Overview of the main types and characteristics of digital images

Following are basic descriptions of digital image types and characteristics:
n Bit depth. The higher an image’s bit depth, the more colors each of its pixels can be;
therefore, the more colors and degrees of brightness the image can contain.
n Bitmap (raster) and vector. A bitmap, or raster, image gets its detail from the number
of pixels it contains. The more pixels a bitmap image has, the more detail the image can
contain. A vector image gets its detail from shapes and lines that are mathematically calculated,
so its detail does not depend on the number of pixels in the image.
n Color mode. The color mode of an image specifies which basic set of colors is used to
make up an image. Some common basic sets of colors used to make up full-color images
are Red, Green, and Blue (RGB mode) and Cyan, Magenta, Yellow, and Black (CMYK
mode; Black is represented by the letter K). Colors within color modes are stored in their
own color channels. You can view the channels in the Channels palette.
CROSS--REF See Chapter 10 for more information about the Channels palette.
n Color space, color profile, gamut. Within the basic sets of colors used to make up each
color mode, such as RGB or CMYK, there are different sets of combinations of these colors.
These subordinate sets can be thought of as color palettes chosen for images so they
will look better when they reach their final destinations.
n File format. Various destinations for an image, such as other software or a computer
screen display, require certain file formats because of the software language or properties
of the device. Some common file formats are JPEG and GIF, usually used for Web page
display, and TIFF, often used for page layout programs and printing.
NOTE The color-mode term Bitmap mode (images that are limited to black and/or white;
no gray; no color) is different from the image-type term bitmap, or raster (images
that get their detail from the number of pixels they contain). Not all bitmap images are Bitmap
mode.
Bit depth
You can see or convert bit depth by choosing Image ➪ Mode ➪ Bits/Channel. At a basic level, all
digital images contain components called pixels. Pixels can contain varying amounts of information
that describe their color. In this case, color refers to the qualities of hue combined with lightness.
The greater an image’s bit depth, the more information each of its pixels can contain, and the more
colors each of those pixels can potentially be. Therefore, the higher an image’s bit depth, the more
colors and degrees of brightness the image can contain (see Table 1.1).

Bit depth is measured in bits per channel (bpc):
n 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.
n 8 bits/channel. An image with a bit depth of 8 has 28, or 256, possible values per channel.
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).
n 16 bits/channel. An image with a bit depth of 16 has 216, or 65,536, possible values per
channel.
n 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.
(Sourse. photoshop bible)