Tài liệu Photoshop cs5 by Dayley part 14 pot

Part I: Getting Started with Adobe Photoshop CS5 102 FIGURE 4.4 The color levels histograms of an RGB image in Photoshop Using the Histogram panel In the previous sections, we discussed the importance of using histograms in understanding the composition of light and color in an image. Photoshop provides a Histogram panel that helps you quickly view the important histograms of an image. Using the Histogram panel, you can view the histograms of each of the channels, all channels together, colors, and luminosity. You also can use the histogram tool to view histograms of specific layers. In addition to the histograms, the Histogram panel can show you numerical statistics about the level composition of each histogram. Tip When you are making adjustments or corrections that have to do with color, hue, tone, contrast, and so on, view the Histogram panel for the image. Look at the overall histogram to check for overexposure/underexposure. Also, look at the individual channels to verify that the color balance is what you would expect for that image. n The Histogram panel, shown in Figure 4.5, is launched by selecting Window ➪ Histogram from the main menu in Photoshop. The following sections discuss how to configure and use the Histogram panel to view histograms and statistics. 08_584743-ch04.indd 10208_584743-ch04.indd 102 5/3/10 10:18 AM5/3/10 10:18 AM Chapter 4: Understanding Colors, Histograms, Levels, and Curves 103 FIGURE 4.5 The Histogram panel in Photoshop Setting Histogram panel options The Histogram panel provides several views that can be set by selecting the menu icon shown in Figure 4.5. The following settings in the Histogram panel menu allow you to configure what infor- mation is shown in the Histogram panel: l Compact view: The compact view, shown on the left in Figure 4.6, shows only the histogram image of the current channel. This view is handy if you just want to dock the histogram with several other panels to view changes as you adjust the image. l Expanded view: The expanded view, shown in the center in Figure 4.6, shows the histogram image of the current channel but also provides the option to select different channels and sources for the histogram. It also displays the statistics if the Show Statistics option is selected. l All channels view: The all channels view, shown on the right in Figure 4.6, shows the histograms of all channels in addition to everything that the expanded view shows. l Show statistics: The show statistics setting toggles the statistics on and off in the expanded and all channels views. l Show channels in color: The show channels in color option toggles the color view of channels on and off. When selected, the histograms of individual channels are displayed in the channel color. This is useful if you are viewing a specific channel, because it is easy to distinguish which channel it is. Selecting channels The Channel menu, shown in Figure 4.7, allows you to select a specific channel to view. The channel that is selected is displayed in the compact and expanded views and at the top of the all channels view. Also, the statistics are based on the channel that is selected. 08_584743-ch04.indd 10308_584743-ch04.indd 103 5/3/10 10:18 AM5/3/10 10:18 AM Part I: Getting Started with Adobe Photoshop CS5 104 FIGURE 4.6 The compact, expanded, and all channels views of the Histogram panel in Photoshop Note What channels are available in the Channel drop-down list depends on the color mode of the current image. For example, if the color mode is RGB, then the channels available are Red, Green, and Blue; if the color mode is CYMK, then the channels are Cyan, Magenta, Yellow, and Black. n FIGURE 4.7 The Channel menu in the Histogram panel of Photoshop 08_584743-ch04.indd 10408_584743-ch04.indd 104 5/3/10 10:18 AM5/3/10 10:18 AM Chapter 4: Understanding Colors, Histograms, Levels, and Curves 105 The following channels are available in the Histogram panel: l All Channels: At the top of the list in the channels menu is the option to select all the channels for the color mode. For example, if you are using RGB, the top item is RGB and it selects all channels to use for the histogram views and statistics. l Single Channel: In addition to all channels, each individual channel is listed in the channels list. l Luminosity: Selecting the luminosity channel calculates the histogram and statistics based on how much general light is coming from a composite of all channels. This is useful in determining the overexposure/underexposure. l Colors: Selecting the colors channel displays a histogram that is a composite of all color channels as well as the overlapping colors they generate. This is useful in seeing the general color composition in the image. Selecting a source The Histogram panel allows you to select different sources to calculate the histograms from. Selecting one of the following options from the Source menu gives you the ability to view a histogram from any layer or adjustment layer or for the entire image: Note The image must have more than one layer or adjustment layer to use the Source menu to select alternate sources. n l Entire Image: This calculates the histogram