Tài liệu Photoshop cs5 by Dayley part 16 docx

Each color model breaks the light into one or more channels and then assigns an intensity level of each channel for each pixel in the image.. To set the color mode of an image, select Im

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

Understanding the different color modes

A color model is simply a method to translate the light captured in an image into a digital form that the computer and other devices can understand Each color model breaks the light into one or more channels and then assigns an intensity level of each channel for each pixel in the image

Photoshop provides several color modes that match the most common color models

Depending on what you are doing with an image, you want to use a specific color mode that pro-vides the best management of the color To set the color mode of an image, select Image ➪ Mode and then select the mode you want to use from the main menu in Photoshop The following sec-tions discuss each of the color modes and what they are for

Bitmap

The bitmap color mode contains only one channel with only two possible levels, 0 and 255 The translation is that a bitmap is a black and white image without color and even without shades of gray In essence, the image becomes a series of black dots on a white background This may not sound very useful However, there are several good uses for these types of images

The most common use of the bitmap mode is outputting the image to a black and white laser printer Laser printers create images as a series of black dots on the page So working with the image as a bitmap lets you make changes to a version that appears exactly how it will when it is printed by the laser printer

Tip

Bitmap images that are printed on low-resolution laser printers often end up darker than you would expect Be

sure to lighten the image before printing it to a low-resolution laser printer n

The bitmap dialog box shown in Figure 4.15 lets you set the resolution and method to use when creating the bitmap from the image You should set the resolution to the same resolution that you will be using to output the image to the printer The following methods can be selected from the drop-down list in the dialog box:

l 50% Threshold: This sets every pixel in the image that is more that 50% gray to black

and every pixel that is 50% gray or lighter to white This is by far the simplest pattern;

however, the end effect is very choppy, and the image typically doesn’t look very good unless you are trying for a special effect, for which it would be better to use the Threshold tool in the Image ➪ Adjustment menu

l Pattern Dither: The pattern dither uses a pattern to mix black and white pixels together,

which results in the appearance of different shades of gray The problem with this method

is that the pattern shows up in the image, so the effect is not very smooth

l Diffusion Dither: This uses an error-diffusion method of converting an image into a

series of dithered pixels that are less structured than the pattern dither The diffusion dither method produces an image similar to a mezzotint, which often is the best option for printing on low-resolution laser printers

l Halftone Screen: This uses a series of dots of varying sizes and spacing that trick the eye

into believing it is seeing a continuous tone When you select the halftone screen option,

an additional dialog box pops up that enables you to select the frequency, angle, and shape of the halftone dots Typically, the best shape to use is the round shape because it is closest to what the printer generates The frequency depends on the resolution of the printer The higher the frequency, the better resolution you get in the image; however, if you set the frequency too high, the patterns overlap when printed and the results do not look good

l Custom Pattern: This allows you to select a custom pattern, either one of those included

with Photoshop or one of your own The custom pattern can be used with similar results

to the pattern dither Typically, this option is used only if you have a specific pattern that you want to show up in the image

FIGURE 4.15

The options for the bitmap mode in Photoshop

Grayscale

The grayscale mode contains only one channel, but unlike the bitmap mode it can have intensity levels from 0 to 255 The grayscale mode is useful if you are outputting the image to a noncolor printer

Tip

You can create a grayscale image from a single RGB channel by selecting that channel in the Channels panel

and converting the image to grayscale Only the selected channel is converted to grayscale This can be useful

for special effects as well as utilizing the detail of a specific channel n

Another advantage of grayscale is that viewing and adjusting the image in grayscale reduces the over-head of dealing with three color channels Photoshop is much faster at performing complex opera-tions on grayscale images than on multichannel images Having a single channel also makes it easier

to make adjustments to an image because you only need to worry about adjusting that channel

To change an image from color to grayscale, Photoshop takes a composite intensity for all three channels and

reduces it to the single grayscale channel This results in a loss of the original color channels Therefore, make

sure you have a backup copy of the file before you save it again n

Duotone

The duotone mode uses on contrasting color of ink over another to produce highlights and middle tones in a black and white image Duotones typically are used to prepare images for printing

Using the Duotones Options dialog box, you can add one, two, three, or four inks to create a monotone, duotone, tritone, or quadtone image in Photoshop

Use the following steps to configure the monotone, duotone, tritone, or quadtone options from the Duotone Options dialog box shown in Figure 4.16:

Note

The Duotone color mode option is available only for grayscale images If you are using a color image you need

to convert it to grayscale before changing to duotone n

1 Select the type of tone from the Type menu.

2 Click the blank swatch for each ink you need to specify.

3 Select the color to use for that tone

Typically, you should use black for the first ink in the list

4 Click the curve for each ink you need to configure to launch the Duotone Curve tool.

FIGURE 4.16

The options for the Duotone color mode and the Overprint Colors dialog box in Photoshop

5 Use the Duotone Curve tool to adjust the tone curve for that color of ink

Typically, you do not need to adjust the curve unless you want a specific effect from that tone of ink

6 Click the Overprint Colors button, shown in Figure 4.16, to bring up the Overprint

Colors dialog box.

