Tài liệu Photoshop cs5 by Dayley part 15 doc

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 eyedr

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

So what do the histogram controls really do? To help clarify, notice that most of the level values in the histogram shown in Figure 4.10 are located in the center levels That means a relatively small number of tones are represented in the image, about 75 out of 255 The result typically is a washed-out image with very little detail

Now move the left control over to the right until it is on the left side of the histogram mountain, and move the control on the right to the left until it is on the right side of the histogram mountain,

as shown in Figure 4.10

On the Web Site

A file with a histogram similar to the one in the image shown in Figure 4.10 can be found on this book’s Web

site as Figure 4-10.tif You can open it in Photoshop Try adjusting the levels and see how changing the

histo-gram works and the results in the image n

Notice the values of the Histogram panel are distributed more evenly between 0 and 255 This means a greater range of tones is represented in the image because it includes values ranging between 0 and 255 instead of just a range between about 100 and 175 The tonal range of the entire image has been extended, providing much more detail with the simple adjustment of the two sliders

FIGURE 4.10

Adjusting the minimum and maximum levels on the Levels tool to match more closely with the actual data

in the image

The middle slider simply adjusts the balance of the midtone levels in the histogram When the his-togram is weighted heavily to one side or the other, the middle adjustment handle can be moved toward that side to balance midtones represented in the image

For example, the Levels tool shown in Figure 4.11 is weighted heavily to the left, which means that all the data for the image is in the darker end of the levels Move the midtone handle to the left toward the middle of the histogram mountain Notice that the histogram mountain moves to the right to balance on the new location of the midtone slider

On the Web Site

A file with a histogram similar to the one in the image shown in Figure 4.11 can be found on this book’s Web

site as Figure 4-11.tif You can open it in Photoshop Try adjusting the levels and see how changing the

histo-gram works and the results in the image n

FIGURE 4.11

Adjusting the midtone levels on the Levels tool to change the midtone levels balance in an image

Using the eyedroppers to adjust the histogram

The Eyedropper tools allow you to adjust the levels of a channel by selecting three points in the image The eyedropper on the left sets the minimum level for the channel, the eyedropper handle

on the right sets the maximum level for the channel, and the middle eyedropper controls the bal-ance of middle tones for the channel between the high and low levels

The eyedroppers work similarly to the control handles except that instead of selecting a specific level for the dark, light, and midtones, you can select pixels directly in the image To use the eyedroppers, simply click the left Eyedropper tool and select a pixel that should appear black in the image Then click the right Eyedropper tool, and select a pixel that should appear white in the image Finally click the middle Eyedropper tool, and select a pixel that should match the midtone for the color channel selected In the case of all channels, select a pixel that should appear gray in the image

Tip

The eyedroppers are fast and extremely accurate if there are items in the image that should appear black,

white, and gray in the image If you are not certain of the colors of the pixels, use the control handles instead

of the eyedroppers n

Adjusting levels with the Curves tool

Another tool that Photoshop provides to help you better see how to adjust and correct images is the Curves tool, available by selecting Image ➪ Adjustments ➪ Curves from the main menu in Photoshop The Curves tool is one of the most difficult tools to quickly grasp, so it has become one

of the most avoided tools in Photoshop However, after you learn how to use it, a whole new world

While the Histogram tool allows you to change the light, dark, and midtone values for the histo-gram, the Curves tool allows you to apply a complex curve equation to the histohisto-gram, giving you unlimited control of the range of levels in the image Using the Curves tool, you can control com-pletely the tonal properties of an image

In this section, we discuss what the curve is and how to use the Curves tool to adjust the levels in the image Using the different Curves tools in Photoshop for specific purposes is covered in later chapters of this book

Understanding curves

To understand how the Curves tool works, you need to understand the curve itself The curve starts as a diagonal line with the value of 0 on the left and 255 on the right, as shown in Figure 4.12 This means that the pixels currently with an intensity level of 0 for that channel have a value

of 0 in the histogram, the pixels at level 1 have a value of 1, and so on up to 255

Note

The grid lines in the curve window allow you to more easily tell what the values are for points on the line The

bottom/left line has a value of 0, the middle line has a value of 127, and the top/right line has a value of 255

