Title: Bit Depth
Course: IN 166 Introduction to Graphics > Section: Module 3

Bit depth quantifies how many unique colors are available in an image's color palette in terms of the number of 0's and 1's, or "bits," which are used to specify each color.  This does not mean that the image necessarily uses all of these colors, but that it can instead specify colors with that level of precision.  For a grayscale image, the bit depth quantifies how many unique shades are available. Images with higher bit depths can encode more shades or colors since there are more combinations of 0's and 1's available.

TERMINOLOGY

Every color pixel in a digital image is created through some combination of the three primary colors: red, green and blue.  Each primary color is often referred to as a "color channel" and can have any range of intensity values specified by its bit depth.  The bit depth for each primary color is termed the "bits per channel."  The "bits per pixel" (bpp) refers to the sum of the bits in all three color channels and represents the total colors available at each pixel.  Confusion arises frequently with color images because it may be unclear whether a posted number refers to the bits per pixel or bits per channel.  Using "bpp" as a suffix helps distinguish these two terms.

EXAMPLE

Most color images from digital cameras have 8-bits per channel and so they can use a total of eight 0's and 1's.  This allows for 2^8 or 256 different combinations—translating into 256 different intensity values for each primary color.  When all three primary colors are combined at each pixel, this allows for as many as 2^8*3 or 16,777,216 different colors, or "true color."  This is referred to as 24 bits per pixel since each pixel is composed of three 8-bit color channels.  The number of colors available for any X-bit image is just 2^X if X refers to the bits per pixel and 2^3X if X refers to the bits per channel.

COMPARISON

The following table illustrates different image types in terms of bits (bit depth), total colors available and common names.

Bits Per Pixel | Number of Colors Available | Common Name(s)
1 | 2 | Monochrome
2 | 4 | CGA
4 | 16 | EGA
8 | 256 | VGA
16 | 65536 | XGA, High Color
24 | 16777216 | SVGA, True Color
32 | 16777216 + Transparency |
48 | 281 Trillion |