Displays, modes, and channels

Displays, modes, and channels

Onscreen, your Photoshop image is a bitmap —a geometric arrangement, or mapping, of dots on a rectangular grid. Each dot, or pixel, represents a diff erent color or shade. If you drag with a painting tool, such as the Brush, across an area of a layerpixels below your pointer are recolored. With a high zoom level chosen for your document, you cansee (and edit) individual pixels.A Bitmap programs like Photoshop are best suited for editing and producing photographic, painterly, or photorealistic images that contain subtle gradations of color, which are called “continuous tones.” Th e images you edit in Photoshop can originate from a digital camera, from a photo print that you have scanned, from a fi le saved in another application, or from scratch using program features, such as painting and cloning tools.
To enable color images to be viewed onscreen, your display projects red, green, and blue (RGB) light. Combined in their purest form, these additive primaries produce white light. If you were ton send your Photoshop fi le to a commercial print shop for four-color process printing, it would be rendered with cyan (C), magenta (M), yellow (Y),and black (K) inks. Because computer displays use the RGB model, they can only simulate the CMYK inks that are used in commercial printing. Th e successful translation of a digital image to a printed one isn’t as simple as you might think. To begin with, the same document may look surprisingly diff erent on diff erent displays due to such

variables as the temperature of the display, the lighting in the room, and even the colors on your wall. Moreover, many colors that you see in the natural world or that can be displayed onscreen can’t be printed (have no ink equivalents), and
conversely, some printable colors can’t be displayed onscreen. Th e color management techniques that we outline in this chapter will help smooth out the
kinks in the color workfl ow from digital input to display onscreen, then fi nally to print.
➤ In Photoshop, you can choose colors using the Grayscale, RGB (red-green-blue), HSB (huesaturation-brightness), CMYK (cyan-magentayellow-black), or Lab (lightness, a-component,and b-component) color model, and you can choose colors from a color matching system,such as PANTONE.


Photoshop channels

All Photoshop images are composed of one, three,or four channels (see the sidebar below). In an RGB image, for example, the three channels store the intensity of red, green, or blue at each pixel as a level of gray. Most likely you will work with images that store 256 levels of gray for each channel. Because the 256 gray levels are represented by 8 bits (short for “binary digits”) of computer data,
the bit depth of such an image is said to be 8 bits per channel. Files that have a higher bit depth of 16 or 32 bits per channel contain more color information
(see page 19).
➤ Open an RGB Color image and display the Channels panel. Click Red, Green, or Blue on the panel to display only that channel, then click the topmost channel name on the panel to restore the composite display. Although you

can make adjustments to individual channels,normally you will edit all the channels at once while viewing the composite image.
In addition to the core channels we just discussed,you can add two other types of channels. You can save a selection as a mask in a grayscale (alpha)channel, or add channels for individual spot colors.A Th e more channels a document contains, the
larger its fi le storage size is. A document in RGB Color mode, which contains three channels (Red, Green, and Blue), will be three times larger than it would be if converted to Grayscale, a single channel mode. If you were to convert it to CMYK Color mode, the fi le would contain four channels (Cyan, Magenta, Yellow, and Black) and would be
larger still.

Photoshop document color modes

A document can be converted to, displayed in,and edited in any of these color modes: Bitmap,Grayscale, Duotone, Indexed Color, RGB Color,CMYK Color, Lab Color, or Multi channel. Th e ones you will work in the most are RGB Color and CMYK Color. To convert a document to a diff erent mode,use the Image > Mode submenu. B If a mode is dimmed on the menu, it means you must convert the fi le to a diff erent mode fi rst in order to make it available. For example, a fi le must be in Grayscale mode to be converted

to Duotone mode. Th e availability of some Photoshop commands and options will vary depending on the color mode of your
document.
Some mode conversions can cause noticeable color shifts. For example, if you convert a fi le from RGB Color mode to CMYK Color mode, printable colors will be substituted for RGB colors. Th e fewer times you convert a fi le, the better, as its color data is altered with each conversion change. Some conversions fl atten layers, such as a conversion to Indexed Color, Multichannel, or Bitmap mode.
Other conversions give you the option to preserve layers via a Don’t Flatten button in an alert dialog. Digital cameras and medium- to low-end scanners produce RGB images. For faster editing, and in order to access all the fi lters in Photoshop, we recommend keeping your fi les in RGB Color mode. In fact, most desktop color inkjet printers, especially those that use six or more ink colors, can process RGB Color fi les directly from Photoshop.
➤ To “soft-proof” your RGB document as a simulation of CMYK Color mode without performing an actual mode change, see pages 404–405.

Th e following is a brief summary of the color modes that a document can be converted to in Photoshop:
In Bitmap mode, pixels are either 100% black or 100% white, and no layers, fi lters, or adjustment commands are available. (To convert a fi le to this mode, you must convert it to Grayscale mode fi rst.)
In Grayscale mode, pixels are black, white, or up to 254 shades of gray (a total of 256). If you convert a fi le from a color mode to Gray scale mode and then save and close it, its luminosity (light and dark) values will be preserved, but its color information will be deleted permanently. In Chapter 13,we’ll show you how to convert a layer to grayscale without changing the document color mode.To produce a duotone, two or more extra plates are added to a grayscale image to enhance its richness and tonal depth. Th is requires special preparatory
steps in Photoshop and expertise on the part of your commercial printer. Files in Indexed Color mode contain a single channel, as well as an 8-bit color table (which contains a maximum number of 256 colors or shades).
When you optimize a fi le in the GIF format via the Save for Web & Devices dialog, the fi le is converted to this color mode automatically (see pages
423–424).
RGB Color A is the most versatile mode of all and the one you’ll use most often. It’s the mode in which digital cameras save your photos, the only mode in which all the Photoshop tool options and fi lters are accessible, the mode of choice for online output, and the mode of choice for export to video
and multimedia programs.
In Photoshop, although you can display and edit your fi les in CMYK Color mode,B we recommend editing them in RGB Color mode, then converting a copy of them to CMYK Color mode only when required for commercial printing or for export to a page layout application. Exceptions to this rule are images that are saved by high-end scanners in CMYK Color mode; keep them as CMYK to preserve their original color data.
Lab Color is a three-channel mode that was developed for the purpose of achieving consistency among various devices, such as printers and displays. Lab Color fi les are device-independent, meaning their color defi nitions stay the same
regardless of how each output device defi nes color. Th e channels represent lightness (the image details), the colors green to red, and the colors blue to yellow. Th e lightness and color values can be edited independently of one another. Although Photoshop uses Lab Color to produce conversions between RGB and CMYK color modes internally,there’s rarely a need for Photoshop users like us to convert our fi les to Lab Color mode.
Multichannel images contain multiple 256-levelgrayscale channels. If you convert an image from RGB Color to Multichannel mode, its Red, Green, and Blue channels are converted to Cyan, Magenta,and Yellow. As a result, the image may become lighter and the contrast may be reduced. Some Photoshop pros assemble individual channels from several images into a single composite image by using this mode, but this takes expertise.
With this foundation in color basics, you’reready to take the plunge into color management.