Working With Images

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Working With Images

• April 21, 2005

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Why this workshop?

• To end the frustrating hours spent trying to
  print posters.
• To maximize the quality of your posters,
  manuscripts, PowerPoint presentations and web
  images.
• To inject some good information into the informal
  support network.

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Agenda

• Image basics
• Resolution
• Color management
• File formats
• Printing
• Incorporating images into documents

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Step 1 Forget everything you know about working
with graphics!

• unless I ask a question

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Image Basics
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Graphic Types

• Two fundamental types of graphics
• Vector graphics
• Raster graphics (a.k.a. bitmaps)

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Vector vs. Raster
Vector Draw a 1024 x 768 pixel rectangle and
fill it with light yellow. Draw a 885 x 628
rectangle with no fill and a 6pt black stroke.
This makes a 504K Illustrator file.
Raster Make a 1024 x 768 pixel bitmap with a
bunch of light yellow pixels and a few black
pixels. This makes a 2.25MB image.
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Graphic Types

• Two fundamental types of graphics
• Vector graphics
• Raster graphics (a.k.a. bitmaps)
• Graphics software used to do one type or the
  other. That is no longer true.
• You need to pay attention to which type of
  graphic is being used and the conversion from
  vector to raster.

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Vector Graphics

• Consist of directions for drawing objects
• File size depends on number and complexity of
  objects.
• Do not have a resolution so they can be scaled
  without affecting image quality
• Traditionally produced by drawing applications
  like Illustrator or Canvas

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Vector Graphics

• Best used for images consisting of geometric
  shapes and lines (i.e. maps, charts,
  stratigraphic sections, etc.)
• Text is generally a vector graphic.
• Its hard to get into too much trouble with
  vector graphics except when you convert from
  vector to raster.
• Beware of overly complex digitized maps.
• Beware of ArcGIS exports!

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Raster Graphics

• Image mapped out by grid (raster) of pixels.

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Raster GraphicsBit Depth

• Each pixel is represented by 1 to 64 bits
  depending on color mode.

Bitmap Grayscale RGB
CMYK
1 bit 21 2 values 0 black 1 white
8 bits 28 256 values 0 Black 255 white This
gray is 119
24 bits 224 16,777,216 8 bits for each channel
(R,G B) This is 255, 153, 0
32 bits 232 4,294,967,296 8 bits for each
channel (C,M,Y K) This is 0,44,95,0
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Image (Color) Modes

• PhotoShop (and other applications) can work in
  numerous image modes.
• Grayscale, Indexed, RGB and CMYK are the most
  common.
• Use grayscale for black and white images.
• Use indexed mode when creating GIFs.
• Use RGB for all other images.

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Raster GraphicsImage size

• Image size (bits) pixel dimension X bit depth
• Divide by 8192 to get Kilobytes
• Increasing the number of pixels geometrically
  increases the image size.
• 500 x 500 24 bit image 732K
• 1000 x 1000 24 bit image 2.9MB
• 2000 x 2000 24 bit image 11.4MB

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Raster GraphicsImage scaling

• Raster graphics are not scalable. Changing image
  size requires adding or removing pixels.

600 x 6003 new pixels created for each original
pixel
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Raster GraphicsFile size

• File size does not usually equal image size.
• Many file formats, such as JPEG and GIF, employ
  compression that shrinks the file size below the
  image size.
• Example a 1024 X 768 red box is 2.25MB, but
  saved as medium quality JPEG the file shrinks to
  4.8KB GIF is 1.5KB.

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Critical take-home point 1

• No matter what size you shrink the file to, the
  image size stays the same!

Critical take-home point 2

• Its the image size that matters when printing or
  displaying images! Compressed files such as a
  JPEGs are uncompressed to the full image size
  before they are used.

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Raster Resolution
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What is resolution?

• It depends on whether you are talking about an
  images specification, a printers print quality,
  a scanners hardware, a monitors pixel dimension
  or a digital cameras CCD.
• Resolution is one of the most misunderstood and
  misused terms in graphics.

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Why is resolution important?

