341 CHAPTER Channels IN THIS CHAPTER Using the Channel Mixer to enhance grayscale Understanding the Channels panel Creating, duplicating, splitting, and merging channels Creating and using alpha channels Adding spot color channels to images C olor channels are a core component of both color and grayscale images. Color images are composed of separate color channels that, when combined, make up the colors in the image. Understanding the color channels and the tools that Photoshop provides will help you make better use of the color data in your images. Photoshop provides two main tools to help you manipulate and manage the color channels in images: the Channel Mixer and the Channels panel. Using these tools, you can see, manipulate, and even create channels in your image to increase your options when making selections and adjustments. This chapter discusses using the Channel Mixer to mix, swap, and combine the color data in your images and using the Channels panel to manage chan- nels, create and use alpha channels, and create spot color channels. Understanding Color Channels In Chapter 4, we discussed how the images that we see are just light emitted from a computer screen or reflecting off a photo. We also discussed that the colors we see can be divided into the levels of three frequency ranges of light-stimulating receptors in the eye. Photoshop uses this same concept to represent digital images. Each pixel in a color digital image contains a series of values that define the actual color of the pixel. For example, each pixel in an RGB image contains a value describing the level of red, a value describing the level of green, and a value describing the level of blue. A channel is the series of values for only one color for the entire image. For the red channel, each pixel contains only one value describing the level of 17_584743-ch11.indd 34117_584743-ch11.indd 341 5/3/10 10:26 AM5/3/10 10:26 AM Part III: Selections, Layers, and Channels 342 red. Splitting the image into individual color channels allows Photoshop to provide some very powerful tools when editing images and creating special effects. Photoshop provides tools to adjust channels based on the color mode of the image. For example, if the color mode is RGB, Photoshop provides red, blue, and green tools; if the color mode is CYMK, Photoshop provides cyan, yellow, magenta, and black tools. Individual channels are treated as grayscale images by Photoshop because color is based on the mix of the channels; with only one channel, Photoshop cannot determine color. That is why when you view a single channel of an image, it is displayed in grayscale. An advantage to breaking color into channels is that each color channel contains part of the detail of the image. Notice the three channels shown in Figure 11.1. Each channel contains different parts of the detail that make up the entire image. Breaking the image out into detail gives you greater options when trying to work with the detail in an image. FIGURE 11.1 The red, green, and blue color channels of an image each contain part of the full detail in the image. Original Red channel Green channel Blue channel 17_584743-ch11.indd 34217_584743-ch11.indd 342 5/3/10 10:26 AM5/3/10 10:26 AM Chapter 11: Channels 343 On the Web Site The image shown in Figure 11.1 can be found on this book’s Web site as Figure 11-1.jpg. You can open it in Photoshop and view each of the color channels. n Using the Channel Mixer One of the most powerful, yet least commonly used, tools in Photoshop is the Channel Mixer. To reach the Channel Mixer, shown in Figure 11.2, select Image ➪ Adjustments ➪ Channel Mixer from the main menu. The Channel Mixer provides a simple interface to mix values between the different channels in the image. You may wonder whether mixing channels is the same thing as adjusting the colors in the image using one of the other adjustment tools. The answer is no. When you mix two or more channels, you are not mixing the colors of the channels so much as you are mixing the detail provided in the channel. Mixing the channels allows you to change the detail levels in each channel, which pro- vides a different result when you later make other adjustments such as changing the hue or tone in the image. Note The Channel Mixer is available only when you are using the RGB and CYMK color modes. If you need to even- tually work in the lab, grayscale, or other color modes, you want to do the channel mixing before changing the color mode from RGB or CYMK. n FIGURE 11.2 The Channel Mixer allows you to mix the channels together to create new channel data. 17_584743-ch11.indd 34317_584743-ch11.indd 343 5/3/10 10:26 AM5/3/10 10:26 AM Part III: Selections, Layers, and Channels 344 The Channel Mixer provides these options when mixing the channels in an image: l Preset: This allows you to select from a previously saved mix of channels. Photoshop pro- vides several black and white preset settings that apply different color filters, including infrared. This is