Pixels: Digital Building Blocks
When it comes to digital cameras, most consumers (and sales-
people) seem obsessed with megapixels. Because “everybody
knows” that having more pixels means better images (it doesn’t
by the way). What’s lacking in all this hoopla is
a clear understanding of what pixels are and just
how many you need. The more pixels you have
(whether they are captured with your digital
camera or acquired with a scanner), the more
RAM you need to buy and extra hard drive
space to store them all. So it behooves you to
understand some of the technology behind the
images you want to capture, manipulate, output,
and store.
In the Beginning…
Essentially, computers and video devices use pixels to express im-
age information. Each pixel is a small square of light. The pixel is
the smallest portion of an image that a computer is capable of dis-
playing or printing. Too few pixels and an image appears “blocky”
2 Chapter 1 Digital Imaging Fundamentals
because there is not enough detail to work with.
Too many pixels and the computer or output
device slows down dramatically because it has to
process so much information.
But where did the term pixel come from? Pixel
is an abbreviation for picture element. The word
was coined to describe the photographic ele-
ments of a television image. In 1969, writers for
Variety magazine took pix (a 1932 abbreviation of
pictures ) and combined it with element to describe
A close-up of TV picture elements, or pixels.
how TV signals came together. There are even
earlier reports of Fred C. Billingsley coining
the word at NASA’s Jet Propulsion Laboratory
in 1965. Although the exact origins of the word
may be disputed, its meaning is not. The word
pixel quickly caught on, first in the scientific com-
munities in the 1970s and then in the computer-
art industry in the mid 1980s.
So What Are Megapixels?
When you shop for a digital camera, you are
bombarded with talk of megapixels. Consumers
are often misled about what megapixels are and
how many are needed. A megapixel is simply a
The red circle shows an enlargement of the image. Notice
unit of storage, whether internal or on a remov-
how you can see actual pixels when you increase the magni-
fication of an image. These squares of light are the building
blocks of all digital photos.
able card. A megapixel is one million pixels and
is a term commonly used to describe how much
data a digital camera can capture. As with your
car, just because your tank can hold more gallons of gas doesn’t
mean it’s more fuel efficient or better than your coworker’s car.
For example, if a camera can capture pictures at 2048 × 1536
pixels, it is referred to as having 3.1 megapixels (2048 × 1536 =
3,145,728). If you were to print that picture on paper at 300 pixels
per inch (ppi), it would roughly be a 7" × 5" print. Professional
Digital cameras
use card-based
storage, like this
compact flash
card, to hold the
captured pixels.
photographers need more pixels than this, but a consumer may
not. It all depends on how the pixels are meant to be displayed.
The more pixels you capture, the larger the image is (both in disk
space and potential print size). Consumer usage such as email or
inkjet prints is less demanding than professional usage such as
Understanding Resolution 3
billboards or magazines. Professionals need more megapixels than
consumers; hence, high-end cameras cost more because they are
targeted at people who make money by taking photos.
TIP
Don’t Believe the
Megapixel Myth
More megapixels does not guar-
Understanding Resolution
antee a better picture. Instead of
picking a camera solely on how
OK, prepare to be confused (but not for long). A lot of terms are
used to describe image resolution. The problem is that many
people (and companies) use the wrong terms, which understand-
ably leads to a great deal of confusion. Let’s take a quick look at
the most common terms and their accurate meanings.
many pixels it will capture, investi-
gate cameras with better lenses or
options that are important to you.
If you are shooting for large-format
output, you’ll need a larger megapix-
el-count camera, but if you’re shoot-
ing for personal use, consider how
Dots Per Inch (dpi)
you output most of your pictures.
The most common term used to describe image resolution is dots
per inch (dpi). Although you’ll hear it used for digital cameras and
TIP
scanners, it is really only appropriate for printers. As a measure-
A Fix for Those with Less Than
ment of resolution, dpi is fairly straightforward.