• Properly specified image resolution is critical
  to print quality, printing speed and printer
  stability.
• Improperly specified resolution can lead to all
  sorts of trouble (i.e. waiting 2 hours while the
  printer decides not to print your document).

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Device Resolution

• For printers it specifies the number of ink spots
  that can be printed per unit area
• For scanners it specifies the physical spacing of
  optical sensors
• For monitors it specifies the total number of
  displayed pixels
• For digital cameras it specifies the total number
  of light-receptive cells in its sensor

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Image Resolution

• Image size has no physical reality it just
  specifies the total number of pixels which have
  no inherent size.
• When an image is displayed or printed it takes on
  a physical size.
• Image resolution controls the conversion between
  the image size and the physical display size.

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The Acronym Game

• DPI Dots Per Inch
• PPI Pixels Per Inch
• SPI Samples Per Inch
• LPI Lines Per Inch (a.k.a. screen frequency)

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Display/Monitor Resolution

• The dpi resolution specified in image files has
  no impact when they are displayed on computer
  monitors or data projectors.
• Monitors can only display at their set
  resolution.
• Each pixel in the image file is mapped to a pixel
  on the monitor.

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Which image is 20dpi and which is 600dpi?
It does not matter because they are both 500 x
350 pixels!
600dpi 20dpi
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Proper Image Dimensions for Display

• Figure out the size of your display device. 1024
  x 768 is most common these days.
• Size your images relative to display size
• 1024 x 768 full screen
• 874 x 618 image with 1 inch border
• 512 x 384 1/4 screen
• Consistent image size adds a look of
  professionalism. Develop a set of preferred sizes
  and stick with them.

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What about 72/96 dpi?

• In the old days the common monitor at native
  resolution had an effective resolution of 72dpi.
  Today it varies.
• Example The display width of my 17-inch CRT
  monitor is 12.25. In 1024 x 768 mode this gives
  an effective resolution of 84dpi.
• Forget about 72/96dpiits meaningless when
  displaying images.

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Scanner ResolutionSamples Per Inch (spi)

• The native resolution of a scanner is determined
  by the spacing of sensors in its scanning bar.
• UMAX 1100 in Image lab 1200spi
• Epson Perfection 2400 Photo in Mac lab 2400spi
• Minolta Dimage slide scanner 2400spi
• Scanning at the native resolution gathers the
  maximum amount of info from the original document.

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Scanner ResolutionSamples Per Inch (spi)

• Always scan at the native resolution and then let
  PhotoShop downsize the image if needed.
• Exception If you are in a hurry and you know
  your image is destined for low resolution use
  like the web or PowerPoint presentations, then
  scan at lower resolution for increased scanning
  speed.
• Never scan at a setting higher than the native
  resolution. This causes the scanner software to
  make up pixel data. Let PhotoShop do the upsizing.

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Printer Basics

• Images are printed as a series of dots. Your
  brain interprets the dots as continuous tones.
• The dots are solid colors (black, cyan, magenta
  or yellow).
• The resulting color on the page is a function of
  dot concentration and the ratio between the
  different colored dots.
• The conversion of your image into the series of
  dots is halftoning.

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Example of Halftones
Enlargement illustrates what halftone would look
like if this image was printed on the color laser
writer or plotter.
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Printer ResolutionDots Per Inch (dpi)

• Only applies when printing a monochrome (one
  color) image.
• Text or 1-color line art (black, cyan, magenta or
  yellow).
• Completely irrelevant when printing all other
  images.
• I repeat, completely irrelevant when printing all
  other images!

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Printer ResolutionLines Per Inch (lpi)

• Also known as line screen or screen frequency
• Specifies the number of halftone cells (spots)
  printed per inch
• The LPI is much lower than the DPI and is
  generally hard to determine because printer
  manufacturers do not advertise it.

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DPI vs. LPI
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Printer ResolutionThe importance of LPI

• Image quality peaks when the resolution is set to
  2 times the line screen.
• Many images look just fine at 1.2 - 1.5 times the
  line screen.
• Sending images at resolutions higher than 2.5
  times the line screen degrades image quality and
  clogs the printer with needlessly large files.

