Tài liệu Photoshop cs5 by Dayley part 60 pps

Levels The most basic adjustment you can make to an image to change the color and lighting is to change the level composition of the color channels in the image.. cus-Using the Levels Ad

FIGURE 13.23

Using the Hue/Saturation tool, you made the red and yellow leaves pop out and changed the color and intensity of the sky and water to give the photo a brilliant color tone

Levels

The most basic adjustment you can make to an image to change the color and lighting is to change the level composition of the color channels in the image Adjusting the level composition can restore detail in areas of an image and fix a variety of lighting problems

Photoshop provides the Levels Adjustment tool to give you direct access to the levels of the color channels You can access the Levels Adjustment tool by selecting Image ➪ Adjustments ➪ Levels from the main menu or by selecting a Levels adjustment in the Adjustments panel

This section discusses how to configure and use the auto levels adjustment as well as making

cus-Using the Levels Adjustment tool

The best way to use the Levels Adjustment tool is to add a Levels Adjustment layer to the image by selecting the Levels icon in the Adjustments panel This loads the Levels Adjustment Layer tool, shown in Figure 13.24

FIGURE 13.24

Using the Level Adjustment tool, you can adjust the levels of color channels to change the color

composi-tion and bring detail back to images

Presets Black eyedropper

Gray eyedropper

White eyedropper

Midtone slider

The Levels Adjustment tool allows you to make adjustments to the composite channel or any sin-gle color channel using the following controls:

l Presets: This allows you to select Photoshop’s default preset or one of your own saved

ones You can save the settings you make in the Levels tool as a preset by selecting the Save Levels Preset from the Adjustments panel menu Photoshop provides the following predefined adjustments to quickly adjust the levels of images: Default, Darker, Increase the Contrast 1, Increase the Contrast 2, Increase the Contrast 3, Lighten Shadows, Lighter, Midtones Brighter, Midtones Darker, and Custom

l Channel: This allows you to select the composite channel or one of the individual color

channels Making levels adjustments to the composite channel changes the entire image

Making changes to an individual color channel allows you to fix level problems associated with a specific color without affecting other colors

Caution

Adjusting the levels of color channels separately changes the color composition in the image If the general

color tones match up well, you should not change the levels of the color channels separately because you may

end up with some colors that do not match the rest of the colors in the image n

l Black slider: The black slider sets the level value in the image that equates to the desired

lowest level in the color channel (essentially black) All values lower than the value of the black slider are clipped to 0 Typically, you want to set the black slider to the left edge of the histogram mountain and ignore the few pixels that are scattered to the left

l White slider: The white slider sets the level value in the image that equates to the desired

highest level in the color channel (essentially white) All values higher than the value of the white slider are clipped to 255 Typically, you want to set the white slider to the right edge of the histogram mountain and ignore the few pixels that are scattered to the right

Tip

If you hold down the Alt key while adjusting the Black and White sliders, the areas of the image that are being

clipped are displayed in the document window as shown in Figure 13.25 This is extremely useful to help you

understand what detail will be lost when the clipping occurs You also can turn this feature on permanently by

selecting Show Clipping for Black/White Points in the Adjustment Panel menu n

l Midtone slider: The levels of the pixels between the black and white slider are distributed

to fill the full 0–255 range The midtone slider sets the balance point used for distributing the pixels The pixels below the value of the midtone slider are distributed between 0 and

127, and the pixels greater than the value of the midtone slider are distributed between

128 and 255 Typically, you want the pixels to be evenly distributed, so you want to move the midtone slider toward the area of the histogram that has the largest number of pixels

FIGURE 13.25

Holding down the Alt key while adjusting the Black and White sliders displays the clip-ping in the document window