Perfect Eyesight
To determine dpi, it is necessary to count the number of dots that
can fit in a 1 inch × 1 inch area. A higher dpi can mean smoother
photographs or line art; for example, newspapers tend to use approxi-
mately 150 dpi, whereas magazines can use up to 600 dpi. Consumer
printers easily print at 600 dpi or even higher, which can produce
Are you working with a high-reso-
lution monitor and having a hard
time seeing your menus in Photo-
shop? You can change the size of
the display text. Press Command/
Ctrl+K to open the Interface Prefer-
extremely good results (when using the right paper). An increase in
ences window. From the UI Font
dpi can produce even better-looking images. You’ll see (and hear) dpi
Size menu choose Medium or Large
used a lot, but it solely refers to print and physical output.
to give your eyes a break.
It’s only in evaluating printers that the term dots per inch
In a commercial printing environment, very high-resolution
(dpi) makes sense.
images are required.
4 Chapter 1 Digital Imaging Fundamentals
Pixels Per Inch (ppi)
When you view your images on a computer
monitor, you are seeing pixels displayed on your
screen. Computer monitors use the concept of
logical inches. The Mac OS used 72 pixels per inch
(ppi) to match the concept of the printing idea of
72 points per real inch on paper. The Windows
OS has traditionally used 96 ppi.
As computer monitors have evolved, they’ve ad-
vanced to support variable resolution settings. As
such, the actual ppi for a screen can vary greatly
Modern computer monitors support
various screen resolutions. Changing
the monitor resolution results in a
different amount of pixels per inch
displayed on your monitor. Do not run
Photoshop at a screen resolution of less
than 1024 x 768, or it will cause user
interface problems.
TIP
Scanner Advice
The most important issue with
scanners is optical resolution versus
interpolated resolution. A scanner
captures optical resolution through
hardware. Interpolated resolution
is what happens after the captured
data is enlarged via software. Most
manufacturers claim very high
numbers of spi (or dpi). However,
these interpolated results use soft-
ware to enlarge the image, which
is undesirable. You should only care
about the optical resolution when
choosing a scanner.
depending on the physical size of the screen
and the resolution being used by the computer’s
graphics card. Worry less about the ration of
pixels on your screen and simply accept that the
standard measurement of resolution in Photo-
shop (and most computer programs) is ppi.
Samples Per Inch (spi)
What about scanners, you might ask? Manu-
facturers often tout the dpi capabilities of their
scanner. This is inaccurate. Scanners don’t use
dots, they use samples. A sample is when a scan-
ner captures part of an image. Samples per inch
(spi) is a measurement of how many samples are
captured in the space of one inch. In general,
an increase in sampling leads to a file that is
truer to its analog original. However, there is a
threshold: Once a certain amount of information
is surpassed, human senses (or electronic output
devices) cannot tell the difference.
Consumer-level scanners can capture optical
resolution ranging between 300 to 4800 spi. Pro-
fessional devices can capture significantly higher
optical resolution. Capturing a large number of samples is crucial
if you need to enlarge an image. More samples per inch translates
into more information available as pixels, which can then be har-
nessed in output when they are converted to dots in the printer. So
if your scanner’s software says dpi, it really means spi, but you can
see how the two are closely related.
Image Mode 5
Lines Per Inch (lpi)
In professional printing environments, you’ll often hear the term
lines per inch (lpi). This is from the traditional process where im-
ages with gradiated tones (such as photographs) are screened for
printing to create a halftone. This was originally performed by lay-
ing film with dots printed on it over the film before exposure. In
the digital age, this process and these terms are used less often, but
it is still good for you to have a basic understanding.
These days, the work of converting an image to lines is performed
by an imagesetter. The dots are arranged in lines, and the lpi mea-
surement refers to the number of lines per inch. An increase in lpi
This image has been converted to a
halftone, as is evident by the visible
results in smoother images. Table 1.1 shows the most common lpi
dot pattern.
settings for different output formats.
