Tài liệu Adobe Photoshop 7.0 for Photographers- P6 doc

Adobe Photoshop 7.0 for Photographers

230

Figure 9.6 Curves corrections can be exactly

the same as making a Levels adjustment. In this

example, the two ends of the curve have been

dragged in as you would in Levels, in order to

set the optimum shadow and highlight points. You

can control both the lightness and the contrast

of the image by adjusting the shape of the curve.

Click on the bottom right box to enlarge the

curves dialog, as shown below.

Figure 9.7 If you click on the horizontal output

ramp, the curve point readouts will be displayed

using density percentages when in RGB and ink

percentages when in CMYK. The curve is also

reversed in CMYK, with the shadow point in the

top right corner and the highlight point in the

bottom left. If you Option/Alt-click anywhere

inside the grid area the grid units will switch from

25% to 10% increments.

For example, Chapter Fourteen shows how coloring effects can be achieved through

individual channel curves adjustments. The default RGB units are measured in bright￾ness levels from 0 to 255. CMYK curves are by default displayed differently (click on the

horizontal output ramp to toggle between displaying with levels or ink percentage read￾outs). This alternate mode (see Figure 9.7) is designed for repro users who primarily

prefer to see the output values expressed as ink percentages.

The next example shows how you adjust the curve to both improve image contrast

and correct the color balance at the same time. Providing the monitor has been cor￾rectly calibrated, this can all be judged by eye on the screen.

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Color adjustments

231

1 The photograph shown here lacks detail in the

shadows. The information is there but in its cur￾rent state the hair will print too dark. We don’t

want to lighten all of the photograph, otherwise

the mood in the original picture will become lost.

Choose Image > Adjustments > Curves. The

dialog box contains a line on a graph on which

you can remap the image tones. Identify which

portion of the curve needs adjustment by mous￾ing down in the image area (such as on the darker

areas of the hair) and watch where the circle

appears on the curve.

3 In this corrected version the midtone and high￾light tone values remain unaltered, while the

shadow detail has been lifted. You can always use

Curves in this way to exert fine control over the

lightening or darkening at precise points on the

tonal scale.

2 Click on the curve line to anchor both the

midpoint and the highlights. Now refer back to

stage one which helped you determine where to

add a point on the shadow end of the curve, click

on the curve and drag upwards to lighten. The

line bends to form a smooth curve and the

shadows are made lighter. Precise positioning of

the curve anchor points is achieved by using

either the keyboard arrow keys or entering

numeric values in the Input and Output boxes

below.

Client: Schwarzkopf Ltd. Model: Maria at M&P.

There is also a more precise way of correcting the color balance. If you exploit the

fine tuning capabilities built into the Curves dialog and combine this with the use of

the color sampler tool, you can correct with absolute precision. Repro professionals

will often rely more on the numeric readouts to judge the color in a digital image.

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Adobe Photoshop 7.0 for Photographers

232

1 The above photograph has a cold color cast.

This is particularly noticeable in the backdrop,

which should be a neutral gray. If I want to cor￾rect the color in this picture, the best method is

to apply a curves image adjustment. Go to the

Image > Adjustments menu and choose Curves...

2 This shows you the curves corrected version.

The main Curves dialog allows you to make tonal

corrections to the composite color channel (in

this case, the RGB composite channel). I dragged

down from the RGB channel to select first the

red color channel and added a couple of control

points to the curve to adjust the color balance

for the midtones and highlights. If you are not

happy about the position of a curve point, it is

very easy to move it around and change the shape.

When a control point is selected, to select the

next point, use Control/Right mouse-click+Tab. To

select the previous control point, use Control/

Right mouse-click+Shift+Tab. To get rid of a point

altogether, drag it to the outer edge of the graph

or Command/Ctrl-click on the point in the grid. I

then went to the Blue channel and added a couple

of points. This time to correct the color for the

shadows and highlights in the Blue channel.

Client: Anita Cox Salon.

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Adobe Photoshop 7.0 for Photographers

234

To add a new color value as a control point on the curve, Command/Ctrl-click inside

the document window and you will see a control point appear on the corresponding

portion of the curve. If you use Shift+Command/Ctrl to click in the document win￾dow, Control points will be automatically added to all three color channels at once.

So when adding control points to color correct the white boot, I would

Shift+Command/Ctrl click on top of each of the color sampler points to add these as

control points in all three color channels in the Curves dialog. When you are editing

the points in the Curves dialog, use Shift-click to select multiple points. As you adjust one

control point the others will move in unison. To deselect all the points, use Com￾mand/Ctrl-D. When a single point is selected you can select the next point using

Control/Right mouse-click+Tab and the previous point by using Control/Right mouse￾click+Shift+Tab.

3 The color sample points can be repositioned

as necessary by dragging on them with the color

sampler (you can access the tool while in an image

adjustment dialog, by holding down the Shift key).

Apply a Curves adjustment and adjust each color

channel curve as necessary. The first RGB read￾out figure in the Info palette tells you exactly where

to position the point on the curve (see the In￾put numeric box). Either manually drag the

point or use the keyboard arrows (Shift+arrow

key moves the control points in multiples of 10)

to balance the output value to match those of

the other two channels. What you adjust at one

point on the curve will affect the shape and con￾sequently the color in another part. This is why

it is advisable to monitor the color values across

the range of tones from light to dark. Remember

that you can shift select image sampled colors

to add points to the curve.

Photograph: Davis Cairns. Client: Red or Dead Ltd.

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Color adjustments

233

2 Follow the Levels adjustment procedure as

outlined in the previous chapter (Assigning

shadow and highlight points, page 216). Expand

the tonal range and neutralize the shadows and

highlights as much as possible. This improves the

picture already and removes most of the cyan cast.

1 When you have a white object photographed

against a white background, any color cast will

always be very noticeable. First of all select the

color sampler tool and click on the image in up

to four places to locate the persistent color read￾outs at different places on the boot.

The follow-on tutorial, featuring the white boot, is a good example of where the Info

palette readout can be used to determine the neutrality of the image tones and is the

ultimate guarantee of perfect color correction. If you match up the RGB values so

that red = green = blue, the resulting color is always a neutral gray. Remember, this

is not the case with CMYK color (see Chapter Four on color management).

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