Tài liệu Photoshop cs5 by Dayley part 13 ppt

Understanding Colors, Histograms, Levels, and Curves IN THIS CHAPTER Understanding color and how Photoshop uses it Understanding how to use histograms to analyze colors Using the Curves

Understanding

Colors, Histograms,

Levels, and Curves

IN THIS CHAPTER

Understanding color and how Photoshop uses it

Understanding how to use histograms to analyze colors Using the Curves tool to adjust tones

Selecting a color mode for

an image Selecting colors in Photoshop

Color is the basic element for everything that you do with images The

purpose of this chapter is to help you understand how Photoshop perceives color so you can use the tools in subsequent chapters more effectively

Photoshop provides some very powerful tools to analyze, adjust, and select

colors in your images This chapter discusses using the histogram to

under-stand the color composition in an image Underunder-standing the histogram is a

basic necessity to make the most out of tools such as the Levels and Curves

adjustments

Another important aspect of this chapter is discussing the color modes

pro-vided in Photoshop and how to select colors based on those color modes

The color modes give you an insight into how Photoshop perceives color and

consequently how to make the most out of the editing tools provided

Color Basics

Color is the basic element of all images The purpose of this section is to

discuss the nature of color in relation to the human eye to give you a good

foundation to work from Understanding color helps you make better

adjust-ments and corrections to your images Also, because Photoshop gives you

the option of working in several different color models, understanding color

helps you choose the best color model

What is color?

They human eye can detect millions of different colors, but really, what are

those colors made of? Put simply, color is made of light Light travels in a

series of waves Visible light is made of waves traveling between a specific set of wavelengths

White light is light that contains waves of all frequencies and therefore contains all colors

The human eye can distinguish the different wavelengths of the light waves The wavelength of each light wave determines the color that the eye detects For example, light waves with frequen-cies on the low end of the visible light spectrum are interpreted as blue, and light waves with fre-quencies on the high end of the visible light spectrum are interpreted as red

When the eye looks at an object, it is detecting the light that is reflecting off the object Depending

on the nature of the surface of the object, some of the frequencies are absorbed by the surface of the object and some are reflected into the eye, producing the colors we see

Color, intensity, and the human eye

The eye detects color in light waves through tiny receptor cells in the retina called cones Light stimulates these receptor cells, and they transfer the data to the brain Of the three groups of cones, some are sensitive to the higher frequencies, some to the lower frequencies, and some to the mid-dle frequencies of light Therefore, no matter how complex the composition of wavelengths in the light that the eye is receiving, the color is broken down into three basic components by the eye

This is important as we discuss topics such as channels, histograms, and color management throughout the book

Another attribute of light that affects the colors we see is the intensity Intensity is the strength of the light reaching the eye Basically, brighter light carries more intensity than does dimmer light

Because the cones in the eye are stimulated by the light waves, less intense waves stimulate them less, resulting in a limited amount of data being collected This limits the number of shades of a color that the eye can discern Understanding this helps you when you are making adjustments and correcting photos

The eye overcomes dim lighting by using additional receptor cells called rods In bright light, the rods perform almost no function; however, in dim light, the rods transfer additional data to the brain The data from the rods is colorless, however, which is why we don’t see much color in very low lighting This also is why indoor photos taken with a flash appear much better than those taken without a flash

Note

The cones in the eye are most sensitive to colors on the upper end of the visible light spectrum Therefore, colors

such as red, green, and yellow are affected less by reducing the intensity than colors such as blue and purple n

Understanding Channels and Levels

Photoshop applies the concepts of digital color in a way similar to the way our eyes work Digital color is divided into channels and levels A channel represents a specific color, and a level repre-sents the intensity of that color Using combinations of different levels of channels, Photoshop can represent millions of colors

To illustrate this better, let’s look at how some colors are represented in Photoshop using the RGB color mode (discussed later in this chapter) Using the RGB color mode, all colors are divided into three channels—red, green, and blue, hence RGB Each channel has an intensity level range of 0 to

255, where 0 is none of the color and 255 is full intensity of the color

Using the RGB color model, the color red is represented as 255 in the red channel, 0 in the blue, and 0 in the green Similarly, green is represented as 255 in the green channel and 0 in the other two Yellow is represented as a combination of the red and green channels at 255 and the blue channel at 0 To get black, all channels are set to 0; to get white, all channels are set to 255 In this way, all colors can be represented as a combination of different levels of the red, blue, and green channels