Table 1.1 Common lpi Measurements
Output Method Typical lpi
Screen printing 35–65
Laser printer (matte paper) 50–90
Laser printer (coated paper) 75–110
Newsprint 60–85
Offset printing (uncoated paper) 85–133
Offset printing (coated paper} 120–150+
High-quality offset printing 150–300
Image Mode
Within Photoshop, you need to choose from one of eight image
modes when working with a document. The mode you pick will
depend on what you need to do with the image and how you
2
VIDEO
TRAINING
Converting Image Modes
intend to output it. The three most common modes are RGB, gray-
scale, and CMYK, but it’s worth taking a quick look at all eight.
6 Chapter 1 Digital Imaging Fundamentals
RGB Color
The most common mode for graphics in Photo-
shop is RGB Color mode. The RGB Color mode
uses additive color theory to represent color (a
100% value of red, green, and blue light creates
white light). Different intensity values of red (R),
green (G), and blue (B) combine to form accu-
rate colors. By mixing intensity values, virtually
every color can be accurately represented.
When working in Photoshop, most designers
choose RGB Color mode for its wide range
of available color (also known as gamut ) and
extensive support for filters and adjustments.
Additionally, computer monitors use RGB mode
to display color, and this is the native color space
for onscreen display. Because you’ll most often
be processing images on a computer, it is easiest
to work in the same color space as your monitor.
CMYK Color
Professional printing uses a four-color process
to simulate color. The four inks are cyan (C),
magenta (M), yellow (Y), and black (K for key ).
The CMYK Color mode uses the subtractive
color model to re-create color. Subtractive color
explains the theory of how ink or dye absorbs
specific wavelengths of light and reflects others.
The object’s color is based on which part of the
light spectrum is not absorbed. Although print
designers use CMYK Color mode for profes-
sional printing, they will work in RGB Color
mode throughout the design stage. CMYK Col-
or mode has a smaller color gamut, so CMYK
conversion is not done until the last stage of
image preparation.
Image Mode 7
Grayscale
A grayscale image uses different shades of gray to represent image
details. For example, an 8-bit image is represented by 256 levels of
gray (see “Bit Depth” later in this chapter). Likewise, a 16-bit im-
age would show 65,536 levels of gray (a substantial improvement,
but it requires an output device that can utilize the data). Gray-
scale mode can be significantly affected by printer conditions,
because the amount of ink coverage can vary, which in turn can
impact how dark the image will print. For example, many newspa-
per images look washed out in Photoshop, but they look fine when
the ink prints on the highly absorbent newsprint. When creating
grayscale images, it is important to perform test prints with the
output device and paper to see how contrast is maintained.
Duotone
A duotone image can actually be monotone, duotone, tritone,
or quadtone. Grayscale images that use a single-colored ink are
called monotones. Duotones, tritones, or quadtones are grayscale im-
ages printed with two, three, or four inks, respectively. Using both
black and gray ink to represent the tonal values, duotones create
better quality-printed grayscales.
The most popular form of duotone is a sepia-tone image (often
seen in historical prints). In modern times, a designer may use a
duotone for style purposes or to save money by using fewer inks.
8 Chapter 1 Digital Imaging Fundamentals
Bitmap
A bitmap image uses only one of two color
values—black or white (no gray) —to represent
the pixel data. These 1-bit images have a very
small file size. To create a bitmap, you first must
convert the image to an 8-bit grayscale formula,
and then convert to the Bitmap mode.
Do not confuse Bitmap mode with a bitmap im-
age, which is another name for raster (or pixel-
based) images. Additionally, avoid confusion
with the BMP file format, which is a standard
Windows file format that dates back to the earli-
est version of Windows. An image in the Bitmap
mode simply uses only black and white to repre-
sent image data.
Indexed Color
Indexed Color mode severely limits the num-
ber of colors used to represent the image. In
Indexed Color mode, 256 colors are available.
To reduce file sizes (and download times), some
Web designers use fewer colors in their graphics.
They will turn to specialized formats like GIF
and PNG-8. Although this mode reduces file size,
it also visibly lowers the quality of the image.