7 Adjust the colors used when one ink is printed over another ink by selecting the

colors for each of the overlapping options.

Indexed color

The indexed color mode contains a single channel with a single set of indexed colors Converting

an image to indexed color reduces the image to a set of most important colors The remaining col-ors are given an index between 3 and 256 in a color lookup table Instead of using the level value

of the pixel to define the intensity of the channel, it points to an index value in the lookup table for the indexed image

To change the color mode of an image to indexed color, select Image ➪ Mode ➪ Indexed Color from the main menu To view the table of indexed colors, select Image ➪ Mode ➪ Color Table from the main menu to bring up the Color Table dialog box shown in Figure 4.17 The following are settings that can be modified in the Indexed Color dialog box:

l Palette: This lets you select which palette to use when choosing the color to be placed in

the index You can choose a palette based on Exact, System, Web, Uniform, Local, Master,

or Custom colors The Local option selects colors local to the image If you are working with multiple images, the Master option selects colors from a master of all images When selecting local or master palettes, you can use a Selective, Adaptive, or Perceptual method

of choosing the colors The Selective method tries to preserve the key colors in the image

The Adaptive method simply preserves the most common colors The Perceptual method intelligently selects colors that will provide the best transitions, rather than just the most popular ones You also have the option to select System colors for images that will be viewed only on a computer The Web option selects only Web-safe colors for images that will be used on Web pages

l Colors: This specifies the number of colors to use in the color lookup table The

mini-mum is 3 colors and the maximini-mum is 256

Tip

When creating Web images with file formats such as gif and png, using fewer colors results in smaller images n

l Forced: This lets you force the conversion to keep certain colors in the image The default

is to force only black and white to be kept The Primaries option protects eight colors:

white, black, red, green, blue, cyan, yellow, and magenta The Web option protects the

216 colors in the Web-safe colors The Custom option allows you to preserve a specific palette of colors that you create When you select the Custom option, a dialog box launches that allows you to specify the colors that you want to preserve

l Transparency: This specifies whether to preserve the transparency in the image.

l Matte: The matte option allows you to specify a matte to use when working with

trans-parency in the image If there is no transtrans-parency in the image, this option is inactive If the transparency option is selected, the translucent areas in the image are filled with the matte

color If the transparency option is deselected, the translucent and transparent areas are filled with the matte color

l Dither: This specifies the method Photoshop uses to calculate replacement colors for

col-ors being discarded from the image The None option simply selects the closest color in the lookup table, which can sometimes result in harder edges but is typically the best option to use The Diffusion option dithers the color randomly, creating a more naturalis-tic effect The Pattern option dithers in geometric patterns, which is usually the least desir-able because the patterns show up in the image The Noise option mixes pixels

throughout the image instead of just the areas of transition

l Amount: This specifies the percentage of diffusion to use when dithering This option is

available only when you select the Diffusion dithering option Lower values decrease the size of the file but result in harsher color transitions

l Preserve Exact Colors: This turns dithering off for areas of solid color when the Diffusion

dithering option is selected This option is not available unless the Diffusion dithering option is selected Using this option helps your images look better even if you have to use dithering

FIGURE 4.17

The options for the Indexed color mode and the Color Table dialog box in Photoshop

RGB color

You likely will use the RGB color model most often The RGB color model is used by computer monitors and the human eye Photoshop processes its wide range of vivid colors quickly

RGB is the model that most closely matches the human eye for two reasons First, RGB uses three colors similar to the receptors in the human eye Second, RGB is additive, meaning that as you add more color, you get more light, in the same way that more color results in the eye seeing white

In the RGB model, colors are divided into three channels of red, green, and blue Each channel has

an intensity level range between 0 and 255 Each color is made up of a combination of intensity levels from these three channels, resulting in the possibility of over 16.7 million different colors

RGB provides by far the most vibrant use of colors of all the color models and is supported by most file formats The one downside to RGB is that it contains more colors, especially the brighter ones, than can be printed This can result in loss of detail in areas of your images when they are printed The solution is to use the CYMK color model if you are going to have your images profes-sionally printed

CYMK color

The CYMK color model is completely different from the RGB model in that it uses a subtractive method, meaning that the more color is added, the less light is seen This is one of the reasons the CYMK model works so well for printing Think about adding ink to a page; if you add all the col-ors, you get black, or rather a really deep brown

Another difference between the CYMK model and the RGB model is that it is made up of four channels: cyan, yellow, magenta, and black The black channel is necessary because adding the ink all together makes a dark brown not black, so if you want the printer to print true black, you must have a separate channel to specify black

