Transcript Pre-press Techniques for the Digital Designer

Module 4: Tools and Techniques
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Module 4:
Tools and Techniques
Instructor: Doughlas Remy
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Module 4: Tools and Techniques
Topics Covered in This Module
• Section 1
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Analog-to-digital conversion
Bit groupings
Bit depth
Color and alpha channels
File formats
Quiz
• Section 2
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Image pixels
Screen pixels (grid cells)
Understanding zooming
Anti-aliasing
Why “treeware” is here to stay.
Quiz
• Section 3
Monitor screen sizes and aspect ratios
Display resolution
Screen resolution and pixel size
Using the Image Size dialog box in
PhotoShop
• Resampling: Interpolation methods
• Printer resolution (DPI), screen frequency
(LPI), and monitor resolution (PPI).
• Quiz
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• Section 4:
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Bitmaps and vector images
Type fonts
Trapping
Quiz
Answer Sheet for Printing
Module 4: Tools and Techniques
Section 1
• Analog-to-digital conversion
• Bit groupings
• Bit depth
• Color and alpha channels
• File formats
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Module 4: Tools and Techniques
The Analog Signal
An analog signal is an effect of electromagnetic (EM) radiation.
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Module 4: Tools and Techniques
Conversion of Analog to Digital
When amplitude reaches a certain threshold, the “off” signal is
switched to its “on” state.
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Module 4: Tools and Techniques
Conversion of the Digital Signal to
Binary Code
The “off” state of the signal is assigned a value of 0 (zero) in the
binary system that computers use. The “on” state has a value of 1.
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Module 4: Tools and Techniques
Values Possible in Groupings of Bits
1 bit
1
1
1
1
0
0
0
1
1 byte
There are 28 (256) ways in which the ones and
zeros can be arranged in a byte of data.
Therefore, a byte can represent 256 values.
These values could be used to represent, e.g.,
shades of grey.
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Module 4: Tools and Techniques
Values in Groupings of Bits
1
1
1
1
0
0
0
1
0
1
1
0
0
0
1
0
This is a grouping of 16 bits, which can represent 65,536 values (216).
Other groupings of bits are:
24 (224 = 16,777,216 values)
32 (232 = 4,294,967,296 values)
64 (264 = 18,446,744,070,709,551,616 values)
Next, to understand the concept of bit depth—the number of color
values that a pixel (picture element) can represent—we must first
understand color channels.
Module 4: Tools and Techniques
Color Channels (in Photoshop)
All four channels of this RGB
image are visible. (The top one
is the “composite” channel.)
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Module 4: Tools and Techniques
Color Channels (in Photoshop)
Here, only the green and the
blue are visible.
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Module 4: Tools and Techniques
Color Channels (in Photoshop)
Here, only the red and the
green channels are visible.
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Module 4: Tools and Techniques
Color Channels (in Photoshop)
This screen capture shows the
CMYK channels (composite
channel at top).
Notice what happens when we
remove the black.
Module 4: Tools and Techniques
Color Channels (in Photoshop)
Here, only the cyan and
magenta are visible.
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Module 4: Tools and Techniques
# Color Channels x Bits/channel = Bit Depth
In most cases, CIELab, RGB, grayscale, and
CMYK images contain 8 bits of data per color channel.
Color model
# of channels
(except
Composites)
Bits per
channel
Bit Depth
Possible
Values
CIELab
3
8
24
16.8 million
RGB
3
8
24
16.8 million
Grayscale
1
8
8
256
CMYK
4
8
32
4 billion
Remember that 8 bits can
produce 256 values, or 28.
16,800,000
is 2563.
Module 4: Tools and Techniques
Creating Extra Channels
In addition to a color mode’s default channels, you can
also create:
• Alpha channels. For storing selections as 8-bit grayscale
images to use as masks. (Masks let you manipulate, isolate,
and protect specific parts of an image.)
• Spot color channels. You can create spot color channels
to specify additional plates for printing with spot color inks.
An image can have up to 24 channels, including all color
and alpha channels.
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Module 4: Tools and Techniques
Saving Color Channels
Color channels (including spot channels) are preserved when you
save the image.
Alpha channels are preserved only when you save the image in
one of the following formats:
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Photoshop
PDF
PICT
TIFF
Raw (from your digital camera)
(not JPEGs, PNGs,
or GIFs)
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Module 4: Tools and Techniques
Best Image File Formats
Exercise
Identify the following:
Photographic images
Logos, line art
(rich color or gradations of hue)
(limited color)
Properties
Continuous tones, 24-bit color
or 8-bit grey, no text, few lines
and edges
Solid colors, up to
256 colors, with text
or lines and sharp
edges
Best quality for
archived master
TIF or PNG (no JPG artifacts)
PNG, GIF, or TIF
(no JPG artifacts)
Smallest file size
JPG with a higher quality factor
can be decent, but JPG is of
questionable quality for
archiving master copies.
TIF (with LZW
compression), GIF,
or PNG
Maximum compatibility
(PC, Mac, UNIX)
TIF, JPG (the simplest
programs may not read TIF
LZW.)