useful if you need to make the same mix to more than one photo or if you want to save a certain mix and then try others. To save the current Channel Mixer set- tings, select Save Preset using the drop-down menu button next the Preset drop-down list. l Output Channel: This allows you to select the output channel that receives the mix of data from the set of source channels. Caution The current data in the output channel is overwritten by the values specified in the set of source channels. n l Red: This specifies the percent of the red channel levels to mix into the output channel. You can use the slider or type in the value directly. l Green: This specifies the percent of the green channel levels to mix into the output chan- nel. You can use the slider or type in the value directly. l Blue: This specifies the percent of the blue channel levels to mix into the output channel. You can use the slider or type in the value directly. l Constant: This specifies the grayscale value of the output channel. When you specify neg- ative values, more black is added to the output channel. When you specify positive values, more white is added. l Monochrome: When this option is selected, the output channel is changed to gray and the preview of the image is in grayscale. This is the perfect option to use when you want to adjust a color image before converting it to grayscale. There are three basic reasons to mix the color channels in an image. The most common is to selec- tively control how color channels are mixed to create a grayscale image. Another reason is to mix channels with more details to enhance channels with less. The third reason is to completely swap channels to create special effects without altering the other channels. Color mixing Using the Channel Mixer, you can specify how much of the level data, or detail, from each of the three channels to mix into one single channel. This allows you to change the actual color channel composition of the image. Color mixing is done by selecting an output channel in the Channel Mixer and then setting the percentages of each color channel that will be used to form the new channel. For example, con- sider a single RGB pixel. If we use the Channel Mixer to change the red channel to be a composite of 0 percent red, 50 percent green, and 50 percent blue, then the red value for each pixel would be changed to 0 percent of the red value plus 50 percent of the green value plus 50 percent of the blue value. 17_584743-ch11.indd 34417_584743-ch11.indd 344 5/3/10 10:26 AM5/3/10 10:26 AM Chapter 11: Channels 345 Color mixing is useful when you have a channel in an image that contains very little detail because it was taken in colored lighting or some other situation that alters the color tone in the image. Using a mix of the other two channels, you can add more detail to channels that are lacking. Caution The Channel Mixer allows you to use any combination percentage values of −200% to 200% for each source channel. That means you can end up with a total percentage that is not 100 percent. If you change the values such that the total does not equal 100 percent, the overall tonal content of the image is altered. This typically is not a desired result. n Swapping colors A great way to apply a special effect in an image is to completely swap color channels. This has the effect of changing the look of the entire image without altering the actual detail in the image. The effect changes depending on the color channels that you swap and the content of the channels in the image. To swap colors in an image, follow these steps: 1. Select Image ➪ Adjustments ➪ Channel Mixer from the main menu to launch the Color Mixer, shown in Figure 11.3. 2. Set the Output Channel to Red. 3. Change the value of the Red channel to 0%. 4. Change the value of the Blue channel to 100%. FIGURE 11.3 Changing the Red channel to 100 percent blue and the Blue channel to 100 percent red 17_584743-ch11.indd 34517_584743-ch11.indd 345 5/3/10 10:26 AM5/3/10 10:26 AM Part III: Selections, Layers, and Channels 346 5. Set the Output Channel to Blue. 6. Change the value of the Red channel to 100%. 7. Change the value of the Blue channel to 0%. 8. Click OK. The Red and Blue color channels are swapped in the image. The result is that the red tones are changed to blue and vice versa, as shown in Figure 11.4. Although you cannot see the full effect on the image in Figure 11.4, you can go to this book’s Web site and fol- low these steps to see the full effect. FIGURE 11.4 Swapping the Red and Blue channels results in a complete change to the color tones in an image that can create some fantastic effects. On the Web Site The image shown in Figure 11.4 can be found on this book’s Web site as Figure 11-4.jpg. You can open it in Photoshop and use the steps in this section to see the changes when you swap the blue and red color channels. As another experiment you could try to swap the green and red color channels. n Converting color to grayscale The most common use of the Channel Mixer is to prepare images to be converted to grayscale. When you convert an image to grayscale, Photoshop uses 40 percent of the red and green channels and 20 percent of the blue channel. For general purposes, those values have produced the best results. The Channel Mixer gives you complete control in how much of each channel is applied to the out- put when converting a color image into grayscale. Use these steps to convert an image to grayscale using the Channel Mixer: 1. Open the image in Photoshop, as shown in Figure 11.5. 