Introduction to Photoshop (Part Two)
Whenever you are working in Photoshop, you are working in a color mode. The default mode is RGB (Red-Green-Blue), which is also the default mode for the Web.
Each color mode defines the colors that combine to make up the color of every pixel in the image. To change color modes, choose Image > Mode.
Color modes are made up of channels. Each channel is a grayscale image.
A grayscale image is an image made up of shades of brightness (as many as 256 shades, from white to black). To view the channels, choose Window > Channels, and the Channels palette will appear. Modes with fewer channels result in smaller file sizes.
Some of the most frequently used color modes are:
One channel, 256 shades of gray
Image size and resolution
Digital images are made up of pixels (picture elements), which can be defined as colored squares. Each pixel is only one color.
A good way to learn about concepts related to image size, resolution, and print size is to experiment with an image using the Image Size dialog box. To do this, open an image in Photoshop. Then choose Image > Image Size, and the Image Size dialog box appears.
Under Pixel Dimensions, Width and Height refer to the number of pixels in an image, which has to do with the size of display on screen. Under Document Size, Width and Height refer to the size of the image when printed.
For Online Display
If you are placing an image on the Web, you may want to resample the image (add or delete pixels) in order to change the display size of the image.
To resample, make sure "Resample Image" is checked, as shown above. Unless you want to distort your image, also be sure that "Constrain Proportions" is checked. Then type in a new number of pixels in either the width or height field of Pixel Dimensions.
Sampling up (increasing the number of pixels) makes images larger on a display screen. For sampling up, you should choose the Bicubic or Bicubic Smoother option from the Resample Image dropdown menu.
Sampling down (decreasing the number of pixels) makes images smaller. For sampling down, we suggest that you choose either Bicubic or Bicubic Sharper from the Resample Image dropdown menu.