The middle left/bottom line has a value of 64, and the middle right/top line has a value of 192 n

FIGURE 4.12

A simple linear curve in Photoshop

The Curve tool works by applying new level values in the histogram for each of the current levels

Think of the Selection tool just as you would a graph with the existing level values specified along the bottom axis from 0 to 255 The new values are equal to the value of the point on the line corre-sponding to each of the old levels

To help you understand the curve better, look at the following example Figure 4.13 shows a mod-ified curve Notice that the points on the left (0), right (255), and the middle (127) all are on the original line, which means that pixels with those level values will not change However, the curve goes above the original line before level 127 so those pixels for each of those levels increase to match the value of the line

Using only two points may be a bit of a problem with the tonal correction Notice that the levels close to 0, 127, and 255 do not change as much as the levels around 64 and 192 To overcome this problem, Photoshop allows you to apply additional points to the line to adjust the curve in several ways

FIGURE 4.13

A curve that maps pixels with levels between 0 and 127 to higher levels and pixels with values between

127 and 255 to lower values

Points

Using the Curves tool

Now that you understand how the curve works, we discuss the features of the Curve tool that enable you to create dynamic curves that can really have an impact on the tones of your images

These tools can be found whether you are adjusting images using the Curves tool, with an adjust-ment layer, or in Camera Raw

Selecting the channel

The first thing you want to do is to select that channel you want to adjust The Channel drop-down menu, shown in Figure 4.14, allows you to select any channel or all the channels

FIGURE 4.14

The Curves tool in Photoshop

Control handles

Point Curve button Freehand Curve button

Click and Drag button

Adding points

Points can be added to the curve by clicking the points curve button and then clicking the curve in the curve window When the points curve button is selected, you can use the mouse to add points by clicking on the curve You can add up to 14 points to the curve, making a total of 16 points including the ends, because the end points also are adjustable Points can be removed from the line by dragging them with the mouse to the bottom-left corner or to the top-right corner of the curve window

Tip

The more points you add to the line, the finer your adjustments to the tonal correction are However, adding

more points makes it more difficult to make adjustments because you may need to adjust all points when you

adjust one Make the bigger adjustments first, and then add more points as needed to speed things up n

Adjusting the input levels

You can adjust the input levels using the control handles along the bottom axis of the levels win-dows This is the same thing as adjusting the minimum dark and maximum light levels in the his-togram This limits the curved lined to operate between those ranges To adjust the input levels, simply grab one of the control handles with the mouse and drag it to the appropriate value

You should adjust the input levels before you create your curve because adjusting the input levels afterward

results in changes to the curve n

Adjusting specific levels from the image

A useful feature of the Curves tool is the ability to select a level directly from the image This allows you to simply select a particular area of the image based on what colors/tones you see

To adjust the levels from the image, click the click-and-drag button in the Curves tool (refer to Figure 4.14) Then use the mouse to select a pixel in the image containing the tone you want to adjust A new point is created on the curve While holding down the mouse button, you can move the mouse up and down to adjust the level of that point

Creating a freehand line

If 16 points on the curve line is not enough, you can create your own line by clicking on the Freehand Curve button (refer to Figure 4.14) The Freehand Curve button allows you to draw a freehand line directly on the curve window instead of using points to create the curve

The freehand curve can be used by itself to create the curve You can use the Freehand Curve tool

to make slight, nonsmooth adjustments to the curve after you have created it with points Only the areas of the curve that you draw with the Freehand tool are corrected

Tip

You can convert a freehand line into a points curve by clicking the points curve button This is helpful if you

need to make some additional adjustments to the points curve after you change it using the Freehand tool n

Using the eyedroppers

The eyedroppers work the same way for the Curves tool as discussed earlier in the Histogram tools section Adjusting the image first with the eyedroppers sometimes gives you a better base to start from when making the tonal corrections with the curve

Working in Different Color Modes

Photoshop provides several color modes that help when working with images Ultimately, as we discussed earlier in this chapter, color is just different intensities of light at different frequencies

However, that data needs to be translated into a quantifiable form that can be understood by Photoshop, the monitor, printers, and ultimately you

This section gives you an overview of the different color modes available for use in Photoshop and why to use them It also helps you understand the bits/channel settings for the image modes