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Bit Depth Basics
To use bit depth, you should understand how the number of bits determines the number of color variations for red, green, and blue.
If two bits are used to describe a pixel, there are four possible combinations: 00, 01, 10, and 11. To calculate the number of tones that can be represented by two bits, you raise 2 to the power of 2 (22), which equals 4.
If you continue this exercise, you see that each additional bit in a binary number doubles the number of possibilities.
The most common bit depth in compositing is 8 bits, which produces 256 (28) levels for each primary color. If you multiply 256 by 256 by 256 (for the three colors), you get 16, 777, 216 (224) possible colors for each pixel.
In combustion, you can work at 8, 10, 12, or 16 bits per channel, or you can use Float. (The Particles operator is only 8-bit, though.)
The Float option uses a floating point number represented by a precise number between 0 and 1 for each image channel. This means the extent to which the range of the color scale is sub-divided into intermediate shades is incredibly huge.
The bit depth used for the color channels is also used for the alpha channel to describe the transparency of each pixel. For more information on the alpha channel, see About Channels.
When describing a file format, the total number of bits is often used instead of the bit depth. For example, if you import a PNG image sequence, the footage details in the Footage Controls panel (or in the Footage Library view) may show "32 bit". This means the images use 8 bits per channel, and they include an alpha channel (24+8=32). If the details show "24 bit", the images do not include an alpha channel. If the details for an RPF image show "48 bit", the file has a bit depth of 16 bits, and it does not include an alpha channel.
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