Tip

Adjusting the midtone slider really takes some trial and error An even distribution doesn’t always generate the

best results Don’t be afraid to try different values for the pixels A good rule to remember is that as you move

the midtone slider to the right, the overall tone of the channel gets darker, and as you move the midtone slider

to the left, the overall tone of the channel gets lighter n

l Output levels: The Output Levels sliders allow you to set the minimum and maximum

values for the distribution of levels in the results from the level adjustments For example,

if you set the black slider to 50 and the white slider to 200, the output is distributed between 50 and 200 instead of 0 and 255, which limits the detail by 100 level values

Typically, you want to change this option only if you are planning on combining this image with another that provides the detail in the omitted level values

Tip

You can trade places between the black and white sliders in the Output Levels option to create a negative

effect This can be very useful when you want to combine selections from different color channels n

l Black eyedropper: This allows you to sample the value to be used for black pixels directly

from the image Using the Eyedropper tool sets the value of the black slider to the level of the selected pixel If you hold down the Alt key while using the Eyedropper tool, the image document shows the clipped portions of the image, as shown in Figure 13.25

l White eyedropper: This allows you to sample the value to be used for white pixels

directly from the image Using the Eyedropper tool sets the value of the white slider to the level of the selected pixel If you hold down the Alt key while using the Eyedropper tool, the image document shows the clipped portions of the image, as shown in Figure 13.25

l Gray eyedropper: This allows you to sample the value of a gray pixel directly from the

image The value of the gray pixel is used to calculate the balance of the color channels because a gray pixel of any level should have an equal amount of red, green, and blue

l Auto: Clicking the Auto button applies the Auto Levels Adjustment to the image The next

section discusses setting the options for the auto level adjustment

Note

Photoshop calculates the histogram based on frequency of levels in the image With some images and layer

adjustments, the normal method for calculating histograms can be off For those images, Photoshop displays an

additional Calculate a more accurate histogram button with a triangle warning icon that allows you to use a

more accurate, if less consistent, method for calculating the histogram Sometimes this option produces better

results, but not always, especially if the histogram is fairly evenly distributed n

Configuring the Auto Levels Adjustment

For many general images, the Auto Levels Adjustment in Photoshop is sufficient to correct the lev-els problems in your images To use the Auto Levlev-els Adjustment option, click the Auto button on the Levels Adjustment tool

Because adjusting levels requires lots of complexity, Photoshop allows you to configure the follow-ing options by selectfollow-ing Auto options from the Adjustments Panel menu, by holdfollow-ing down the Alt key (which changes the Auto button to Options) and clicking the Options button, or by clicking the Auto options button in the Non-layer Adjustment tool to display the Auto Color Correction Options dialog box shown in Figure 13.26:

l Enhance Monochromatic Contrast: When this option is selected, all channels are

clipped identically, which preserves the overall color relationship while making highlights appear lighter and shadows appear darker This option is used by the Image ➪ Auto Contrast command

l Enhance Per Channel Contrast: When this option is selected, Photoshop tries to

maxi-mize the tonal range in each channel This results in a much more dramatic correction and provides the greatest amount of detail However, you run the risk of introducing a color cast This option is used by the Image ➪ Auto Tone command

l Find Dark & Light Colors: When this option is selected, Photoshop finds the average

lightest and darkest pixel in the image and uses those pixels to maximize the contrast while still preserving as much detail through minimal clipping This option is used by the Image ➪ Auto Color command

l Snap Neutral Midtones: If you select this option, Photoshop finds the average most

neu-tral color in the image and adjusts the midtone values to make that color neuneu-tral

l Midtones: This allows you to set the color used for the midtone balance when performing

the Auto Levels Adjustment

l Highlights: This allows you to set the color used for the lightest highlights when

perform-ing the Auto Levels Adjustment You also can specify the amount of clippperform-ing that can occur in the highlights during the Auto Levels Adjustment

l Save as defaults: When this option is selected, the values you specify are used as the

defaults when you use the Auto Levels Adjustment These values are preserved even if Photoshop is closed and opened again

FIGURE 13.26

Using the Auto Color Correction Options dialog box, you can adjust the algorithms and settings that

Photoshop uses when performing an Auto Levels Adjustment

Adjusting levels to increase detail in images

Adjusting the levels in an image can help lighten dark areas, reduce the brightness of highlighted areas, restore detail to images, and remove color casts The following example takes you through the process of using the Levels Adjustment tool to adjust the levels in a photo:

The photo is dim due to poor lighting when it was taken

FIGURE 13.27

The lighting in this image is poor because it was taken so close to sundown, so it’s lack-ing contrast and detail

Adjustments panel shown in Figure 13.28.

layer to the image and open the Levels tool.

