Tài liệu Photoshop cs5 by Dayley part 14 pot

FIGURE 4.4The 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 th

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/underexpo-sure 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

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

gram image of the current channel This view is handy if you just want to dock the histo-gram 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

histo-gram 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 nel that is selected is displayed in the compact and expanded views and at the top of the all chan-nels view Also, the statistics are based on the channel that is selected

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

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

chan-nels 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 gen-eral 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 histo-gram 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

histo-grams of each different layer to see how well the colors match up Making the histohisto-grams 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,

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

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 chan-nel 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