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发信人: killest (victor), 信区: Graphics
标  题: PHOTOSHOP(20)-getting image into photoshop
发信站: 紫丁香 (Mon Mar 23 16:12:41 1998), 转信

RESOLUTION, FILE SIZE, AND OUTPUT
The file size of an image refers to the total amount of pixel
information in the
image. A 6-inch-by-6-inch high-resolution image has more pixels,
and therefore a larger file size, than a 6-inch-by-6-inch
low-resolution image with the same dimensions. In print, a
high-resolution image produces more detail than the same image
at low resolution. In Photoshop a high-resolution image appears
larger on-screen (see page 38). The enlarged on-screen
display does not affect the dimensions of the printed image.
6 in x 6 in @ 72 ppi; file size 547K 100% view on-screen
6 in x 6 in @ 300 ppi; file size 7.6 MB 100% view on-screen
43
Displaying the file size and resolution of
an image
Adobe Photoshop displays the file size for each
image in the lower left of the window. You can also
display the dimensions, number of channels, and
resolution information about a file.
To display the file size:
Position the pointer over the triangle in the bot-tom
border of the image window (Macintosh) or
program window (Windows), hold down the
mouse button, and choose Document Sizes from
the menu.
The file size values appear in the box at the lower
left of the window.
The first value indicates the file size of the final file
as it would be sent to the printer, a flattened file
that contains no layer data. The second value
shows the size of the file including all layers and
channels, that is, the file size if the file were saved
with all its layers. For more information on how
layers affect file size, see 揔eeping track of file
sizes� on page 270.
To display dimensions, channel information, and
resolution information:
Press Option (Macintosh) or Alt (Windows), posi-tion
the pointer over the box in the lower left of the
window where the file size is displayed, and hold
down the mouse button.
The box displays the height and width of the image
(both in pixels and in the units of measurement
currently selected for the rulers), the number of
channels, and the image resolution.
Previewing the page layout and print size
Before you change the size of an image, you may
want to preview how the image will appear on the
printed page. You may also want to preview the
print size of the image梩hat is, the physical size
that the image will have when printed.
CHAPTER 3 44
Getting Images into Photoshop
To preview a page:
Position the pointer over the box in the lower left
of the window where the file size is displayed and
hold down the mouse button.
The dimensions of the page shown in the page pre-view
box correspond to the page size that you
select when you choose File > Page Setup. Options
selected in the Page Setup dialog box, including
registration marks, calibration bars, labels, and
captions, appear as gray boxes.
To preview the print size:
Choose View > Print Size.
Photoshop adjusts the magnification of the image
to display it at its actual printed size, as specified in
the Print Size section of the Image Size dialog box
For more information, see 揅hanging print
dimensions and resolution of an image� on
page 46.
About resampling
Resampling refers to changing the pixel dimen-sions
(and therefore file size) of an image. In
Adobe Photoshop, you can change the pixel
dimensions directly, or you can change the pixel
dimensions by changing either the print dimen-sions
or the resolution while the Resample Image
option is selected (see page 45). When you down-sample
(or decrease the number of pixels), Photo-shop
deletes information from the image. When
you resample up (or increase the number of pixels),
Photoshop creates new pixel information based on
the color values of the existing pixels. In both
cases, Photoshop uses an interpolation method to
determine how pixels are added or deleted (see
揅hoosing an interpolation method� on page 48).
Downsampling and then resampling up to the
original resolution causes a deterioration in the
quality of the image. This is because once an image
has been downsampled, some of the original color
information is lost. When Photoshop resamples
the image back up, it attempts to reconstruct the
file based on the new color information. Because
the added pixels are interpolated from the new
image data, they can make the image appear blurry
or out of focus, resulting in a poorer quality image
than the original.
It抯 best to scan in or create your image using a
high enough resolution so that you don抰 need to
increase the pixel dimensions from within Adobe
Photoshop. If you want to preview the effects of
changing the pixel dimensions on-screen, or print
Registration
marks
Calibration
bar
Caption
Label
45
CHANGING PRINT SIZE By default, changing the dimensions or resolution
of an image changes the number of pixels in
the image, a process called resampling. Changing the total pixel count
will probably result in a loss of quality in the printed
image, because Photoshop must determine the color values of the pixels
to be added or deleted using an approximation or
interpolation method. On the other hand, if you deselect the Resample
Image option in the Image Size dialog box, you can
actually improve an image抯 print quality by increasing its resolution.
With Resample Image off, Photoshop maintains the total
pixel count in an image by adjusting the print dimensions to compensate
for the change in resolution and vice versa.
Original file at 11 x 14 @ 72 ppi, file size 2.28 MB
No resampling; total pixel count maintained; resolution
increased
Image resampled; total pixel count decreased
with loss of quality
5.5 x 7 @ 72 ppi, file size 585K 5.5 x 7 @ 144 ppi, file size 2.28 MB

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※ 来源:．紫丁香 bbs.hit.edu.cn．[FROM: victor.hit.edu.c]

Graphics 版 (精华区)