TIF (without LZW
compression) or GIF
Worst Choice
256-color GIF: Limited color;
larger file than 24-bit JPG.
JPG. Compression
adds artifacts,
smears text and
lines and edges.
1. The worst choice for a
photographic image.
2. The best choices for
logos and line art.
3. The best cross-platform
compatibility for a
photograph.
4. The smallest file size
for a photograph.
LZW is a highly efficient
compression method.
Module 4: Tools and Techniques
Bit Depth and File Size: Working with
High Dynamic Range (HDR) images
PhotoShop normally works with
images at 8 bits per channel (bpc),
but it can also work with 16-bpc and
32-bpc images. The higher bit depths
provide finer distinctions in color but
result in much larger file sizes. In
addition, some Photoshop tools* are
not available for the higher bpc
images.
Note: 8 bpc is all you need for output to
print. If you are working with a 16-bpc or
32-bpc image, click Image, click Mode,
and then click to select 8 Bits/Channel.
*E.g., the Art History Brush tool, the Extract and the Pattern Maker features, and some filters.
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Module 4: Tools and Techniques
Section 1 Quiz
Instructions: More than one answer may be correct.
Use the highlighter to mark your options. (Right-click
anywhere, click Pointer Options, and then click Highlighter.
When you finish, restore the arrow pointer.)
1. The digital values of one and zero are
assigned to a signal based on its
varying…
a. speed
b. amplitude
c. frequency
d. wavelength
4. Which of the following color models
has the most color channels in
Photoshop?
a. CIELab
b. RGB
c. Grayscale
d. CMYK
2. A grouping of 8 bits gives you____
values.
a. 8
b. 16
c. 64
d. 256
5. If a color model has three color
channels and uses 8 bits per channel,
then its bit depth is…
a. 32
b. 16
c. 24
d. 256
3. The term “_____” refers to the number
of bits used to display or to print a
pixel.
a. aspect ratio
b. binary weight
c. byte size
d. bit depth
6. In Photoshop, channels may also be
used for storing…
a. the document history
b. paths
c. masks
d. spot colors
7. Which of the following file formats will
save an alpha channel in Photoshop?
a. JPEG
b. Photoshop
c. TIFF
d. GIF
8. The best file format(s) for an archived
master is/are…
a. GIF
b. JPEG
c. TIF
d. PNG
e. PDF
9. A ___ is the worst choice for line art or
logos.
a. GIF
b. JPG
c. TIF
d. PNG
10. The optimal setting for output to print is
___ bits per channel.
a. 8
b. 16
c. 32
Module 4: Tools and Techniques
Section 2
• Image pixels
• Screen pixels (grid cells)
• Understanding zooming
• Anti-aliasing
• Why “treeware” is here to stay
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Module 4: Tools and Techniques
What is a pixel, really??
The digitalized photographic image is a grid of picture
elements, or pixels. These pixels are defined in the
binary code of your computer and only become
translated into visible points of light when you turn on
your monitor and open your image file.
In other words, pixels themselves cannot be described
in terms of size until they are represented on your
monitor’s screen. Before that, they are only code.
(By using an image editing application such as
Photoshop, you can “zoom in” to edit individual pixels
if desired.)
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Module 4: Tools and Techniques
What is a pixel, really??
To get a bit more technical,…
You can think of a pixel as the smallest complete sample of an image, either
before or after output to print or screen.
When
How the pixel is expressed
Before output
Electromagnetic waveforms
Example
(e.g., phone lines to computer, computer to CRT)
Digital values
After output
0010100111001001
Dots (or groups of dots*) on a printed page
(*A 600 dpi inkjet printer might print a 200 ppi
image.)
Cells, parts of cells, or groups of cells* in the
screen grid of a computer monitor
(CRT [cathode ray tube] or LCD [liquid crystal display])
(Note: These cells are sometimes called “screen
pixels.”)
Triads of CRT cells
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Module 4: Tools and Techniques
Monitor technology
CRT
LCD
Color quality
Best
OK
View from any angle
Yes
No
Power consumption
More
Less
Screen flicker
More
Less
Weight
Heavy
Light
Footprint
Large
Small
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Module 4: Tools and Techniques
Screen Pixels (Grid Cells)
How a CRT Monitor* Works (1)
The inside of the tube is
coated with a
conductive material.
Three cathodes (heated
filaments) produce a
stream of electrons
(negative)
The anode, with a positive
charge, attracts the
electrons from the
cathode.
*These diagrams illustrate the kind of CRT that uses a
“shadow-mask” grid. Another type uses an “aperture
grill,” which we’ll describe shortly.
Shown here are three
electron beams, colorcoded to indicate the color
of the phosphor dot they
are aimed at.
Copyright Doughlas Remy 2007
The inside of the
screen is coated with
red, green, and blue
phosphors that glow
when struck by the
electron beams.
The red line represents the grid through
which the beams pass
before they strike the
phosphors.
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Module 4: Tools and Techniques
Screen Pixels (Grid Cells)
How a CRT Monitor Works (2)
The electron beams move back and forth across the back of
the CRT screen, from the top rows to the bottom ones. This
movement is called scanning.