Tip

Often, when working with multiple images, images on multiple computers, or in different applications, you

need to make certain you use the same color in all places If you note the level value in each channel, you can

easily reproduce the same color, no matter where you are working For example, you may be working in RGB

mode and need to create specific color of blue where the red channel has a level of 26, the green channel has

a level of 74, and the blue channel has a value of 158 n

Each of the different color modes utilizes different color channels However, all color modes use the concept of varying levels of each channel to represent different colors or tones Photoshop pro-vides several tools that use the concept of channels and levels to adjust images and apply special effects Understanding how channels and levels represent color helps you utilize those tools in a much broader scope

Adjusting with Histograms

One of the most useful tools Photoshop provides when adjusting colors and tones in your images

is the histogram Using histograms, you can visually see the composition levels in one or more channels At first histograms may seem a bit daunting to understand; however, after you under-stand what is being represented, your Photoshop life will never be the same In this section, we discuss what histograms are and how to use the histogram tools provided in several areas of Photoshop to quickly adjust the levels of channels

Understanding histograms

At first glance, a histogram looks like the silhouette of a mountain range, as shown in Figure 4.1 A histogram is really just a vertical bar chart The chart is constructed by looking at each pixel in the image and counting the number of pixels that contain a value of 0 for that channel, then the value

of 1, and so forth up to the value of 255

Using the bar chart, histograms show how the levels of each channel are distributed in the image

So what does this mean? It means that you have a visual representation of the color and intensity distributing in your image

FIGURE 4.1

A histogram of the RGB levels of an image in Photoshop

You can interpret the visual data in the histograms in many ways to understand the composition of colors and light in an image to help you make the most out of Photoshop’s adjustment tools To illustrate this, the following sections cover how to use histograms to determine the exposure and color balance in an image

Determining overexposure and underexposure in an image

One of the most useful features of a histogram is the ability to quickly determine how overexposed or underexposed an image is Some photos are obviously overexposed or underexposed, and others just don’t quite look good due to overexposure or underexposure, but it is difficult to tell by just looking

at an image Using histograms helps you quickly tell if an image is overexposed or underexposed

Tip

When adjusting color, contrast, levels, and tone of an image, first check to see if the image is overexposed or

underexposed You want to adjust the exposure first as discussed in Chapter 9 n

Images that are overexposed have mostly higher levels for all channels and almost no lower levels because more light was recorded in the image Therefore, the histogram looks like a mountain on the right and not much on the left Figure 4.2 shows an example of an image that has been overex-posed Notice that most of the data in the image falls to the higher end of the histogram

FIGURE 4.2

A histogram of the RGB levels of an overexposed image in Photoshop

Images that are underexposed have mostly lower levels for all channels and almost no higher levels because not enough light reached the image Therefore, the histogram looks like a mountain on the

left side instead of the right Figure 4.3 shows an example of an image that has been underexposed

Notice that most of the data in the image falls to the lower end of the histogram

FIGURE 4.3

A histogram of the RGB levels of an underexposed image in Photoshop

Determining color balance in an image

Histograms allow you to ascertain the color balance in an image Understanding the color balance helps when you are trying to correct color and tonal issues in images For example, if you look at the histograms of an image that should be balanced and see that the image has a disproportionate amount of red, you can easily adjust the red

The best way to determine color balance in an image is to look at the histograms of each color channel individually Viewing the histograms of each channel shows how much of that color is present in the image compared to the other color channels as well as a distribution of the levels of each channel

Note

When viewing the color channel histograms of an image, keep in mind what colors should be present For

example, if you are working with an RGB image of a boat on the ocean with a blue sky background, you should

see lots of blue, but not much green, and almost no red However, if the image contains people, with green

trees in the background and a blue sky, then there should be a fairly even distribution of red, green, and blue n

Figure 4.4 shows the channel histograms of an RGB image Notice that the red channel has almost

no values, and the values that are present are in the lower levels Conversely, the green channel contains lots of data and is fairly distributed The blue channel contains a moderate amount of data but is definitely skewed to the lower levels

Obviously, the color levels in this image are not balanced, but what does that mean? Well, if the image is of a green plant, then it just means that the image contains much more green than any other color However, if the image is a snapshot of a person’s face, then it means that their skin tone is severely out of whack, and you’ve got your work cut out trying to fix it because you need red to get the pink back in the cheeks

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