Sử dụng photoshop cs5 part 19 pptx

Changing the document resolution and dimensions In this section, you’ll encounter three related terms: ➤ A file’s pixel count or pixel dimensions is arrived at by multiplying its pixel

IN THIS CHAPTER Changing the document resolution

and dimensions 121

Changing the canvas size 126

Cropping an image 127

Flipping or rotating an image 131

Straightening a crooked image 132

7

editing commands, you need to make

sure your document has the proper size

and orientation and is cropped to your

liking In this chapter, you will learn how to change

a document’s resolution, dimensions, and canvas size,

and how to crop, flip, rotate, and straighten it

Changing the document resolution

and dimensions

In this section, you’ll encounter three related terms:

➤ A file’s pixel count (or pixel dimensions) is arrived

at by multiplying its pixel height and width (as in

3000 x 2000 pixels)

➤ The resolution (or “res,” for short), is the fineness

of detail in a document, and is measured in pixels

per inch (as in 250 or 300 ppi)

➤ The process of changing a file’s pixel count (adding

or deleting pixels) is known as resampling

Some input devices (e.g., digital cameras that capture

8 megapixels of data or more and high-end scanners)

produce files with a higher pixel count than is needed

for most standard printing devices In Photoshop,

you can take advantage of a file’s high pixel count to

increase its print size or print resolution You can

keep the pixel count constant as you increase the print

size (and thereby lower the resolution) or increase

the resolution (and thereby lower the print size) No

resampling occurs in either case, so the image quality

isn’t diminished

You will need to resample a file if it contains too few

or too many pixels to meet the resolution requirement

of your target output device If you resample a file as

you increase its resolution, pixels will be added to it

and its storage size will increase accordingly Resample

a file as you decrease its resolution (downsample it),

and pixels will be deleted The only way to get those

pixels back is by clicking a prior state on the History

panel before closing the file Even more important,

resampling reduces the image clarity This can be a

problem for print output, depending on the output

resolution and how drastically the file is resampled,

although it can be remedied somewhat by applying

a sharpening filter afterward (see pages 296–300)

Resampling isn’t a problem for Web output

We’ll show you how to resize three common types

of files for print output — low res/large dimensions,

high res/small dimensions, and medium res/small

dimensions — and how to resize a file for Web output

PIXELS

Pixels, short for “picture elements,”

are the building blocks that make

up a digital image — the tiny individual dots that a digital camera uses to capture a scene or that a computer uses to display images onscreen When working

in Photoshop and for Web output, you’ll need to be aware of the pixel dimensions, or pixel count, of an image For print output, you’ll need

to be aware of the resolution of your image — the number of pixels per unit of measure, which is nor-mally per inch, or “ppi.”

122 Chapter 7

By default, photographs from a digital SLR camera

have a low resolution (72 to 180 ppi) and very large

width and height dimensions They contain a

suf-ficient number of pixels for high-quality output

(prints as large as 8" x 12"), provided you increase

their resolution to the proper value You can do this

via the Image Size command in Photoshop

To change the resolution of a digital photo

for print output (low res/large dimensions):

1 With the file open in Photoshop, choose Image >

Image Size (Ctrl-Alt-I/ Cmd-Option-I).

2. The Image Size dialog opens.A Because you

need to increase the image resolution, uncheck

Resample Image When you lower the Width

and Height in step 4, the resolution will increase

automatically

3 In the Document Size area, choose a unit of

measure from the menu next to the Width field

(we chose inches); the same unit will be chosen

automatically for the Height

4 Enter the Width or Height for the desired print

size; the Resolution value increases

5 If the resolution is now between 240 and 300

pixels per inch, you’ve achieved your goal — just

click OK.B The pixel dimensions didn’t change,

so you won’t need to resharpen the image (A–B,

next page)