• CRT monitors: Progressive scanning
• CRT TV screens: Interlaced scanning
(two simultaneous scans of alternate
rows: better for analog video). For
more information, click here.
electron beams
(color coded)
cathodes
anode
Refresh rate: The number of times in a second that the display hardware
draws the data that it is being given. (Usually around 60-100, expressed as
60-100 Hz) Larger monitors require the higher ranges. For more
information, click here.
Screen flicker:
• Caused by a low refresh rate. (low relative to the monitor size)
• May cause discomfort, headaches.
Beam scanning
moves across
each successive
row from top to
bottom.
Module 4: Tools and Techniques
Screen Pixels (Grid Cells)
Note LCD monitors operate differently than CRTs:
• No scanning
• No refresh rate per se.
LCDs use “shutters” to control illumination from a
backlight that is always on. These shutters allow the
amount of RGB light to vary. (For example, a
juxtaposition of 100% red, 100% green, and 100% blue
produces white.)
However, the shutters change their configuration at a
rate of around 60Hz. This is called the “frame rate.” In
addition, the backlight does pulse on and off, typically at
~200Hz. So the LCD also produces a kind of flicker, but
not for the same reasons as the CRT does.
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Screen Pixels (Grid Cells)
How a CRT Monitor Works (3)
The metal grid through
which the RGB beams
pass on their way to the
phosphor coating is
called a shadow mask.
Copyright Doughlas Remy 2007
The shadow mask consists of very small holes arranged in triangular
formations (triads). The RGB beams pass through these holes to strike
the phosphors.
The triad allows the round holes to be packed in more tightly than they
would be in a vertical/horizonal grid.
Module 4: Tools and Techniques
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Screen Pixels (Grid Cells)
Aperture Grills (an alternative CRT technology)
Aperture grilles were developed for TV monitors in the late 1960s by
Sony under the Trinitron brand name. They were once thought to be
superior to shadow-masks, but then shadow-mask technology was
improved to deliver equal or better quality. Now, LCDs are
challenging both technologies and are rapidly replacing CRTs
altogether.
Aperture grilles consist of fine vertical wires that separate the
different colored phosphors into strips. The electron beams fired
from the three guns at the back of the monitor strike phosphors of
the appropriate color.
Aperture grille CRTs are flatter and can achieve higher resolutions
than shadow-mask ones, but they also use two horizontal stabilizing
wires which are visible and annoying to some users.
The letter “e” as shown on an
aperture grille CRT (magnified).
From Wikipedia.
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Module 4: Tools and Techniques
Screen Pixels (Grid Cells)
Note In LCD monitors, RGB liquid crystals
are not arranged in triangles. They are
arranged in a vertical/horizontal grid.
Because the liquid crystals are so small, the
RGB values appear to merge, creating
millions of colors per RGB triad.
Wikipedia’s logo as seen
on an LCD monitor.
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Module 4: Tools and Techniques
Screen Pixels (Grid Cells)
How a CRT Monitor Works (4)
Dot pitch is a measurement of the
space between the triads.
Image sharpness increases with
density of these dots. When
shopping for a monitor, look for the
dot pitch specs.
Copyright Doughlas Remy 2007
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Module 4: Tools and Techniques
Screen Pixels (Grid Cells)
How a CRT Monitor Works (5)
The coating on the inside of the
screen consists of one phosphor dot
for each hole in the shadow mask.
Precise positioning of the mask
allows each electron beam to strike
the phosphor dot of the correct color.
Because the triads are so small (72
per linear inch), each triad is
perceived as a single colored screen
pixel.
Copyright Doughlas Remy 2007
1 triad = 1 screen pixel
Phosphor dot
Module 4: Tools and Techniques
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Screen Pixels (Grid Cells)
How a CRT Monitor Works (6)
Notice that the arrangement of “pixels” (RGB triads) is
hexagonal. (The six larger triangles, plus the one in the
center, fit into a hexagon.)
This is just one of several ways of representing pixels.
Photoshop uses these hexagonal groupings to represent
the pixels as squares. (Because of the fineness of the grid,
the eye may be tricked into seeing straight lines.)
This is not a
screen pixel. It is
an image pixel
that is displayed
by using roughly
250 screen
pixels.
Copyright Doughlas Remy 2007
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Module 4: Tools and Techniques
Image Pixels vs. Screen Pixels
The pixels that form a digitized image (e.g., a
JPG displayed on a Web page) may or may not
be in a one-to-one correspondence with screen
pixels.
Each RGB triad =
one “screen” pixel
A "screen pixel" (or grid cell)
can display a single image
pixel at any ratio, e.g., 1:1,
1:2, or 2:1.
Ratio of screen
pixels to image
pixels
Image resolution
One screen pixel
displays___image
pixel(s).
1:1
72 ppi
1
1:2
144 ppi
2
2:1
36 ppi
1/2
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Module 4: Tools and Techniques
Setting the ppi Ratio for your Image
If your image is to be used only for Web output, set
your image resolution (in PhotoShop) to match that
of your monitor (usually 72 ppi). This will keep file
size to a minimum. (I.e., The monitor can’t use the
extra pixels; it will just “average” them.)
