Datasheet

Working with Digital Images

Bit depth is measured in bits per channel (bpc):

1 bit/channel. An image with a bit depth of 1 has pixels with only two possible values,

black or white. A Bitmap mode image has one channel with pixels that can be either

black or white. There is no 1 Bit/Channel option that can be selected in Photoshop, but

Bitmap mode images are 1-bpc images.

8 bits/channel. An image with a bit depth of 8 has 28, or 256, possible values per chan-

nel. An 8-bpc Grayscale mode image’s pixels can only be black, white, or shades of gray.

Since a Grayscale mode image has only one channel, each 8-bpc Grayscale mode image

can have only 256 possible values. An 8-bpc RGB mode image has 256 possible values in

each of its three color channels (red, green, and blue). It is possible for each pixel in an

RGB image to be any mixture of those three colors. Therefore, each 8-bpc RGB image has

16,777,216 possible values (256 × 256 × 256).

16 bits/channel. An image with a bit depth of 16 has 216, or 65,536, possible values per

channel.

32 bits/channel. Also known as a high dynamic range (HDR) image, an image with a bit

depth of 32 has 232, or 4,294,967,296, possible values per channel.

TABLE 1.1

Limits of Photoshop’s Support for Bit Depths Above 8

16-bpc Images 32-bpc Images

Cannot use Bitmap, Duotone, or Indexed

Color mode

Can use only RGB and Grayscale modes

Cannot use Art History Brush tool Cannot use some blending modes

Cannot use Variations image adjustment

Cannot use some commands from the Image

➪ Adjustments

Cannot use some filters Cannot use some filters

Can save these formats only:

Photoshop (PSD, PSB)

Radiance (HDR)

Portable Bit Map (PBM)

OpenEXR

TIFF

Cannot fully use a few commands from the Image menu

Cannot use a few commands from the Layer menu

Cannot use a few commands from the Select menu

Cannot use some of the tools in the Toolbox