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Proper Image Resolution for Printing

• Use 100- 140dpi for normal images lacking fine
  detail or where the fine details are not
  critical.
• Bump up to 140-200dpi when the fine details
  count or you are printing small images.
• Go up to 200-255dpi when printing small detailed
  images.
• Never use a resolution 255dpi.
• Experiment with resolutions images such as backgrounds.

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Impact of Proper Image Resolution

• Lets take the example of a color image we want
  printed at 10 x 8 inches
• At 140dpi the image is 4.5MB
• At 200dpi the image is 9.4MB
• At 600dpi the image is 84.4MB
• The 600dpi file is 19x larger than 140 dpi and 9x
  larger than the 200dpi file.

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Impact of Proper Image ResolutionThe Extreme Case

• Lets take the extreme example of a color image
  12 tall spanning a 5 poster
• At 120dpi the image is 30.4MB
• At 160dpi the image is 54MB
• At 600dpi the image is 759.4MB
• 120dpi is fine for almost any image youll print
  at this size. 160dpi is plenty for even finely
  detailed images. 600dpi makes the plotter go BOOM!

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Critical take-home point 3

• Less is more when setting image resolution. Never
  use an image resolution higher than than you need!

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Sizing Images in PhotoShop
Use these boxes when sizing for display
Use these boxes when sizing for printing
Turn Resample Image off when you want to change
resolution without changing image size
Use Bicubic Smoother when upsampling and use
Bicubic Sharper when downsampling
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Time to play with the Image Size dialog box
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Color Management

• There be monsters here

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What color is this?
Purple?
Nope. Its 100110000001001111111111
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The Monsters of Color

• Computers cannot see color. To them images are
  only a collection of 1s and 0s.
• Monitors cannot see color. To them images are
  just electrical signals that tell them how to
  adjust electron flow.
• Printers cannot see color. To them images are
  only a set of directions on how to control
  printheads.

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The RGB vs. CMYK Complication

• Red, Green, Blue
• Used by monitors and data projectors
• Works by adding colors together (additive)
• RedGreenBlue White

• Cyan, Magenta, Yellow, Black
• Used by printers
• Works by absorbing color (subtractive)
• CyanMagentaYellow Black
• Imperfections in ink absorption causes CMY
  muddy brown
• Black (K) is added to fix the problem

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The Reality of Color

• Image color is device-dependent. It varies
  depending on the physical characteristics and
  configuration settings of the computer, monitor
  and printer.
• Example In a store the TVs are often all tuned
  to the same program. They are all receiving the
  same input signal yet they all display slightly
  different colors.

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Color Management

• Color management is the science of displaying and
  printing images with consistent color across
  different computers and printers.
• Its as mysterious as theoretical physics, but
  fortunately we can get by without knowing too
  many of the details.

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Color Spaces

• Color spaces are 3D models used to represent a
  range of real-world colors using numbers.
• Each color space contains a different sub-set of
  real-world colors (check out the Color Sync
  utility).
• Color profiles define the color space and are the
  basis for all color management operations.

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Working Space vs. Device Spaces

• The Working Space Profile defines the color space
  within the application.
• You must specify the working space.
• In Illustrator, PhotoShop, and InDesign each
  image has its own working space profile.
• Device Space Profiles define the color behavior
  of the device (monitor, printer, scanner, digital
  camera, etc.).
• For most devices you just select the proper
  manufacturers profile.
• However, you should create your own monitor
  profile.

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Working Space

• Keep images in RGB mode
• Specify the sRGB color space (sRGB IEC
  61966-2.1)
• This color space closely matches the color range
  displayed by monitors.
• The color laser printer and both HP plotters are
  configured to color match images created in the
  sRGB color space.

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Setting Working Space Profile

• Open the color settings dialog box

Set RGB working space to sRGB
Set RGB color management policy to convert to
working space
Turn on Ask when opening and Ask when pasting
Illustrator color setting dialog box. Other
applications have similar dialog boxes.
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Monitor Profile

• To display color accurately, applications must
  know how your monitor behaves.
• what color is white
• how it displays color tones
• what actual color it produces when fed pure red,
  green or blue signals.
• The monitor profile tells applications how your
  monitor behaves.
• Most factory CRT monitor profiles do not
  accurately represent the monitor's behavior .