If your output is to print, 300 ppi is a good choice, but
always check with the printers beforehand.
Best for Web
output
Ratio
Resolution
One screen pixel
displays___image
pixel(s).
1:1
72 ppi
1
Image does not suffer, but file size is needlessly large.
1:2
144 ppi
2
Image loses precision, clarity.
2:1
36 ppi
1/2
For Web output:
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Module 4: Tools and Techniques
Zooming in Microsoft Office Applications
What happens when you “zoom into” a bitmap
image in a typical MS Office application?
B
A
To simulate the effect of zooming, image
B has been scaled up to 4” x 4” in
PowerPoint.
Note the following:
Image A (a JPEG)
is 72 ppi
and is shown here
at a screen-toimage ratio of 1:1.
(It is 1” square)
• Image A uses 5184 screen pixels;
Image B uses 82,944 screen pixels.
(Size ratio = 1:16)
• You can get the same effect by using
the PowerPoint Zoom tool.
• Image B does not show the individual
pixels of the original JPEG.
PowerPoint has “smoothed” them
through mathematical algorithms—
sampling, averaging, etc.
Scaled image, simulating zoom effect.
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Module 4: Tools and Techniques
Zooming in PhotoShop
What happens when you “zoom into” a bitmap
image in an image-editing application such as
PhotoShop?
C
A
Image C was not made by scaling
image A in Powerpoint. It is a screen
capture of image A at 400%*
magnification in Photoshop.
Image A (72 ppi,
shown here at a
screen-to-image
ratio of 1:1.)
(It is 1” square)
Notice the individual pixels. (Photoshop
allows you to edit single pixels, whereas
Powerpoint does not.)
This image, like image B, uses 82,944
screen pixels. But the image that is
represented is a screen capture
showing only 5,184 pixels.
Screen capture of
zoomed image in
PhotoShop
*The magnification ratio applies to
width or height, not area.
Compare the
“smoothed”
enlargement.
B
Module 4: Tools and Techniques
A
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Anti-Aliasing
Anti-aliasing (definition from Wikipedia)*
“In digital signal processing, anti-aliasing is the
technique of minimizing the distortion artifacts
known as aliasing when representing a highresolution signal at a lower resolution. Antialiasing is used in digital photography, computer
graphics, digital audio, and many other domains.”
By squeezing grid A
at the top, we force it
into an area of fewer
pixels. Notice that the
aliasing increases as
you approach the top
in version B.
In the image domain, aliasing artifacts can appear
as wavy lines or bands, or moiré patterns,
or popping, strobing, or as unwanted sparkling.
B
* http://en.wikipedia.org/wiki/Antialias
(Visit the site for some clear examples of anti-aliasing and for
descriptions of several anti-aliasing techniques.)
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Module 4: Tools and Techniques
Monitors / Displays
In this example, anti-aliasing works by introducing grey scale pixels to replace or to supplement the B/W
ones. Various mathematical algorithms are used to achieve this, and most rely on methods of sampling.
A
Before antialiasing.
After antialiasing
B
Note Images A and B are 4.5” wide
at 96 ppi. Next to and below each
one are screen captures from
Photoshop, showing magnifications.
Notice the reduced moiré effect in
the anti-aliased version. Higher
image resolution would reduce that
effect even more.
C
This is the anti-aliased image at 96 ppi
and only 2 inches wide! Notice the
increased moiré.
Module 4: Tools and Techniques
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Why “dead-tree” media* are not dead yet
Even the best monitors are limited when it comes to showing fine detail.
Unless you are getting continuous sets of proofs along the way (which is unlikely), you
will have to largely imagine what your printed output will look like. This may not be too
difficult in some cases, but if your document contains very data-intense, highresolution images such as topographic or DNA maps, you will not be able to view the
detail without zooming, which is incompatible with integrated viewing.
For example, there is simply
no hope of showing this entire
scatterplot effectively on a
monitor screen. A highresolution digital photo or
scan might capture the detail,
but the monitor cannot show
it without increasing the scale
to the point that the entire
image is no longer viewable
on the screen.
*treeware
3-D Scatterplot reproduced in “Beautiful Evidence,” by Edward Tufte, Graphics Press LLC,
2006, page 57. This JPG is from a 600 dpi scan.
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Why “dead-tree” media* are not dead yet
One reason why monitor output is unlikely ever to entirely replace printed output is that
print can produce resolutions greater than 1,200 dpi*, as opposed to a monitor’s 96 ppi
(new monitors only). This means that we can pack about 12 times more charts like the
ones below onto a sheet of paper than onto a monitor screen of equal area.
A
These charts may
seem extremely
detailed, but notice
how CC begins to
break up.
B
C
*Most office printers output at 600 dpi.
Printed output has the
capability to represent
such line graphs as
sparklines, embedded
within a stream of text
in a 10 point font.
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Why “dead-tree” media* are not dead yet
A3 size paper (11” x 17”) can display the data-equivalent of about 15
large computer screens or 300 PowerPoint slides. [This data] is adjacent
in space rather than stacked in time. By showing vast amounts of data
within the eyespan, spatial adjacency assists comparison, search,
pattern-finding, exploration, replication, and review.
