Introduction: Bits Per Pixel and Related Ideas

The terminology for image formats can be confusing because there are often several ways of describing the same format. This topic explains what the terms mean.

48 bit Images are similar to 24-bit images. They have three RGB color channels, but have 16-bits per channel. Each channel is capable of 65,536 shades of color. When these are combined the image will have 281.5 trillion mixed colors.

64 Bit Images have three color channels similar to 48-bit images. However, they have an additional channel. This is used for an alpha channel in the case of RGB color spaces. The alpha channel holds additional information such as a transparency indicator

The advantage of using high-bit-rate images is when image processing such as leveling is applied to an image. Leveling compresses or stretches the tonal range of an image. When these are performed on a True Color images - having 256 shades per channel - gaps may result. This is called posterization and appears as jumps or bands in color and brightness. When starting with 65,536 shades posterization is less likely to occur. High-bit-rate is also an advantage when small details in an image are of interest. An example would be in a medical image. When looking for immature tumors, the small details are cropped or rounded in an 8-bit per channel images.

If an image is 24 bits per pixel, it is also called a 24-bit image, a true color image, or a 16M color image. Sixteen million is roughly the number of different colors that can be represented by 24 bits, where there are 8 bits for each of the red, green, and blue (RGB) values.

A 32-bit image is a specialized true-color format used in image files, where the extra byte carries information that is either converted (e.g. case of CMYK color space), ignored when loaded as 24-bit or loaded as alpha channel to memory. The extra byte is used for an additional color plane in CMYK files, which are specialized files for color printing. In that case, LEADTOOLS, by default, converts the values to 24-bit RGB values when loading the image. The additional byte may also be used for an Alpha channel, which carries extra information such as a transparency indicator.

If an image is 16 bits per pixel, it is also called a 16-bit image, a high color image, or a 32K color image. Thirty-two thousand is roughly the number of different colors that can be represented by 16 bits, where there are 5 bits for each of the red, green, and blue values, the 16th bit can be the alpha bit. (Devices that specify 64K color support are also referring to 16-bit images, but they are counting the left-over bit.)

If an image is 8 bits per pixel, it is also called an 8-bit image or a 256-color image. Two hundred fifty-six is the number of different colors that can be achieved by using the image data as 8-bit indexes to an array of colors called a palette.

If an image is 4 bits per pixel, it is also called a 4-bit image or a 16-color image. Sixteen is the number of different colors that can be achieved by using the image data as 4-bit indexes to a palette.

If an image is 1 bit per pixel, it is also called a 1-bit image, a black and white image, a 2-color image, or a bitonal image. Two is the number of different colors that can be achieved by using the image data as 1-bit indexes to a palette. The palette can contain colors other than black and white, although black and white are most common.

If an image is grayscale, its red, green, and blue values are all the same, and the values are incremented from the lowest to the highest. For example, an 8-bit grayscale image has 256 shades of gray, with values from 0 to 255. LEADTOOLS supports 4, 8, 12, 16, and 32-bit black to white ordered grayscale, reverse ordered grayscale, and unordered grayscale images. Support for the 12, 16 and 32-bit grayscale images is only available in the Document/Medical only. Most of this same terminology is applied to video cards. For example, an 8-bit card is one that is capable of displaying 256 colors.