If the resolution is greater than 300 ppi, check

Resample Image, then enter a Resolution of

300 Also, from the menu at the bottom of the

dialog, choose a resampling method for the way

in which Photoshop reassigns color values based

on the values of existing pixels The Bicubic (Best

for Smooth Gradients), Bicubic Smoother (Best

for Enlargement), and Bicubic Sharper (Best for

Reduction) options cause the least reduction in

image quality As a result of resampling, the pixel

dimensions of the image will have changed

6 Click OK If the image was resampled, you should

now resharpen it (see pages 296–300)

➤ To restore the settings that were in place when

you opened the Image Size dialog, Alt-click/

Option-click Reset (the Cancel button becomes

a Reset button)

➤ To specify a default Image Interpolation method

for Photoshop features, such as the Image Size

dialog, see page 386

JPEGS FROM BRIDGE TO CAMERA RAW

From Bridge, you can open JPEG fi les into Photoshop

by way of the Camera Raw dialog By default, Camera Raw assigns a resolution of 240 ppi to all fi les it opens into Photoshop To achieve that 240 ppi resolution, Camera Raw preserves the pixel count but alters the Width and Height (Document Size) To increase the resolution of a JPEG fi le (say, to 300 ppi) or to reduce its Document Size dimensions, follow the steps on this page

A In the Image Size dialog, uncheck Resample Image to make the Width, Height, and Resolution interdependent (as shown by the link icon).

B When we changed the Width value to 8, the Height value changed automatically and the Resolution value increased to

300, but the Pixel Dimensions stayed the same.

In many cases, scanned images have a high

resolu-tion and small dimensions and contain a sufficient

number of pixels for large printouts

To resize a scanned image for print output

(high res/small dimensions):

1 Choose Image > Image Size (Ctrl-Alt-I/

Cmd-Option-I) The Image Size dialog opens

2 Make sure Resample Image is unchecked

3 Increase the Width or Height to the size needed

for your printout The Resolution will decrease

If the Resolution falls between 240 and 300 ppi,

you’re done; click OK Because no resampling

occurred, no resharpening is necessary

If the Resolution is still greater than 300 ppi,

check Resample Image, C then lower the

Resolution to 300 From the menu at the

bottom of the dialog, choose Bicubic Smoother

(Best for Enlargement) as the interpolation

method You’ve just resampled the image, so you

should resharpen it after clicking OK (see pages

296–300)

4. Click OK

C Our first attempt at resizing our photo left us with too high a resolution, so here we’ve checked Resample Image and will lower just the Resolution (not the Width and Height).

A The original photo has dimensions of 25 x 35.556 inches

(way too large for our printer) and a resolution of 72 ppi. B When we reduced the photo size to 6 x 8.533 inches via

the Image Size dialog, the resolution increased automatically

to 300 ppi Because the pixel count didn’t change, the image size and quality were preserved

PERMITTING STYLES TO SCALE … OR NOT

When Resample Image and Constrain Proportions are checked in the Image Size dialog, you can check or uncheck Scale Styles to control whether any layer styles that were applied to the image will be scaled to fi t the new size (to learn about styles, see Chapter 21)

124 Chapter 7

Small files (with a resolution of, say, 180 to 200 ppi)

lack a sufficient number of pixels to be enlarged

without resampling, so they must be resampled to

achieve the dimensions needed for print output

This is not an ideal scenario, as it reduces the image

sharpness, and you’ll certainly need to apply a

sharp-ening filter afterward

To resize a scanned image for print output

(medium res/small dimensions):

1 Choose Image > Image Size (Ctrl-Alt-I/

Cmd-Option-I) The Image Size dialog opens

2 Check both Resample Image and Constrain

Proportions.

3 Enter the Width desired for your printout The

Height value will change proportionately and

the file storage size and pixel dimensions will

increase

4. Click OK Since the image was resampled, you

should now use a sharpening filter to resharpen

it (see pages 296–300)

Because Web images are viewed on computer

dis-plays, which are low-resolution devices, they should

have a lower pixel count than images designed

for print output In most cases, you will need to

downsample your files (discard image pixels) to make

them the correct size for output

To change the pixel dimensions of an image

for Web output:

1 Use File > Save As to make a copy of your file,

then choose Image > Image Size (Ctrl-Alt-I/

Cmd-Option-I) The Image Size dialog opens.A

2 Make sure Resample Image is checked

3 To preserve the width-to-height ratio of the

image, check Constrain Proportions.