From “Beautiful Evidence,” by Edward Tufte, Graphics Press LLC, 2006, page 63.
But—to put in a kind word for pixel media—paper does
not allow focus to the extent that pixel media do, at
least not without expenditure of an important natural
resource. For example, you can use an entire
Powerpoint slide to focus on a very limited and discrete
chunk of information, whereas an entire printed page
devoted to that same chunk might require the axing of a
tree somewhere if your print volume is high.
Module 4: Tools and Techniques
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Some Conclusions
• Photographic images that you include in your document and send out for
printing should be much higher than your monitor screen resolution (e.g., 300
ppi vs. 96 ppi) unless you are trying to achieve certain rough effects. Find out
about the printing process and the DPI resolutions that it is capable of
producing.
• Don’t be too concerned about moiré patterns that show up on screen
renderings of images containing grids, stripes, etc. These effects are unlikely to
show up in the printed output, but check the proofs anyway.
• When dealing with customers, be aware that they may not be making the
mental adjustments that you’re making when viewing a highly detailed image on
the monitor screen. Show customers a printed proof
if possible.
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Section 2 Quiz
Mark your answer on a separate sheet of paper. More than one option may be correct.
1. A pixel is a ____ element and has
____.
a. hardware … a fixed size
b. hardware … no fixed size
c. software … a fixed size
d. software … no fixed size
4. The inside of the CRT screen is
coated with...
a. cathodes
b. anodes
c. phosphors
d. a conductive material
2. An image pixel may be expressed
as...
a. digital values (e.g.,
01001111)
b. waveforms.
c. dots on a printed page.
d. cells in the screen grid of a
monitor.
e. (all of the above)
5. CRT monitors use ____ scanning.
a. interlaced
b. progressive
c. alternate
d. intermediate
3. What are the advantages of LCD
monitors over CRTs?
a. Superior color quality
b. Lower power consumption
c. Smaller footprint
d. May be viewed from any
angle
6. The CRT’s shadow mask is a grid
of small holes arranged in...
a. squares
b. triangles
c. circles
d. pentagons
7. Image sharpness in a CRT
depends on...
a. anode size
b. scanning rate
c. dot pitch
d. image pixel size
8. Which of the following image
resolutions might be best for
normal print output?
a. 300 ppi
b. 96 ppi
c. 72 ppi
d. 1200 ppi
9. Which of the following applications
will allow you to view and edit
individual image pixels?
a. PowerPoint
b. PhotoShop
c. MS Word
10. Displaying a high-resolution image
at a lower resolution may produce
an effect known as ...
a. aliasing
b. anti-aliasing
c. sampling
d. interpolation
11. Which of the following media can
display the most detail per unit of
area?
a. an LCD monitor
b. a CRT monitor
c. a printed page
Module 4: Tools and Techniques
Section 3 • Monitor screen sizes and aspect ratios
• Display resolution
• Screen resolution and pixel size
• Using the Image Size dialog box in
PhotoShop
• Resampling: Interpolation Methods
• Printer resolution (DPI), screen frequency
(LPI), and monitor resolution (PPI).
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Module 4: Tools and Techniques
Monitor Screen Sizes and Aspect Ratios
The monitor’s screen
size is the diagonal
measurement (the
hypotenuse of the
triangle).
Popular CRT screen
sizes:
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The aspect ratio of most CRT (cathode ray tube)
monitors is 4:3.
5
3
15”
4
9”
12”
Math refresher:
The hypotenuse is equal
to the sum of the
squares of the two sides,
e.g.,
(52 = 32 + 42)
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Module 4: Tools and Techniques
Monitor Screen Sizes and Aspect Ratios
Another common aspect ratio is 16:9.
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18.35
This aspect ratio is used in cinematic film. It was not adopted
for TV screens when they were first developed, but it has now
become common in LCD monitors and plasma TV displays.
16
LCD flat-panel monitor
Plasma TV display
Module 4: Tools and Techniques
Digital Publishing
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This diagram shows
you the variety of
available monitor
sizes and aspect
ratios. Four aspect
ratios are indicated by
the diagonal lines.
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Module 4: Tools and Techniques
Display Resolution
The monitor’s display resolution is the number of pixels that it
can display per unit of length on the monitor. So monitor
resolution is usually measured in pixels per inch (ppi).
Most new monitors can display up to 96 ppi.
At 96 ppi, a 15” (9”x12”) monitor displays 10,368 pixels. (96 x
9 x 12 = 10,368)
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Module 4: Tools and Techniques
Display Resolution
The monitor’s grid (aperture grill, shadow mask, or liquid crystal array)
maps to the highest resolution offered by the monitor.
For example, a display with a physical grid of 1280 rows by 1024
columns can obviously support a maximum resolution of 1280 x 1024
pixels. It usually also supports lower resolutions such as 1024 x 768,
800 x 600, and 640 x 480.
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Screen Resolution and Pixel Size
Your display may
offer you several
screen resolutions.
To view these settings in
Windows Vista, go to
Control Panel, click the
Personalization icon,
and then click Display
Settings.