2. Select Image ➪ Adjustments ➪ Channel Mixer from the main menu to launch the Color Mixer. 17_584743-ch11.indd 34617_584743-ch11.indd 346 5/3/10 10:26 AM5/3/10 10:26 AM Chapter 11: Channels 347 FIGURE 11.5 Color image converted to grayscale using the Channel Mixer 3. Check the Monochrome check box. This changes the output channel to Gray. The Red and Green source channels are set to 40% and the Blue channel is set to 20%, as shown in Figure 11.6. The image will be con- verted to a monochrome grayscale. The image results of the default values are not very appealing, as also shown in Figure 11.6. FIGURE 11.6 The initial results of selecting the Monochrome box are somewhat disappointing. 4. Adjust the Red slider, which has the effect of adjusting the red levels in the image. In this example, the value is changed to 0%, as shown in Figure 11.7. 17_584743-ch11.indd 34717_584743-ch11.indd 347 5/3/10 10:26 AM5/3/10 10:26 AM Part III: Selections, Layers, and Channels 348 5. Adjust the Green slider, which also has the effect of adjusting the image, but in this case it is a different set of detail in the green levels. In this example, the value is changed to 110%, as shown in Figure 11.7 6. Adjust the Blue slider, which has the effect of adjusting the blue levels in the image bringing back some of the detail lost from increasing the red and green channels. In this example, the value is left at 20%, as shown in Figure 11.7 7. Click OK to apply the channel mixer settings to the image, as shown in Figure 11.7. The results are much better contrast and tone than was achieved with the default values in Figure 11.6. Setting the values in the Channel Mixer varies greatly between different images. You will want to play with the sliders trying different values while previewing the results in the images. 8. After you have applied the Channel Mixer settings, select Image ➪ Mode ➪ Grayscale to convert the color model of the image to Grayscale. Note When you use Image ➪ Mode ➪ Grayscale, Photoshop suggests using Image ➪ Adjustments ➪ Black & White adjustment to convert the image to grayscale. You do not need to use the Black and White adjustment tool because the channels have already been adjusted. The Black and White adjustment tool is discussed in Chapter 12 and is a bit simpler than using the channel mixer, but you may get better results using the Channel Mixer method. n FIGURE 11.7 The results of using custom values in the Channel Mixer to create the Grayscale channel are much better than those with the default settings. 17_584743-ch11.indd 34817_584743-ch11.indd 348 5/3/10 10:26 AM5/3/10 10:26 AM Chapter 11: Channels 349 On the Web Site The image shown in Figure 11.5 can be found on this book’s Web site as Figure 11-5.jpg. You can open it in Photoshop and use the steps in this section to see the effects of mixing the channels to create a better grayscale image. n Using the Channels Panel The Channels panel shown in Figure 11.8 provides access to all the channels contained in the images. To open the Channels panel, select Window ➪ Channels from the main menu or use the F7 shortcut key and select the Channels tab. Note When working with RGB and CYMK images, the top entry in the Channels panel is a composite channel of the RGB or CYMK channels. When you select the composite channel, all the component channels are selected. n Using the Channels panel, you can view and select individual channels, remove channels, and cre- ate new channels. The following sections discuss using the Channels panel to work with the color channels in an image. FIGURE 11.8 The Channels panel allows you to manage individual color channels in images. Composite channel Visibility Channels panel menu Load Channel as Selection Delete Channel Save Selection as Channel Create New Channel 17_584743-ch11.indd 34917_584743-ch11.indd 349 5/3/10 10:26 AM5/3/10 10:26 AM . 0 percent red, 50 percent green, and 50 percent blue, then the red value for each pixel would be changed to 0 percent of the red value plus 50 percent of the green value plus 50 percent of the. green tools; if the color mode is CYMK, Photoshop provides cyan, yellow, magenta, and black tools. Individual channels are treated as grayscale images by Photoshop because color is based on the. images and creating special effects. Photoshop provides tools to adjust channels based on the color mode of the image. For example, if the color mode is RGB, Photoshop provides red, blue, and green