based on a composite of all layers. Basically, this is the histogram of the image if you flatten out all your changes. l Selected Layer: This calculates the histogram based on the selected layer in the Layer panel. You use this to calculate histograms on only a single layer. l Adjustment Composite: This calculates the histogram based on the adjustments made in the selected adjustment layer and all layers below it. This is useful to understand the adjustments you have made to an image in graphical form. Tip When combining elements from multiple images, add the selection from one image as a layer in the second. Then you can use the selected layer option in the source menu of the Histogram panel to compare the histograms of each different layer to see how well the colors match up. Making the histograms match better helps the composite image look better. n Understanding statistics The statistics data in the Histogram panel, shown in Figure 4.8, displays the numerical values that are represented in the histogram image as well as some additional items. Most people would never need to use the statistics because image editing is really more of an art than a science. However, having specific numerical data about the histograms can be useful if you are tightly comparing images. 08_584743-ch04.indd 10508_584743-ch04.indd 105 5/3/10 10:18 AM5/3/10 10:18 AM Part I: Getting Started with Adobe Photoshop CS5 106 Tip Many of the statistics change to match the level under the mouse cursor as you move the mouse over the image. n FIGURE 4.8 The statistics data in the Histogram panel Mouse cursor Statistics data The following statistics are available from the statistics view in the Histogram panel: l Mean: This represents the average level for the selected channel. l Standard Deviation: This represents the variability of the selected channel—in other words, how varied in intensity levels the color in the selected channel is. l Median: This shows the exact middle value of the intensity levels for the channel. In other words, if you took the pixels that are more intense and placed them on one side of a scale and the pixels that are less intense on the other side of the scale, they would be evenly balanced. l Pixels: This shows the number of pixels in the selected channel. This can be useful to see how much data is contained in a specific layer—if you create a selection mask as a layer, for example. l Level: This shows the intensity level value directly under the mouse pointer when the mouse is over the histogram. l Count: This shows the total number of pixels that contain a level of intensity equal to the level under the mouse cursor. 08_584743-ch04.indd 10608_584743-ch04.indd 106 5/3/10 10:18 AM5/3/10 10:18 AM Chapter 4: Understanding Colors, Histograms, Levels, and Curves 107 l Percentile: This displays the percentage of pixels with intensity levels that are at or below the level under the mouse cursor. The percentage is calculated based on a percentage of all pixels in the image, so the level to the farthest left is 0% and the level to the farthest right is 100%. l Cache Level: This displays the setting for current cache level if the Use Cache for Histograms is selected in the Preferences dialog box. Adjusting images with the histogram tools Some Photoshop tools provide histogram tools to help you better see how to adjust and correct images. Although we cover using the tools, such as the levels adjustments, in subsequent chapters, you need to know generally how they work. The tools that Photoshop provides all have the same basic components. As an example, look at the Levels tool in Figure 4.9. The input levels are represented as a histogram. Below the histogram view are three triangular control handles that allow you to easily adjust the input levels of the channel in the image. To the right of the histogram are three eyedropper tools that allow you to select dark, light, and midtone points in the image to quickly adjust the input levels of the channel. The following sections discuss briefly how to use these tools. We discuss the tools in more detail in subsequent chapters as we get into correcting and adjusting images. FIGURE 4.9 The Levels tool showing the histogram utility that Photoshop provides in several different tools Control handles Using the handles to adjust the histogram The dark handle on the left controls the minimum level for the channel, the white handle on the right controls the maximum level for the channel, and the middle gray handle controls the balance of middle tones between the high and low levels. 08_584743-ch04.indd 10708_584743-ch04.indd 107 5/3/10 10:18 AM5/3/10 10:18 AM . Part I: Getting Started with Adobe Photoshop CS5 102 FIGURE 4.4 The color levels histograms of an RGB image in Photoshop Using the Histogram panel In the. 5/3/10 10:18 AM5/3/10 10:18 AM Part I: Getting Started with Adobe Photoshop CS5 104 FIGURE 4.6 The compact, expanded, and all channels views of the Histogram panel in Photoshop Note What channels. images. 08_584743-ch04.indd 10508_584743-ch04.indd 105 5/3/10 10:18 AM5/3/10 10:18 AM Part I: Getting Started with Adobe Photoshop CS5 106 Tip Many of the statistics change to match the level under the mouse