This document discusses image resolution and its relationship to print size. It states that resolution is measured in pixels per inch (PPI) or dots per inch (DPI), with higher resolutions providing more detail. An image's resolution and print size are directly related, such that changing one affects the other. It recommends a resolution of 200-300 PPI for good output quality and discusses resampling images to change their dimensions while maintaining resolution.

1.  State the constraints between output device resolution,
monitor resolution and original image.

2. In This Chapter, you’ll learn on:
 Identify the relationship between an image's
resolution and an image's print size.
 Adjust monitor and image bit depth
 Resample an image based on a given print size.

3. Identify the relationship between an Image’s
Resolution and an Image’s Print Size
Image Resolution
 Resolution is the number of pixels in a linear inch—
pixels per inch (or PPI), but it is most commonly
referred to as dots per inch (DPI). The more pixels, or
“dots,” per inch, the higher your image resolution
will be.

4.  With color images, each pixel can be one of 16 million
different colors.
 For black and white images, there are 256 gradations of
gray pixels ranging from black to white: 0 (black)
through 255 (white).
 More pixels means higher resolution, which creates
better image quality because you end up with more
realistic representations of color, better gradations of
both individual colors and gray tones, and crisper
images in general.
 The most important thing to understand about resolution
is the relationship between an image’s resolution (DPI)
and an image’s print size (the actual width and height).

5.  These two images give some idea of the difference in quality
obtained from the same size print at two different resolutions.
The first one represents 360 ppi, the second 60 ppi. The latter is
clearly blurred, but it is surprising how low a resolution is
possible before differences are perceived by the naked eye.
Generally, however, more pixels per inch mean more detail
and more subtlety of color.

6.  An image resolution of 200-300 ppi is quite sufficient
for good output, although the optimum for most
printers is 300 ppi.
 An image significantly below 150 ppi begins to blur
(see above) and will eventually look "pixilated" (see
next page);
 and resolutions above 360 ppi usually waste
processor time, as the printer has to resample
down.
 If resampling is required it is usually better to do it in
Photoshop.

7.  A detail showing "pixilation". Separate pixels, the
building blocks of which the image is composed,
can be clearly seen. Notice the jagged, or stepped,
edges apparent on curves, and the incoherent
detail on the mouth of the baby.

8.  Once you scan an image or take a picture with your
digital camera, it becomes digitized—made up of
hundreds of thousands of pixels. Pixels are nothing more
than very tiny colored squares (there are 72 pixels in an
inch) that you can see if you increase the magnification
of any image to its maximum:

9.  If you were to scan a 35mm film at an optical
resolution of 2700 ppi, this will generate a file size of
about 26MB.
 When resizes to fit A4 paper size the image
resolution drops to about 360 ppi, although the file
size remains the same.
 At A3 size the resolution would drop to around 240
PPI, but still gives an acceptable print.
 It is possible to maintain the resolution when
enlarging, by choosing the resampling option in
Photoshop.
 This allows the insertion of extra pixels - interpolation
- by duplicating existing pixels, but can impair
quality if overused.

10. Resample an image based on a given print size.
Resample Image refers to changing the pixel dimension (and
therefore display size) of an image.
 When off, allows you to change any of the dimensions or
resolution options without altering the quality of the image.
 When on, the computer will recalculate the pixel information
when you adjust the settings, altering image quality and file
size. When you down sample (or decrease the number of
pixels) information is deleted from the image and when you
resample up (or increase the number of pixels) new
information is added based on color values of existing pixels.
You specify also an interpolation method to determine how
pixels are added or deleted.

11. When the Resample Image box is checked, any
changes you make to an image’s width or height will
not change the image’s resolution, and as such, any
changes you make to an image’s resolution will not
affect the image’s width and height.
 Keep in mind, however, when you increase width
and height, or resolution, with the Resample Image
box checked, you are adding pixels to your image.
These pixels don’t actually exist so Photoshop must
create them. As such, you will succeed only in
degrading the quality of your image.

12. If you want to increase an image’s width and height, or
resolution, then uncheck the Resample Image box. Now
any changes you make to the image’s width and height
will change the image’s resolution, and vice versa:
 If you decrease resolution, the width and height will
increase.
 If you increase resolution, the width and height will
decrease.
 If you increase the width or height, the resolution will
decrease.
 If you decrease the width or height, the resolution will
increase.

13. Bit Depth
 A "bit" in computer language refers to the smallest amount of
information that a computer can understand. It is an
electronic pulse that contains one of two characters. It
contains either a 1 or a 0 and can either signal on or off. Bit
depth refers to the number of bits found in a graphics file or
that can be displayed on a monitor.
 Pixels, which make up a monitor's screen to display an image,
are made up of bits. The more bits that are contained in the
pixel, the greater the bit depth. The greater the bit depth, the
more colors can be displayed at one time. This is also true of a
graphics file. Bit depth of a graphics file indicates the amount
of color in that file. As you might expect, greater bit depth
brings greater file size. A 4" x 5" RGB photo (24-bit depth) will
have a bigger file size than a grayscale image (8-bit depth) of
the same size and resolution.

14.  The best way to increase the width and height of a
scanned image is to scan the image in at a high
resolution (about twice what your final resolution
should be), and with the Resample Image box
unchecked, decrease the resolution.
 Once the image width and height is where you
want it, you can then check the Resample Image
box and type in the resolution you want if the
resolution is too high. At this point, as long as you
don’t increase resolution, or width and height, your
image quality will not suffer.

15.  The only way to determine what your image's
actual print size will be is through the Image Size
dialog box or by calculating it manually from the
formula (size x dpi = pixel size) i.e. print size= Pixel size
divided by the resolution in dpi).

16. Resample an image based on a given print size.
 We have changed image size from 720x576 pixels to
360x288 pixels and as we also now have marked
"Resample Image",

17. • the prints size is also only half while keeping the
resolution constant. So how getting the size you
want without ruining the native resolution? By
adjusting your print size in editor software and use
PRINT size dialogs not IMAGE size dialogs.

18. You should however be aware of that the width and the height of
the image as you view it on your monitor is not necessarily
representative of the image's actual width and height — the size it
would print out at (Print Size).
Average monitor resolution is 72 dpi. If you view a 72 ppi image at
100% in Photoshop, chances are that it will appear on your screen in
its actual print size. However, this is not true when viewing a 300 ppi
image. A 300 ppi image viewed on-screen at 100% will be enormous.

19. Don't get tricked into believing that what you see on your
monitor is what you'll get when you print or place the
image into another application. The only way to determine
what your image's actual print size will be is through the
Image Size dialog box.
Print size is the size of the representation of an image when
it is printed. The size that it is represented on the screen is
found by multiplying the size in inches by the dpi (size * dpi
= pixel size). An image of 720x576 pixels that is set to 288dpi
would print at 2.5x2 inches.

20. While the image size and resolution have been halved
the print size is still the same.