You may test these
settings on your
monitor. Please be sure
to restore the settings
afterwards.
Let’s discuss this.
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Module 4: Tools and Techniques
Screen Resolution and Pixel Size
When you lower the screen resolution setting in Control Panel, your
images fill more of the screen. Each image consists of a set number of
pixels, so when the image fills more of the screen, the pixels are larger.
15” monitor
Lower screen resolution
=
larger pixels
=
larger image
Screen resolution:
832 x 624
(higher)
640 x 480
(lower)
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Module 4: Tools and Techniques
Screen Resolution
high res setting
15” monitor
high res setting
20” monitor
low res setting
Module 4: Tools and Techniques
Using the Image Size Dialog Box in
Photoshop
This is the Image Size
dialog box in Photoshop.
Notice the major sections,
the units of measurement,
and the check boxes at the
bottom.
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Module 4: Tools and Techniques
Using the Image Size Dialog in Photoshop
• Pixel dimensions: the number of
pixels in an image (e.g., 60 x 90 =
5400)
• Image dimensions: the size of
the image as it is meant to appear
in print or online. (Measured in
inches, centimeters, etc.)
• Image resolution: the ratio of
pixels per unit of measurement.
E.g., 72 ppi.
Pixel dimensions
Image dimensions
Image resolution
Zoom
200%
Zoom
200%
60 pixels wide x
90 pixels high
250 pixels wide x
375 pixels high
0.83 x 1.25 in.
0.83 x 1.25 in.
72 ppi
300 ppi
Module 4: Tools and Techniques
58
Using the Image Size Dialog in Photoshop
Based on information in the blue section (“Results”), which option is best for the following
scenarios?
1. Increase the image size without increasing the file size.
2. Decrease the image size without lowering the image quality.
3. Increase the image quality.
4. Copy a photo from the Internet and make it suitable for a billboard.
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Module 4: Tools and Techniques
Resampling
Resampling refers to changing the pixel dimensions (and
therefore display size) of an image. When you downsample
(or decrease the number of pixels), information is deleted
from the image. When you resample up (or increase the
number of pixels), new pixels are added based on color
values of existing pixels. You specify an interpolation method
to determine how pixels are added or deleted.
Resampled up
Original
Sampled down
Notice that there is no
difference between the
first two images. This is
because resampling “up”
only adds pixels and some
smoothing, but it does not
add any additional detail.
The only way to get more
detail is to start with a
higher res photo.
Module 4: Tools and Techniques
Why are interpolations necessary?
Let’s say you want to
increase the resolution of
this image (…which, by the
way, will not improve the
image quality but only add
more pixels to play with).
This might be simple if you
chose to increase the
resolution by a factor of 9…
… because each original
pixel would be surrounded
by 8 pixels of exactly the
same color.
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Module 4: Tools and Techniques
Why are interpolations necessary?
However, let’s say you want
to increase the resolution by
a factor of 6.
Your software cannot just
place 5 more blue pixels
pixels around the original
one,…
… because the other pixels in the grid would have
to be some color—maybe that of an adjoining
pixel?—and the image would no longer look the
same.
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Module 4: Tools and Techniques
Why are interpolations necessary?
Software engineers found ways of providing
additional pixels without noticeably altering
the image. These are mathematical models
called “interpolations,” and—for our
purposes—you can think of them as a kind
of “averaging.”
In Photoshop, these three methods are called…
• Bilinear
• Bicubic (default setting)
• Nearest neighbor
Exactly how this would look would depend
on the colors of the pixels surrounding the
original grey pixel.
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Module 4: Tools and Techniques
Resampling – Interpolation Methods
With any image file open in Photoshop,
find the Image Size dialog. The resampling
drop-down menu is at the bottom. The
three options are…
Bicubic (resampling up)
(smoother tonal gradations,
slow processing, large file size)
Bilinear (medium)
Nearest neighbor (resampling
down) (jagged, hard edges,
fast processing, small file size)
Resampled up
Original
Resampled down
Module 4: Tools and Techniques
Printer Resolution, Screen
Frequency, and Monitor Resolution
• DPI – Dots/inch -- Printer resolution
• LPI – Lines/inch – Screen frequency, aka “line ruling” -- lines of cells
per inch in a halftone screen
• PPI -- Pixels/inch – (1) a measure of monitor resolution, and (2) a
“working measure” in imaging editing programs such as PhotoShop.
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Module 4: Tools and Techniques
DPI (Dots per Inch)
• Printer resolution: the number of ink dots per
inch (dpi) produced by a laser printer. 600 dpi is
fine for most purposes.
Inkjet printers produce a spray of ink, not actual dots.
However, most ink jet printers are said to have an
approximate resolution of 300 to 600 dpi. Best results
are produced by printing images that have been output
at 150 ppi.
Inkjet printer (Dell)
Module 4: Tools and Techniques
66
LPI (Lines per Inch)
• LPI (Screen frequency): the number of printer dots or halftone cells per
inch used to print grayscale images or color separations in non-digital
printing processes only. Also known as screen ruling or line screen,
screen frequency is measured in lines per inch (lpi)—or lines of cells per
inch in a halftone screen.
