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.

Using High-bit Rates

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 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 8-bit per channel images.

1-bit Image

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.

4-bit Image

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.

8-bit Image

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.

16-bit Image

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.

24-bit Image

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.

32-bit Image

There are two types of 32-bit images:

• 32-bit CMYK is somewhat equivalent to 24-bit RGB, where the extra byte contains only color information (additional color plane). These specialized true-color images are often used for color printing. LEADTOOLS, by default, converts the CMYK values to 24-bit RGB values when loading these images.
• RGB+A is a 24-bit RGB image with an additional 8-bit alpha channel, normally used for transparency.

48-bit Image

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 Image

64-bit images are similar to 32-bit and 48-bit images. They have three RGB color channels of 16-bits, and an additional 16-bit channel (alpha channel), which is usually used for transparency information.

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 SDK.

Refer to Grayscale Images for more information.