ptg Understanding Colors and Channels Introduction In the world of design, color is one of the most important ele- ments. When you're creating a brochure, advertisement, or banner using Adobe Photoshop, good use of color attracts the attention of the viewer. It also helps draw the elements of your design into one cohesive unit. Color is a strong motiva- tor and is used in all aspects of our daily life. Since color is so important to design, Photoshop lets you use industry-standard color sets, or you can create and save your own customized color panels. You can also color-correct a photograph by removing the color entirely or selectively remove colors from portions of the image. In addition, Photoshop gives you ways to select areas based on color, and then fill those areas with any color you choose. Not only is it important to understand how color is used, it's also important to understand how Photoshop manages color information and that's where the Channels panel comes into the picture. Channels are where color information is stored. The number of channels in an image is based on its color mode , or color model, such as RGB (Red, Green, and Blue) or CMYK (Cyan, Magenta, Yellow, and Black). A firm understanding of channels and color modes, and their func- tion in Photoshop, will go a long way in helping you control and manage color. When adjusting your image, you can use various com- mands—Auto Contrast and Color, Curves, Color Balance, Brightness/Contrast, and Desaturate, just to name a few. You can also use the Match Color and Selective Color adjust- ments to further fine-tune your image. Photoshop also pro- vides a Photo Filter adjustment, as well as a Shadows and Highlights adjustment to correct those overexposed or under- exposed images. With all of the commands and adjustments available, the real dilemma will be, where do you begin? 8 8 What You’ll Do Work with 8-, 16-, and 32-Bit Images Work with the Channels Panel Work with Color Modes Understand the Various Color Modes Use the Replace Color Adjustment Work with the Color Panel Work with the Swatches Panel Use the Stroke and Fill Commands Create Spot Color Channels Use Auto Contrast and Auto Color Use Levels Adjustment Commands Use the Exposure Adjustment Use Curves and Color Adjustments Use Hue and Saturation Use Match and Selective Color Use Channel Mixer and Gradient Map Use Photo Filter and Shadows/Highlights Use the Invert and Equalize Commands Use the Threshold and Posterize Adjustments Use the HDR Toning Adjustment Use the Black & White Adjustment 175 From the Library of Wow! eBook From the Library of Michele Renth ptg 176 Chapter 8 It's all about the numbers, and that's a fact. The number of colors available for displaying or printing each pixel in an image is called bit depth —also known as pixel depth or color depth. A higher bit depth means more avail- able colors and more accurate color represen- tation in an image. A bit depth setting of 2 bits displays 4 colors, 4 bits displays 16 col- ors, 8 bits displays 256 colors, 16 bits displays 32,768 colors, and 24 bits and 32 bits both display 16.7 million colors. Most digital images currently use 8 bits of data per chan- nel. For example, an RGB image with 8 bits per channel is capable of producing 16.7 mil- lion (a 24-bit RGB image: 8 bits x 3 channels) possible colors per pixel. While that may seem like a lot of color information, when it comes to color correction and adjustment, it isn't. In response to Photoshop users needing more control, Photoshop supports 16-bit and now 32-bit—known as High Dynamic Range (HDR) —images. High Dynamic Range images with 32 bits per channel have a more extended dynamic range than lower bit depth images. Dynamic Range describes the ability of a channel to capture maximum information from the black to white and dark and bright areas of an image. An 8-bit channel image has a dynamic range of 250:1 (per channel), simi- lar to the dynamic range of printed paper or a computer display. A 16-bit channel image has a dynamic range of 65,000:1, and a 32-bit channel image has a dynamic range of over 200,000:1. The greater dynamic range trans- lates into better control over an image when making fine color and contrast adjustments using Levels and Curves (shown below). Working with HDR images is very similar to using raw files and applying exposure changes after the fact. Photographers can capture the full dynamic range of a scene with multiple exposures and merge the files into a single image. Working with 8-, 16-, and 32-Bit Images When you correct a 8-bit image, it can lose tonal values. 16- and 32-bit images hold more image data and therefore provide more to work with during correction operations. From the Library of Wow! eBook ptg Chapter 8 Understanding Colors and Channels 177 Changing Bits Per Channel The ability to work with 32-bit images is rela- tively new in Photoshop, and initially you had a limited use of adjustments and filters. However, in Photoshop many more adjust- ments and filters have become available for 32-bit images, such as Hue/Saturation, Levels, Gaussian Blur, Add Noise, Smart Sharpen, Vibrance, and more. When adjusting the color or contrast of an image, first convert a standard 8-bit image to 16 bits, and then make your corrections. This helps prevent loss of color information, and banding between light and dark shades. Once all the color/contrast adjustments have been made, you can (if necessary) convert the image back to 8 bits. It's that simple. You can change an image's bit depth by displaying the image, clicking the Image menu, pointing to Mode, and then clicking 8 Bits/Channel, 16 Bits/Channel, or 32 Bits/Channel. When you convert a 32-bit image to 8 or 16 bits per channel, if you choose to merge your layers before changing the bit depth, Photoshop opens the HDR Conversion dialog box to let you make exposure and contrast corrections so the image retains the dynamic range you want. The Exposure and Gamma option lets you manually adjust brightness and contrast. Drag the Exposure slider to adjust the gain and drag the Gamma slider to adjust the contrast. The Highlight Compression option automatically adjusts highlight values to fit within the range for 8- or 16-bit images. The Equalize Histogram option automatically preserves image con- trast. The Local Adaptation option adjusts the tonality (local brightness regions) in the image. Drag the Radius slider to specify the size of the local brightness regions and then drag the Threshold slider to specify the dis- tance between tonal values before they are included in the brightness region. If you want to reuse these settings in the future, you can save them, and then load them again as needed. Viewing 32-Bit Images The dynamic range of HDR images exceeds the display capabilities of standard monitors. When you view a 32-bit HDR image, the high- lights and shadows may look dark or washed out. To correct the problem, Photoshop allows you to adjust 32-bit preview options so 32-bit images display properly on your monitor. The preview