Note When you send a digital image file to a printer for offset printing, or for printing by
any non-digital process (non-digital processes require the making of printing plates) find
out from the printer what resolution is best for the image.
65 lpi: coarse screen typically used to print newsletters and grocery coupons.
85 lpi: average screen typically used to print newspapers.
133 lpi: high-quality screen typically used to print four-color magazines.
177 lpi: very fine screen used for annual reports and images in art books.
Module 4: Tools and Techniques
A Word About File Size
• Measured in kilobytes (K), megabytes (MB) or gigabytes (GB)
• Larger file sizes are slower to edit and print, and require more disk
space to store.
• Doubling the resolution more than doubles the file size.
• How to reduce file size:
• Compression
• Keep color bit depth at 8 bits rather than 16.
• Reduce the number of layers and channels in the image.
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Module 4: Tools and Techniques
68
Section 3 Quiz
Mark your answer on a separate sheet of paper. More than one option may be correct.
1. Most CRT monitors have an
aspect ratio of ...
a. 4:3
b. 8:5
c. 16:9
d. 5:4
2. What is the display resolution
of most new monitors?
a. 10,368 pixels
b. 96 ppi
c. 1280 x 1024
3. When you increase your
screen resolution in Control
Panel, an image’s pixels
appear...
a. larger
b. smaller
c. clearer
d. fuzzier
4. The amount of detail in an
image is determined by...
a. image pixel size
b. screen dot pitch
c. screen resolution
d. the number of image
pixels
7. Which of the following is NOT
a way of reducing file size?
a. Set color bit depth from
8 to 16.
b. Use compression.
c. Lower the resolution.
d. Use fewer channels.
5. How can you improve the
quality of an image file in
PhotoShop?
a. Increase its resolution.
b. Resample it.
c. Increase its file size.
d. None of the above
8. Which of the following printers
does NOT use dots to produce
images?
a. Laser
b. Dot-matrix
c. Inkjet
6. “LPI,” or “screen frequency,”
refers to the number of ___ per
inch, used in ___ printing
processes only.
a. pixels ... digital
b. pixels ... non-digital
c. halftone cells ... digital
d. halftone cells ... nondigital
Module 4: Tools and Techniques
Section 4
• Bitmaps and vector images
• Type fonts
• Trapping
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Module 4: Tools and Techniques
Bitmap Images and Vector Graphics
Two categories of computer graphics:
• Bitmap images
• Vector Graphics
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Module 4: Tools and Techniques
Bitmaps
A bitmap is an array of pixels. It is less frequently called a “raster
image.”
• Black and white bitmaps require only one data bit for each pixel
on a monitor screen.
• Gray-scale bitmaps require 8 bits per pixel, producing 256
shades of gray.
• Full-color bitmaps need as many as 24 bits per pixel, producing
16.8 million different colors. (Other bit depths are 8 and 16.)
Paint and photo imaging programs like Photoshop use bitmaps.
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Module 4: Tools and Techniques
Bitmaps
• Also called “raster images”
• Grids of colored pixels
• Most common electronic medium for mass printing of continuous-tone images,
such as photographs or digital paintings
• Resolution-dependent
• Some loss of quality occurs when you manipulate them too much.
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Module 4: Tools and Techniques
Vectors
Vectors are mathematically-described
objects consisting of paths. They are
used in “object-oriented” drawing
programs like Illustrator.
Starting point
Vectors store data as a list of drawing instructions.
A vector consists of a starting point, a direction, and a length.
Examples of vectors:
• A line drawn with a pencil or a pen.
• An outline type font.
Direction
Length
Module 4: Tools and Techniques
Vectors
• Lines defined by mathematical vectors
• Resolution-independent
• Best choice for images that may be repeatedly scaled by remote
users (e.g., logos)
• Images do not lose quality when you change them.
• Ideal for technical drawings
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Module 4: Tools and Techniques
Type Fonts
Two kinds:
Caution:
• bitmapped (screen fonts)
Avoid the terms “screen font”
and “printer font” if possible.
Since the introduction of
Adobe Type Manager,
screen fonts can be used for
printers, and outline fonts
can be used for screen
display.
• outline (printer fonts)
Module 4: Tools and Techniques
76
Type Fonts
Bitmapped fonts…
Outline fonts…
• store each point on the grid as a
pixel spot.
• are primarily for output to a printer or
imagesetter.
• do not resize well.
• are composed of bezier curves (vectors)
that can be linked together with
seamless joints.
• require lots of storage.
• need to be installed in all sizes
intended for use, unless Adobe
Type Manager (ATM) is used.
• can be used in printers, but
produce ragged, uneven results.
• contain size and shape information of
each character.
• allow clear, precise printing of type faces
at any size, angle, resolution, or shade
of gray.
• With ATM, can be used for screen
display.
Module 4: Tools and Techniques
77
True Type
TrueType is an outline font technology developed jointly by Microsoft and
Apple. Because TrueType support is built into all Windows and Macintosh
operating systems, anyone using these operating systems can create
documents using TrueType fonts.
Since its introduction in 1991, TrueType has quickly become the dominant font
technology for everyday use, and is even displacing PostScript in many
publishing environments.