Indexed Color mode works well for illustrations
or logos but not so well for photos on the Inter-
net. Instead of converting the image to Indexed
Color mode via the Image menu, you can access
this mode by using the Save for Web command
(File > Save for Web). This will convert the file
to a GIF or PNG-8 (which both use the Indexed
Color mode), but leaves the original image at the
higher-quality, RGB Color mode.
Image Mode 9
Lab Color
L*a*b* Color is the most complete color mode used to describe
the colors visible to the human eye. The three parameters of color
are L for luminance of the color, a represents the color’s position
between red and green, and b represents its position between yel-
low and blue.
The Lab Color mode was created to serve as a device-indepen-
dent, absolute model to be used for a reference. Lab Color mode is
most commonly used in Photoshop to work with Photo CD imag-
es. Lab attempts to simulate the full gamut of color; however, it is a
three-dimensional model and can’t be represented properly within
Photoshop. Hence, the * after the L , a , and b is used to signify
that it is a derivative model. Lab images can only be printed on
PostScript Level 2 and Level 3 printers: For all other professional
printers, Lab images must first be converted to CMYK mode. The
Lab Color mode is generally only used by imaging professionals
seeking the truest color fidelity, because it supports all the colors in
both the RGB and CMYK Color modes.
Multichannel
Multichannel mode is a
highly specialized mode used
for complex separations for
professional printing. You may
never need to use it. Photo-
shop automatically converts to
Multichannel mode when you
delete a channel from an RGB
or CMYK image. The color
onscreen is no longer accurate
because Photoshop cannot
describe it. This is sometimes
done for an effect or as part of
the image repair process if one
channel did not capture prop-
erly (such as from a malfunc-
tioning digital camera). Most
likely, you’ll never want to work
in Multichannel mode.
10 Chapter 1 Digital Imaging Fundamentals
Bit Depth
Besides resolution (the number of pixels) and color mode (the way
colors are processed) there is one other variable that affects image
quality. Bit depth measures how much color is available for display
or printing of each pixel. A greater bit depth means each pixel
contains more information for describing the color. A pixel with a
bit depth of one can display the pixel as either black or white. The
most common bit depth is 8-bit mode, which has a possible value
of 256 intensity levels per color channel. However, depending on
the version of Photoshop you are working with, you can access 8,
16, or 32 bits per channel. It’s important to note that a large bit
depth can limit image adjustment commands.
Time to Move On
There’s a lot more ground to cover, but you’ll explore the topics
discussed here and others in greater depth in each chapter. You’ll
feel a bit more comfortable with the language used to describe im-
ages and color as you read on. With the knowledge you’ve gained
so far, you can jump into using Photoshop and start to navigate
around its interface.
Photoshop’s
Interface
Photoshop’s interface can be pretty intimidating. Among all those
2
windows, tools, and menu commands it’s easy to get lost. How-
ever, it’s worth it to master these components. Adobe Photoshop
is by far the most-used image editor and knowing how to harness
its power unlocks a world of
design opportunities. Working
professionals use it for a variety
of tasks, from enhancing maga-
zine photos to designing Web
animations and from creating
television graphics to perform-
ing medical imaging.
Open the file Ch02_Red_Rock.
psd from the Chapter 2 folder
on the CD included with this
book. Many of the windows in
Photoshop require an image
to be open before they display
any detail.
Most important is to learn the essential features you need right
away, and then gradually learn the rest as needed. I frequently
tell students of all levels that often there are three or more ways to
3
VIDEO
TRAINING
Setting Preferences
perform the same task in Photoshop. Adobe’s software designers
have tried their best to make the program intuitive (and everyone
certainly doesn’t think the same way). Additionally, new features
are often unveiled with product updates, yet the old features re-
main for those who resist change or prefer the older method.
Learning Photoshop is a very doable task, especially if you take
a balanced and measured approach, balancing learning new
features with practical application. At this point in my career, I
have seen older professionals as well as young students become
proficient Photoshop users. In fact, learning Photoshop is the best
way to learn other Adobe programs, such as Illustrator and After
Effects, as well as learn how to complete diverse tasks like color
correction for video or Web page design.