Which model should you use for general color image editing? The answer is RGB The RGB model provides the widest range for tonal adjustment and correction The scanner, monitor, and most other devices (except printers) work in the RGB model Also, editing images in Photoshop in the RGB mode is much faster than in CYMK

Tip

Even if you are using the RGB model, you can select View ➪ Proof Colors from the main menu to toggle the

view to a simulated CYMK model This way you can periodically check to see how the image will look when

printed Just remember to turn it off n

Lab color

The Lab color model is very different from RGB and CYMK The Lab model does have three chan-nels, but instead of all three dedicated to colors, only two—a and b—are dedicated to color; the third—Lightness—is dedicated to luminosity

The a channel maps colors ranging from deep green at level 0 to gray at level 127 to a rich pink at

255 The b channel maps colors ranging from bright blue at level 0 to gray at level 127 to a dim yel-low at 255 The Luminosity channel maps the brightness of each pixel from dark at 0 to white at 255

The Lab channel is additive like the RGB model, but it has only two channels of color mixing, and the levels of those channels are not mapping to intensity but rather tones of color The tones add together to form brighter colors, and only the luminosity channel provides data to darken the tone that is created by the other two channels

Editing images in Lab color is about the same speed as RGB and much faster than CYMK, so it is a fun alternative if you want to adjust your thinking of mixing colors

Multichannel

The Multichannel mode separates out the channels in the current color model into spot channels

Spot channels can be used to store parts of an image that you want to print in specific inks or spot colors For example, you can print specific inks from a Pantone library

When you convert an image to the Multichannel model, the current channels are changed to spot channels The channels created in Multichannel mode depend on the original color mode of the image For example, the RGB mode gets converted to cyan, magenta, and yellow spot channels, the CYMK model gets converted into cyan, magenta, yellow, and black spot channels, and the Lab model gets converted into three Alpha channels

Note

The spot channels overlap, so if you do not want ink from one channel to be printed on ink from another

chan-nel, the data in those areas of the channel cannot overlap n

Bits per channel

How many bits should you use per channel? The quick answer is 8 bits per channel, but let’s look

a bit closer What does bits per channel mean? A bit is a single item of information for a computer with a value of 0 or 1 That doesn’t mean much in terms of an image, but if you string millions of bits together, it can mean a lot

Using 8 bits of information, we can define an intensity level of 0 to 255 For three channels, we can define about 16.7 million different colors for each channel If we use 16 bits per channel, that goes up to over 281.4 trillion colors; if we go to 32 bits per channel, well, you get the idea

So why not just use 32 bits per channel and maximize our information? The answer is disk space and speed An image with 32 bits per channel takes up much more disk space and much more effort to edit on Photoshop’s part Plus, the human eye can’t even detect all the colors in the 8 bits per channel

And that leads to the question of why not just use 8 bits per channel, because it is more than enough for the human eye? The answer lies in what happens during adjustments, corrections, and conversions Each time you make a correction to an image, change the levels, add a filter, and so

on, you lose a little bit of the distinguishing detail If you do enough corrections on an image with

8 bits per channel, you may lose noticeable detail in the image However, if you are using an image with 16 bits per channel, the data lost is in levels that cannot be detected by the human eye, so when you convert the image back to 8 bits per channel, there is no data loss

An image must be in the RGB or Lab color modes to convert it to 16 bits per channel To change your image to 16 bits per channel, select Image ➪ Mode ➪ 16 Bits/Channel After you have changed your image to 16 bits per channel, you can change it to 32 bits per channel by selecting Image ➪ Mode ➪ 32 Bits/Channel

An image with 32 bit per channel is considered an HDR (High Dynamic Range) image Typically, these images

are used in 3D rendering and advanced CGI animation effects n

Choosing Colors

Now that you understand about color and how it relates to different modes in Photoshop, the last thing you need to know is how to actually select a color in Photoshop You will be working with many different tools in Photoshop that require you to select colors to use

Remember that even in 8 bits per channel, you can choose from more than 16 million colors The following sections discuss the main methods that you will use to set and select colors

Using the Color Picker tool

The most common method of choosing a color is using the Color Picker tool shown in Figure 4.18 The Color Picker tool is launched by clicking the foreground or background tool in the Photoshop toolbox

The Color Picker tool allows you to select any color possible in Photoshop and gives you a wealth

of information The main areas of the Color Picker tool are the color chooser pane, the range slider, the new/current color view, and the color settings

The Color Picker lets you use the mouse to select the color of any pixel displayed in the pane The range slider lets you use the mouse to adjust the range of colors displayed in the Color Picker pane The new/current color view simply shows the current color on the bottom and the newly selected color on top for comparison purposes

FIGURE 4.18

Selecting colors using the Color Picker tool