Module 4: Tools and Techniques
78
Type Fonts: Adobe Postscript
PostScript is an object-oriented language, meaning that it treats images, including fonts,
as collections of geometrical objects rather than as bit maps. PostScript fonts are
called outline fonts because the outline of each character is defined. They are also
called scalable fonts because their size can be changed with PostScript commands.
Given a single typeface definition, a PostScript printer can thus produce a multitude of
fonts. In contrast, many non-PostScript printers represent fonts with bit maps. To print
a bit-mapped typeface with different sizes, these printers require a complete set of bit
maps for each size.
The principal advantage of object-oriented (vector) graphics over bit-mapped graphics is
that object-oriented images take advantage of high-resolution output devices whereas
bit-mapped images do not. A PostScript drawing looks much better when printed on a
600-dpi printer than on a 300-dpi printer. A bit-mapped image looks the same on both
printers.
Module 4: Tools and Techniques
Trapping
The image to the right
simulates a printed image.
Describe what you see.
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Module 4: Tools and Techniques
Trapping
Now compare these two images
(also simulations of printed images).
Do you see any difference between
them?
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Module 4: Tools and Techniques
Trapping
The top one is actually a
crescent and a disk.
The bottom one consists
of two overlapping disks.
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Module 4: Tools and Techniques
Trapping
These two images
represent two ways of
printing adjacent, filled
shapes when spot colors
are used.
knockout*
overprint
The first is called
“knockout,” and the
second is called
“overprint.”
*The yellow “knocks out” to leave a
white space for the green disk.
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Module 4: Tools and Techniques
Trapping
Knockout has an
important advantage over
overprint.
knockout
overprint
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Module 4: Tools and Techniques
Trapping
However, there is a
potential problem with the
knockout method.
Because the two colors
are printed using spot
colors, the paper
receives the impressions
from two different
stations on the press.
There may be registration
issues.
knockout
Module 4: Tools and Techniques
Trapping
Solution:
Trapping digital files is the process of compensating for misregistration on the printing press by printing small areas of
overlapping color where objects meet.
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Module 4: Tools and Techniques
Trapping: Choking and Spreading
How to trap
To compensate for minor
misalignments on the printing
press, you can slightly overlap
your touching colors. Then, if
one color shifts slightly, you
won’t get a white space
between colors.
You can overlap the colors by
either of these two methods:
In both choking and spreading, lighter colors expand
into darker colors.
Choking
Spreading
Module 4: Tools and Techniques
87
Manual vs. Automatic Trapping
If you decide to do the trapping yourself, there are three ways you can go:
Illustration Software
In vector-based drawing programs such as Adobe Illustrator you can create simple
manual traps by applying outlines (overprinting strokes) to objects or use plug-ins
that apply the appropriate trap.
Page Layout Software
High-end tools such as QuarkXPress, Adobe PageMaker, and Adobe InDesign
have built-in trapping. Each program may have multiple ways in which trapping is
applied.
Dedicated Trapping Software
Usually used by service bureaus and printers, these programs are designed to do
nothing but trapping. TrapWise and Trapper are two such programs.
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Module 4: Tools and Techniques
Trapping Alternatives
Alternative 1: Keep colors apart.
The best alternative to trapping is to
keep colors apart when possible.
Then, if there is a misregistration, it
won’t be noticed.
Alternative 2: Use black borders.
Module 4: Tools and Techniques
89
Section 4 Quiz
1. Vectors consist of …
a. pixels
b. bits
c. paths
d. channels
2. Which of the following applications
would be best for designing a
company logo or a technical
drawing?
a. Illustrator
b. Photoshop
c. Flash
d. InDesign
3. Vector drawings …
a. are resolution-independent
b. may be repeatedly scaled
c. do not lose quality when
edited
d. consist of grids of colored
pixels
4. Outline fonts …
a. are composed of vectors
b. can be used for screen
display
c. do not resize well
d. are not for output to printers
5. The software that allows outline fonts
to be used for screen display and
bitmapped fonts to be used for
output to printers is called …
a. Type Wizard
b. Adobe Type Manager
c. Abracadabra
d. Adobe Illustrator
e. Adobe Fontissimo
6.
“DPI” is a measure of _____
resolution.
a. screen
b. monitor
c. printer
d. printing press
7.
“LPI” describes the resolution of …
a. the monitor screen
b. desktop printer output
c. halftone images
d. continuous tone photos
8. Trapping compensates for…
a. dot gain
b. color mismatching
c. misregistration
d. faulty cropping
9. Trapping may be necessary
whenever...
a. process colors are used.
b. different spot colors are
adjacent to each other.
c. the page is not correctly
cropped.
d. color schemes are highly
contrastive.
10. As an alternative to trapping, you
can...
a. avoid process colors.
b. avoid spot colors.
c. keep the colors apart.
d. use black borders around
colors.
11. Which of the following techniques
can you use to avoid trapping
issues?
a. choking
b. spreading
c. overprint
d. knockout
Module 4: Tools and Techniques
End of
Module 4
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Module
4: Tools and Techniques
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quizzes,
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4: Tools
and Techniques
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