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Critical take-home point 4

• You must pay attention to your monitor profile if
  you want accurate colors!

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Monitor Calibration

• Both Mac and Windows users have the ability to
  create custom profiles for their monitors.
• The utility is built into the Mac Displays
  system preference dialog box.
• On Windows you need Adobe Gamma which is a
  control panel installed along with Photoshop.

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Monitor CalibrationHow to

• Set a familiar desktop picture you can use to
  assess color quality.
• Mac Open the Displays control panel, select the
  color dialog box and click on calibrate.
• Windows Open the Adobe Gamma control panel
• Follow the utilities directions.

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Monitor calibration demonstration.
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File Formats

• This used to be simple

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File FormatsNew Complications

• Traditionally raster-only formats like TIFF now
  support vector graphics and other image features
  like layers.
• Traditionally closed formats like PDF are now
  editable.
• Applications handle the new format features
  differently.
• The best I can do is give some general guidelines.

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Loss-Less vs. Lossy Compression

• Loss-Less Files are compressed without altering
  the pixels. Repeated saves do not degrade the
  image. Examples RLE, LZW and ZIP
• Lossy The act of compressing the file
  permanently alters the pixels. Each iteration of
  saving does more damage. Example JPEG
• Rule of thumb Dont use lossy compression on
  working images nor on images destined for
  printing.

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Formats for Working Images

• The retention of editing flexibility is
  paramount.
• Native application formats (i.e. PhotoShop or
  Illustrator) are best.
• TIFFs are a good choice if you have to move
  images across many applications.

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Formats for Archiving Images

• You want a format that preserves as much editing
  flexibility as possible.
• You want a format that will be readable in the
  future by many applications.
• TIFF is the hands-down favorite.
• PDF and EPS files are also good.
• Native application formats are the most flexible,
  but have long-term compatibility concerns.

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Formats for the Web

• File must compress well to minimize network
  bandwidth requirements.
• File format must be readable by web browsers.
• JPEG, GIF and PNG are the industry standards. PDF
  is also common.
• Use care when compressing JPEGs.
• PNG supports both raster and vector data.

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Formats for Printing

• The format must support color matching.
• The format must be easily digestible by printers
  and page layout applications.
• TIFF, EPS, PDF and PostScript are the industry
  standards.
• Native application formats also work well.
• Watch out for compression and downsampling of
  raster data.

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Printing
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The Printing Process

• Printers only print raster images.
• When you print complex files like posters, the
  computer sends the printer a Postscript file that
  contains both raster and vector data.
• The printer then converts (RIPs) the file into
  one giant raster image.
• The amount of memory needed for the RIP depends
  on the file size and complexity.

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Images in Documents
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Embedded Files

• Embedding files incorporates the image data into
  the document file
• Creates a single file for the entire document
• Quickly leads to very large files
• Slow to load, save and display
• Cumbersome to edit

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Linked Files

• Linking a file incorporates a small version of
  the image into the document as a place holder.
• When printing or displaying, the computer links
  back to the original file.
• Images are edited in their native application
  and the changes are automatically incorporated
  in the document.

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Linked Files

• Documents with linked files are relatively small
  so they open fast and do not bog down the
  computer.
• Caution Changing the relative location of the
  document and link files breaks the link.
• Linked files can often be embedded later if you
  need the entire document in one file.

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Images in PowerPoint

• File links in PowerPoint are very fragile.
• Embed all files in PowerPoint.
• If you need to use linked files, like video
  clips, save your PowerPoint presentation as a
  PowerPoint Package.
• Use JPEG, GIF or PNG format.
• Scale images to the proper pixel resolution at
  104dpi to simplify importing.

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Images in Illustrator and InDesign

• Use linked images to improve workflow and
  versatility.
• Use native format for PhotoShop files, TIFF for
  rasters from other programs.
• Scale images as needed in document, but then
  rescale originals.
• Embed linked images to make portable file.

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Lets look at building documents.