options are stored in the image file, so each file retains its own settings. To set pre- view options, open a 32-bit HDR image, click the View menu, and then click 32-Bit Preview Options. In the 32-bit Preview Options dialog box, select the preview settings you want (described earlier in this topic), and then click OK. From the Library of Wow! eBook ptg 178 Chapter 8 The Channels panel is Photoshop's storage locker for color and selec- tion information. For example, when you open an RGB image, the Channels panel displays color channels of red, green, and blue. When you open a CMYK image, the color channels are cyan, magenta, yellow, and black. These primary color channels are defined as the native color channels of the image. The Channels panel can also contain spot color channels and selection masks. In addition to color information and selection masks, the Channels panel contains a composite chan- nel. The composite, when selected, lets you view the full-color image in the document window. Selecting any of the individual native color channels changes the active view of the image to display the selected color channel. The Channels panel stores color information using shades of gray, and each color channel is capable of displaying 256 gradations from black to white. A zero-value pixel displays as black, and a 255-value pixel displays as white. The darker the shade of gray, the less of the selected ink color is used to create the visible colors within the image. Working with the Channels Panel Work with the Channels Panel Open a color document. Select the Channels panel. Click on the individual channels to view the native color channels of the active document. Click the composite channel to view the full-color image. 4 3 2 1 3 4 2 See Also See “Creating Spot Color Channels” on page 196 for more information on using the Channels panel. See “Using Channels to Create and Store Selections” on page 92 for more information on using channels. From the Library of Wow! eBook ptg Chapter 8 Understanding Colors and Channels 179 Color modes define the colors represented in the active document. Although you can change the color mode of a document, it is best to select the correct color mode at the start of the project. Photoshop's color modes are Bitmap, Grayscale, Duotone, Indexed Color, RGB (Red, Green, and Blue), CMYK (Cyan, Magenta, Yellow, and Black), Lab, and Multichannel. See "Selecting Color Modes and Resolution" on page 13 for information on the best use for each color mode. The number of channels in an image depends on its color mode. For example, a CMYK image contains at least four channels, one for each color. Color modes determine the number of col- ors, the number of channels, and the file size of an image. For example, an RGB image has at least three channels (like a printing plate), one for red, green, and blue color information. Color modes not only define the working color space of the active document, they also represent the color space of the output docu- ment. It's the document output (print, press, or monitor) that ultimately determines the document color mode. Color modes do not just determine what colors the eye sees; they represent how the colors are mixed, and that's very important because different output devices use different color mixes. Therefore, when selecting a color mode, know the file format of the document and where it will be used. An image taken with a digital camera and then opened in Photoshop would most likely be in the RGB color mode. An image displayed on a monitor would be RGB, or possibly Indexed Color. A photograph scanned on a high-end drum scanner would most likely be in the CMYK color mode. An image being sent to a 4-color press would be CMYK, too. If you were creating a Photoshop document from scratch, the color mode you choose should represent the eventual output destination of the document, such as on a web page, to an inkjet printer, or a 4-color press. Switching Between Color Modes Unfortunately, images do not always arrive in the correct format. For example, you take several photographs with your digital (RGB) camera, but the images are being printed on a 4-color (CMYK) press, or you want to colorize a grayscale image. Changing color modes is a snap, but changing the color mode of an image isn't the problem. The problem is what happens to the digital color information when you change color modes. For example, if you open an RGB image with the intent of sending it out to a 4-color press (CMYK), the smartest course of action is to remain in the RGB color mode through the pro- cessing of the image, and then convert the image into the CMYK mode at the end. The reason has to do with how Photoshop moves between those two color spaces. For example, if you move a color-corrected CMYK image into the RGB color mode, and then back to CMYK, the colors shift because Photoshop rounds color values during the change process. On top of that, a CMYK image is 25% larger than an RGB image, and the RGB color mode repre- sents the color space of your monitor, not a printing press. It is impossible to view subtractive CMYK color on an RGB device. If, however, the image originally came to you as a color-corrected CMYK image, then stay in and work inside that color mode. Working with Color Modes From the Library of Wow! eBook ptg 180 Chapter 8 The RGB color mode is probably the most widely used of all the color modes. RGB generates color using three 8-bit channels: 1 red, 1 green, and 1 blue. Since each channel is capable of generating 256 steps of color, mathematically, that translates into 16,777,216 possible colors per image pixel. The RGB color mode (sometimes referred to as Additive RGB) is the color space of computer monitors, televisions, and any electronic display. This also includes PDAs (Personal Digital Assistants), and cellular phones. RGB is considered a device- dependent color mode. Device-dependent means that the colors in images created in the RGB color mode will appear differently on vari- ous devices. In the world of computer monitors and the Web, what you see is very seldom what someone else sees; however, understanding how Photoshop manages color information goes a long way to gaining consistency over color. Understanding the RGB Color Mode Convert an Image to RGB Color Open an image. Click the Image menu, point to Mode, and then click RGB Color. Photoshop converts the image into the RGB color mode. 2 1 1 RGB color mode From the Library of Wow! eBook . of the active document, they also represent the color space of the output docu- ment. It's the document output (print, press, or monitor) that ultimately determines the document color. represented in the active document. Although you can change the color mode of a document, it is best to select the correct color mode at the start of the project. Photoshop& apos;s color modes. be CMYK, too. If you were creating a Photoshop document from scratch, the color mode you choose should represent the eventual output destination of the document, such as on a web page, to