4. From the menu at the bottom of the dialog,

choose the Bicubic Sharper (Best for Reduction)

resampling method, which will degrade the

image the least

5 Enter a Resolution of 72 ppi

6 In the Pixel Dimensions area, choose pixels from

the menu (the default unit), then enter the exact

Width and/or Height dimensions needed.B

7 Click OK On pages 421–426, you will learn how

to optimize Photoshop files for output to the

Web

A These are initial Image Size values of a typical digital photo To prepare this photo for Web output, we will need to lower its pixel count.

B We checked Resample Image, changed the Resolution to

72, and set the Width (under Pixel Dimensions) to 288 The file size, which is listed at the top, is now smaller because we lowered the document’s pixel count The image now has an appropriate size for online viewing.

COPYCAT

In some dialogs that have Width and Height fi elds, if you choose a unit of measure from the menu for the Width, the same unit is chosen automatically for the Height, and vice versa If you want to prevent this from happening, hold down Shift while choosing a unit; the unit will change just for that dimension

PIXEL COUNTS, RESOLUTION, AND IMAGE SIZE COMPARED

The moral here: Don’t judge the output size of an image based on its onscreen size Instead, consider these two factors: the current zoom level of the image in the document window and the image resolution

The illustrations below explain the concept of

reso-lution and how it affects image size Figures A–B

compare the same image at two different

resolu-tions, and figures C–D compare the print sizes for

those resolutions

B This is the same image with the same dimensions, except here the resolution is 150 ppi, which is half that of the image at left When viewed at the same zoom level of 25%, the low-resolution image displays at only half its original size because it now contains fewer pixels (has a lower pixel count).

A This original image has a resolution of 300 ppi (the

resolution value is listed on the Status bar when Document

Dimensions is chosen for the bar).

C For this 300 ppi image, we chose View > Print Size,

which zoomed the image to an onscreen approximation of

the printout size (that is, the Document Dimensions, as

listed on the Status bar) Note the zoom level is 24%.

D We also chose View > Print Size for this low-res (150 ppi) version of the same image; note that the zoom level here is 48% Although this image and the image shown at left will print at the same size, the print quality

of this one will be lower because it has a lower pixel count.

126 Chapter 7

Changing the canvas size

By using the Canvas Size command, you can

enlarge or shrink a document’s live, editable area

Pixels can be added to or removed from one, two,

three, or all four sides of the image This is useful,

say, if you want to make room for type, as in the

example shown on this page, or to accommodate

imagery from other documents (see Chapter 14)

To change the canvas size:

1 Choose Image > Canvas Size (Ctrl-Alt-C/

Cmd-Option-C) The Canvas Size dialog opens

2 Optional: Choose a different unit of measure

from the Width menu

3. Do either of the following:

Enter new Width and/or Height values The

dimensions are independent of one another;

changing one won’t affect the other.A–B

Check Relative, then in the Width and Height

fields, enter positive values to increase those

dimensions or negative values to decrease them

4 Optional: The gray square in the center of the

Anchor arrows represents the existing image area Click an arrow to reposition the image rel-ative to the canvas The arrows point to where the new canvas area will be added

5 From the Canvas Extension Color menu,

choose a color for the added pixels Or to choose a custom color, choose Other or click the color square next to the menu, then click

a color in the Color Picker (see page 186) or

in the document window If the image doesn’t have a Background (take a peek at the Layers panel), this menu won’t be available

6 Click OK C Any added canvas area will auto-matically be filled with the color you chose in the preceding step, unless the image contains layers but not a Background, in which case the added canvas area will be transparent

➤ You can also enlarge the canvas area manually

by dragging with the Crop tool (see page 129)

A This is the original image.

C After adding pixels to the top of the canvas,

we created some editable type.

B To add canvas area to the top of the image, in the Canvas

Size dialog, we increased the Height value, then clicked the

bottom Anchor arrow to move the gray square downward.