4 Select the Red channel, and adjust the black slider to the left edge of the histogram

mountain and the white slider to the right edge of the histogram mountain, as shown in Figure 13.29, to drop the lower levels that are not needed

5 Select the Blue channel, and adjust the black slider to the left edge of the histogram

mountain and the white slider to the right edge of the histogram mountain, as shown in Figure 13.29, to drop the lower levels that are not needed

FIGURE 13.28

Selecting the Levels tool from the Adjustments panel in Photoshop adds an Adjustment layer to the image and launches the Levels Adjustment tool

FIGURE 13.29

Adjusting the black and white sliders in each of the color channels drops pixel values that are not really needed and opens up room in those levels for additional detail

Adjusting the midtone range of the green channel removes the green color cast in the image

6 Select the Green channel, and adjust the black slider to the left edge of the

histo-gram mountain and the white slider to the right edge of the histohisto-gram mountain, as shown in Figure 13.29, to drop the lower levels that are not needed

green cast that existed in the image due to the color of the sky and ocean

FIGURE 13.30

Using the Levels Adjustment tool, you restored lots of the detail and contrast in the image and removed the green color cast

On the Web Site

The project used to create the image shown in Figure 13-30 can be found on this book’s Web site as Figure

The most powerful tool Photoshop has for adjusting color and lighting is the Curves Adjustment tool The Curves Adjustment tool provides a dynamic and flexible interface that allows you to make dramatic changes to the lighting and color in images Although the Curves Adjustment tool takes a bit of practice and patience to learn, after you are effective with it, you can make changes in images that you may have thought were not possible

The Curves Adjustment tool can be accessed by selecting Image ➪ Adjustments ➪ Curves from the main menu or by selecting a Curves adjustment in the Adjustments panel This section discusses how to configure and use the Curves Adjustment tool to make changes to the color, lighting, and contrast in your images, as well how to configure and use the Auto Curves Adjustment

Cross-Ref

The concept of using curves to adjust color channel levels was introduced in Chapter 4 You should read that

chapter before reading this section because you need to understand the concepts from Chapter 4 to get the

most out of using the Curves tool n

Using the Curves Adjustment tool

The best way to use the Curves Adjustment tool is to add a Curves Adjustment layer to the image

by selecting the Curves tool in the Adjustments panel This loads the Curves Adjustment layer tool shown in Figure 13.31

The Curves Adjustment tool allows you to make adjustments to the composite channel or any sin-gle color channel using the following controls:

l Presets: This allows you to select one of Photoshop’s predefined presets or one of your

own saved ones You can save the settings you make in the Curves tool as a preset by selecting the Save Curves Preset from the Adjustments Panel menu Photoshop provides the following predefined adjustments to quickly adjust the levels of images: Default, Color Negative, Cross Process, Darker, Increase Contrast, Lighter, Linear Contrast, Medium Contrast, Negative, Strong Contrast, and Custom

l Color channel: This allows you to select the composite channel or one of the individual

color channels Making curves adjustments to the composite channel changes the entire image Making changes to an individual color channel allows you to fix level problems associated with specific color without affecting other colors

Caution

Adjusting the curves of color channels separately changes the color composition in the image If the general

color tones match up well, you should not change the levels of the color channels separately because you may

end up with a color cast or some colors that do not match the rest of the colors in the image n

l Black eyedropper: This allows you to sample the value to be used for black pixels directly