of each of the primary
colors—red, green, and blue. In this case, there are 2
8
(256) shades of each color
component. This creates more than 16.7 million possible colors (256 x 256 x 256 > 16.7
million). The bit depth of an RGB image can be 24 (8 bits for each color), and the bit
depth of an RGBA image (red, green, blue, and an alpha channel) can be 32 (8 bits for
each color + alpha channel). The bit depth of an alpha channel describes the
transparency of each pixel. Although these images are 24- and 32-bit, such color
images are often referred to as 8-bit (because of the 8 bits per channel).
Note: An RGB image does not necessarily imply 8 bits per pixel.
Motion’s bit depth setting is bits-per-channel. In an 8-bit Motion project, the 256 levels
of color are represented on an integer scale of 0–255 (where 0 represents black and 255
represents white). All of your operations are clamped within that 0–255 range. There is
a one-to-one ratio between each number and its represented color. In 8-bit mode, 16.7
million colors can be represented—equalling the number of possible combinations of
256 different color values from each Red, Green, and Blue channel. Although that is a
large number of colors, it is often helpful to have finer gradations of colors available.
Using floating point calculations, color shades can be subdivided into an enormous
amount of intermediate colors, providing orders of magnitude more colors available to
your project palette. Incredibly small increments of color can be represented in 16-bit
float, and even finer increments in 32-bit float.
The bit depth of your source footage will often determine the bit depth of your project.
Even if your source footage is 8-bit, you may want to work in a project with a higher bit
depth to achieve better results. When you increase the bit depth of your project, you
are not introducing any new color information to the original images. However,
operations such as keying, color correction, applying blur or other filters with high
parameter values, or creating graphics that require very smooth color gradients can
benefit from the new number of possible color levels.
Important: There is a price for working in higher bit depths, however. And that price is
paid in processing time. Remember also that because Motion is hardware dependent,
most systems have a limitation on the size of imported files. For more information on
the required hardware, visit the Motion website at http://www.apple.com/
finalcutstudio/motion.